vendor/llvm-project/llvmorg-10-init-17538-gd11abddb32f

145 files changed, 4899 insertions, 1094 deletions

diff --git a/clang/include/clang/AST/ASTConcept.h b/clang/include/clang/AST/ASTConcept.h
index 896d857d8c96..30c4706d2a15 100644
--- a/clang/include/clang/AST/ASTConcept.h
+++ b/clang/include/clang/AST/ASTConcept.h
@@ -22,10 +22,25 @@
 #include <utility>
 namespace clang {
 class ConceptDecl;
+class ConceptSpecializationExpr;
+
+/// The result of a constraint satisfaction check, containing the necessary
+/// information to diagnose an unsatisfied constraint.
+class ConstraintSatisfaction : public llvm::FoldingSetNode {
+  // The template-like entity that 'owns' the constraint checked here (can be a
+  // constrained entity or a concept).
+  NamedDecl *ConstraintOwner = nullptr;
+  llvm::SmallVector<TemplateArgument, 4> TemplateArgs;
+
+public:
+
+  ConstraintSatisfaction() = default;
+
+  ConstraintSatisfaction(NamedDecl *ConstraintOwner,
+                         ArrayRef<TemplateArgument> TemplateArgs) :
+      ConstraintOwner(ConstraintOwner), TemplateArgs(TemplateArgs.begin(),
+                                                     TemplateArgs.end()) { }
 
-/// \brief The result of a constraint satisfaction check, containing the
-/// necessary information to diagnose an unsatisfied constraint.
-struct ConstraintSatisfaction {
   using SubstitutionDiagnostic = std::pair<SourceLocation, StringRef>;
   using Detail = llvm::PointerUnion<Expr *, SubstitutionDiagnostic *>;
 
@@ -37,9 +52,13 @@ struct ConstraintSatisfaction {
   /// invalid expression.
   llvm::SmallVector<std::pair<const Expr *, Detail>, 4> Details;
 
-  // This can leak if used in an AST node, use ASTConstraintSatisfaction
-  // instead.
-  void *operator new(size_t bytes, ASTContext &C) = delete;
+  void Profile(llvm::FoldingSetNodeID &ID, const ASTContext &C) {
+    Profile(ID, C, ConstraintOwner, TemplateArgs);
+  }
+
+  static void Profile(llvm::FoldingSetNodeID &ID, const ASTContext &C,
+                      NamedDecl *ConstraintOwner,
+                      ArrayRef<TemplateArgument> TemplateArgs);
 };
 
 /// Pairs of unsatisfied atomic constraint expressions along with the
diff --git a/clang/include/clang/AST/ASTContext.h b/clang/include/clang/AST/ASTContext.h
index fb269cef1ce8..f8403cf13c4a 100644
--- a/clang/include/clang/AST/ASTContext.h
+++ b/clang/include/clang/AST/ASTContext.h
@@ -88,6 +88,7 @@ class AtomicExpr;
 class BlockExpr;
 class BuiltinTemplateDecl;
 class CharUnits;
+class ConceptDecl;
 class CXXABI;
 class CXXConstructorDecl;
 class CXXMethodDecl;
@@ -211,7 +212,7 @@ class ASTContext : public RefCountedBase<ASTContext> {
   mutable llvm::FoldingSet<ObjCObjectPointerType> ObjCObjectPointerTypes;
   mutable llvm::FoldingSet<DependentUnaryTransformType>
     DependentUnaryTransformTypes;
-  mutable llvm::FoldingSet<AutoType> AutoTypes;
+  mutable llvm::ContextualFoldingSet<AutoType, ASTContext&> AutoTypes;
   mutable llvm::FoldingSet<DeducedTemplateSpecializationType>
     DeducedTemplateSpecializationTypes;
   mutable llvm::FoldingSet<AtomicType> AtomicTypes;
@@ -1542,7 +1543,9 @@ public:
 
   /// C++11 deduced auto type.
   QualType getAutoType(QualType DeducedType, AutoTypeKeyword Keyword,
-                       bool IsDependent, bool IsPack = false) const;
+                       bool IsDependent, bool IsPack = false,
+                       ConceptDecl *TypeConstraintConcept = nullptr,
+                       ArrayRef<TemplateArgument> TypeConstraintArgs ={}) const;
 
   /// C++11 deduction pattern for 'auto' type.
   QualType getAutoDeductType() const;
diff --git a/clang/include/clang/AST/ASTNodeTraverser.h b/clang/include/clang/AST/ASTNodeTraverser.h
index e0ebb020e697..9ebf64a12af5 100644
--- a/clang/include/clang/AST/ASTNodeTraverser.h
+++ b/clang/include/clang/AST/ASTNodeTraverser.h
@@ -548,8 +548,8 @@ public:
   }
 
   void VisitNonTypeTemplateParmDecl(const NonTypeTemplateParmDecl *D) {
-    if (const auto *TC = D->getPlaceholderTypeConstraint())
-      Visit(TC->getImmediatelyDeclaredConstraint());
+    if (const auto *E = D->getPlaceholderTypeConstraint())
+      Visit(E);
     if (D->hasDefaultArgument())
       Visit(D->getDefaultArgument(), SourceRange(),
             D->getDefaultArgStorage().getInheritedFrom(),
diff --git a/clang/include/clang/AST/DeclCXX.h b/clang/include/clang/AST/DeclCXX.h
index b716ea453a5a..2e8e31dbf4c7 100644
--- a/clang/include/clang/AST/DeclCXX.h
+++ b/clang/include/clang/AST/DeclCXX.h
@@ -1893,6 +1893,37 @@ public:
   static bool classofKind(Kind K) { return K == CXXDeductionGuide; }
 };
 
+/// \brief Represents the body of a requires-expression.
+///
+/// This decl exists merely to serve as the DeclContext for the local
+/// parameters of the requires expression as well as other declarations inside
+/// it.
+///
+/// \code
+/// template<typename T> requires requires (T t) { {t++} -> regular; }
+/// \endcode
+///
+/// In this example, a RequiresExpr object will be generated for the expression,
+/// and a RequiresExprBodyDecl will be created to hold the parameter t and the
+/// template argument list imposed by the compound requirement.
+class RequiresExprBodyDecl : public Decl, public DeclContext {
+  RequiresExprBodyDecl(ASTContext &C, DeclContext *DC, SourceLocation StartLoc)
+      : Decl(RequiresExprBody, DC, StartLoc), DeclContext(RequiresExprBody) {}
+
+public:
+  friend class ASTDeclReader;
+  friend class ASTDeclWriter;
+
+  static RequiresExprBodyDecl *Create(ASTContext &C, DeclContext *DC,
+                                      SourceLocation StartLoc);
+
+  static RequiresExprBodyDecl *CreateDeserialized(ASTContext &C, unsigned ID);
+
+  // Implement isa/cast/dyncast/etc.
+  static bool classof(const Decl *D) { return classofKind(D->getKind()); }
+  static bool classofKind(Kind K) { return K == RequiresExprBody; }
+};
+
 /// Represents a static or instance method of a struct/union/class.
 ///
 /// In the terminology of the C++ Standard, these are the (static and
diff --git a/clang/include/clang/AST/DeclTemplate.h b/clang/include/clang/AST/DeclTemplate.h
index 7a55d04a0f35..7a9f623d8152 100644
--- a/clang/include/clang/AST/DeclTemplate.h
+++ b/clang/include/clang/AST/DeclTemplate.h
@@ -1102,6 +1102,17 @@ public:
   /// template.
   ArrayRef<TemplateArgument> getInjectedTemplateArgs();
 
+  /// Return whether this function template is an abbreviated function template,
+  /// e.g. `void foo(auto x)` or `template<typename T> void foo(auto x)`
+  bool isAbbreviated() const {
+    // Since the invented template parameters generated from 'auto' parameters
+    // are either appended to the end of the explicit template parameter list or
+    // form a new template paramter list, we can simply observe the last
+    // parameter to determine if such a thing happened.
+    const TemplateParameterList *TPL = getTemplateParameters();
+    return TPL->getParam(TPL->size() - 1)->isImplicit();
+  }
+
   /// Merge \p Prev with our RedeclarableTemplateDecl::Common.
   void mergePrevDecl(FunctionTemplateDecl *Prev);
 
@@ -1215,7 +1226,6 @@ public:
                                       bool ParameterPack,
                                       bool HasTypeConstraint = false,
                                       Optional<unsigned> NumExpanded = None);
-
   static TemplateTypeParmDecl *CreateDeserialized(const ASTContext &C,
                                                   unsigned ID);
   static TemplateTypeParmDecl *CreateDeserialized(const ASTContext &C,
@@ -1374,7 +1384,8 @@ class NonTypeTemplateParmDecl final
     : public DeclaratorDecl,
       protected TemplateParmPosition,
       private llvm::TrailingObjects<NonTypeTemplateParmDecl,
-                                    std::pair<QualType, TypeSourceInfo *>> {
+                                    std::pair<QualType, TypeSourceInfo *>,
+                                    Expr *> {
   friend class ASTDeclReader;
   friend TrailingObjects;
 
@@ -1429,10 +1440,12 @@ public:
          ArrayRef<TypeSourceInfo *> ExpandedTInfos);
 
   static NonTypeTemplateParmDecl *CreateDeserialized(ASTContext &C,
-                                                     unsigned ID);
+                                                     unsigned ID,
+                                                     bool HasTypeConstraint);
   static NonTypeTemplateParmDecl *CreateDeserialized(ASTContext &C,
                                                      unsigned ID,
-                                                     unsigned NumExpandedTypes);
+                                                     unsigned NumExpandedTypes,
+                                                     bool HasTypeConstraint);
 
   using TemplateParmPosition::getDepth;
   using TemplateParmPosition::setDepth;
@@ -1543,20 +1556,22 @@ public:
     return TypesAndInfos[I].second;
   }
 
-  /// Return the type-constraint in the placeholder type of this non-type
+  /// Return the constraint introduced by the placeholder type of this non-type
   /// template parameter (if any).
-  TypeConstraint *getPlaceholderTypeConstraint() const {
-    // TODO: Concepts: Implement once we have actual placeholders with type
-    //                 constraints.
-    return nullptr;
+  Expr *getPlaceholderTypeConstraint() const {
+    return hasPlaceholderTypeConstraint() ? *getTrailingObjects<Expr *>() :
+        nullptr;
+  }
+
+  void setPlaceholderTypeConstraint(Expr *E) {
+    *getTrailingObjects<Expr *>() = E;
   }
 
   /// Determine whether this non-type template parameter's type has a
   /// placeholder with a type-constraint.
   bool hasPlaceholderTypeConstraint() const {
-    // TODO: Concepts: Implement once we have actual placeholders with type
-    //                 constraints.
-    return false;
+    auto *AT = getType()->getContainedAutoType();
+    return AT && AT->isConstrained();
   }
 
   /// \brief Get the associated-constraints of this template parameter.
@@ -1566,8 +1581,8 @@ public:
   /// Use this instead of getPlaceholderImmediatelyDeclaredConstraint for
   /// concepts APIs that accept an ArrayRef of constraint expressions.
   void getAssociatedConstraints(llvm::SmallVectorImpl<const Expr *> &AC) const {
-    if (TypeConstraint *TC = getPlaceholderTypeConstraint())
-      AC.push_back(TC->getImmediatelyDeclaredConstraint());
+    if (Expr *E = getPlaceholderTypeConstraint())
+      AC.push_back(E);
   }
 
   // Implement isa/cast/dyncast/etc.
diff --git a/clang/include/clang/AST/ExprCXX.h b/clang/include/clang/AST/ExprCXX.h
index 2c29409e0ca5..cea360d12e91 100644
--- a/clang/include/clang/AST/ExprCXX.h
+++ b/clang/include/clang/AST/ExprCXX.h
@@ -14,7 +14,6 @@
 #ifndef LLVM_CLANG_AST_EXPRCXX_H
 #define LLVM_CLANG_AST_EXPRCXX_H
 
-#include "clang/AST/ASTConcept.h"
 #include "clang/AST/Decl.h"
 #include "clang/AST/DeclBase.h"
 #include "clang/AST/DeclCXX.h"
@@ -4836,99 +4835,6 @@ public:
   }
 };
 
-/// \brief Represents the specialization of a concept - evaluates to a prvalue
-/// of type bool.
-///
-/// According to C++2a [expr.prim.id]p3 an id-expression that denotes the
-/// specialization of a concept results in a prvalue of type bool.
-class ConceptSpecializationExpr final : public Expr, public ConceptReference,
-      private llvm::TrailingObjects<ConceptSpecializationExpr,
-                                    TemplateArgument> {
-  friend class ASTStmtReader;
-  friend TrailingObjects;
-public:
-  using SubstitutionDiagnostic = std::pair<SourceLocation, std::string>;
-
-protected:
-  /// \brief The number of template arguments in the tail-allocated list of
-  /// converted template arguments.
-  unsigned NumTemplateArgs;
-
-  /// \brief Information about the satisfaction of the named concept with the
-  /// given arguments. If this expression is value dependent, this is to be
-  /// ignored.
-  ASTConstraintSatisfaction *Satisfaction;
-
-  ConceptSpecializationExpr(const ASTContext &C, NestedNameSpecifierLoc NNS,
-                            SourceLocation TemplateKWLoc,
-                            DeclarationNameInfo ConceptNameInfo,
-                            NamedDecl *FoundDecl, ConceptDecl *NamedConcept,
-                            const ASTTemplateArgumentListInfo *ArgsAsWritten,
-                            ArrayRef<TemplateArgument> ConvertedArgs,
-                            const ConstraintSatisfaction *Satisfaction);
-
-  ConceptSpecializationExpr(EmptyShell Empty, unsigned NumTemplateArgs);
-
-public:
-
-  static ConceptSpecializationExpr *
-  Create(const ASTContext &C, NestedNameSpecifierLoc NNS,
-         SourceLocation TemplateKWLoc, DeclarationNameInfo ConceptNameInfo,
-         NamedDecl *FoundDecl, ConceptDecl *NamedConcept,
-         const ASTTemplateArgumentListInfo *ArgsAsWritten,
-         ArrayRef<TemplateArgument> ConvertedArgs,
-         const ConstraintSatisfaction *Satisfaction);
-
-  static ConceptSpecializationExpr *
-  Create(ASTContext &C, EmptyShell Empty, unsigned NumTemplateArgs);
-
-  ArrayRef<TemplateArgument> getTemplateArguments() const {
-    return ArrayRef<TemplateArgument>(getTrailingObjects<TemplateArgument>(),
-                                      NumTemplateArgs);
-  }
-
-  /// \brief Set new template arguments for this concept specialization.
-  void setTemplateArguments(ArrayRef<TemplateArgument> Converted);
-
-  /// \brief Whether or not the concept with the given arguments was satisfied
-  /// when the expression was created.
-  /// The expression must not be dependent.
-  bool isSatisfied() const {
-    assert(!isValueDependent()
-           && "isSatisfied called on a dependent ConceptSpecializationExpr");
-    return Satisfaction->IsSatisfied;
-  }
-
-  /// \brief Get elaborated satisfaction info about the template arguments'
-  /// satisfaction of the named concept.
-  /// The expression must not be dependent.
-  const ASTConstraintSatisfaction &getSatisfaction() const {
-    assert(!isValueDependent()
-           && "getSatisfaction called on dependent ConceptSpecializationExpr");
-    return *Satisfaction;
-  }
-
-  static bool classof(const Stmt *T) {
-    return T->getStmtClass() == ConceptSpecializationExprClass;
-  }
-
-  SourceLocation getBeginLoc() const LLVM_READONLY {
-    return ConceptName.getBeginLoc();
-  }
-
-  SourceLocation getEndLoc() const LLVM_READONLY {
-    return ArgsAsWritten->RAngleLoc;
-  }
-
-  // Iterators
-  child_range children() {
-    return child_range(child_iterator(), child_iterator());
-  }
-  const_child_range children() const {
-    return const_child_range(const_child_iterator(), const_child_iterator());
-  }
-};
-
 } // namespace clang
 
 #endif // LLVM_CLANG_AST_EXPRCXX_H
diff --git a/clang/include/clang/AST/ExprConcepts.h b/clang/include/clang/AST/ExprConcepts.h
new file mode 100644
index 000000000000..2a64326e8604
--- /dev/null
+++ b/clang/include/clang/AST/ExprConcepts.h
@@ -0,0 +1,540 @@
+//===- ExprConcepts.h - C++2a Concepts expressions --------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+/// \file
+/// Defines Expressions and AST nodes for C++2a concepts.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_AST_EXPRCONCEPTS_H
+#define LLVM_CLANG_AST_EXPRCONCEPTS_H
+
+#include "clang/AST/ASTContext.h"
+#include "clang/AST/ASTConcept.h"
+#include "clang/AST/Decl.h"
+#include "clang/AST/DeclarationName.h"
+#include "clang/AST/DeclTemplate.h"
+#include "clang/AST/Expr.h"
+#include "clang/AST/NestedNameSpecifier.h"
+#include "clang/AST/TemplateBase.h"
+#include "clang/AST/Type.h"
+#include "clang/Basic/SourceLocation.h"
+#include "llvm/Support/TrailingObjects.h"
+#include <utility>
+#include <string>
+
+namespace clang {
+class ASTStmtReader;
+class ASTStmtWriter;
+
+/// \brief Represents the specialization of a concept - evaluates to a prvalue
+/// of type bool.
+///
+/// According to C++2a [expr.prim.id]p3 an id-expression that denotes the
+/// specialization of a concept results in a prvalue of type bool.
+class ConceptSpecializationExpr final : public Expr, public ConceptReference,
+      private llvm::TrailingObjects<ConceptSpecializationExpr,
+                                    TemplateArgument> {
+  friend class ASTStmtReader;
+  friend TrailingObjects;
+public:
+  using SubstitutionDiagnostic = std::pair<SourceLocation, std::string>;
+
+protected:
+  /// \brief The number of template arguments in the tail-allocated list of
+  /// converted template arguments.
+  unsigned NumTemplateArgs;
+
+  /// \brief Information about the satisfaction of the named concept with the
+  /// given arguments. If this expression is value dependent, this is to be
+  /// ignored.
+  ASTConstraintSatisfaction *Satisfaction;
+
+  ConceptSpecializationExpr(const ASTContext &C, NestedNameSpecifierLoc NNS,
+                            SourceLocation TemplateKWLoc,
+                            DeclarationNameInfo ConceptNameInfo,
+                            NamedDecl *FoundDecl, ConceptDecl *NamedConcept,
+                            const ASTTemplateArgumentListInfo *ArgsAsWritten,
+                            ArrayRef<TemplateArgument> ConvertedArgs,
+                            const ConstraintSatisfaction *Satisfaction);
+
+  ConceptSpecializationExpr(EmptyShell Empty, unsigned NumTemplateArgs);
+
+public:
+
+  static ConceptSpecializationExpr *
+  Create(const ASTContext &C, NestedNameSpecifierLoc NNS,
+         SourceLocation TemplateKWLoc, DeclarationNameInfo ConceptNameInfo,
+         NamedDecl *FoundDecl, ConceptDecl *NamedConcept,
+         const ASTTemplateArgumentListInfo *ArgsAsWritten,
+         ArrayRef<TemplateArgument> ConvertedArgs,
+         const ConstraintSatisfaction *Satisfaction);
+
+  static ConceptSpecializationExpr *
+  Create(ASTContext &C, EmptyShell Empty, unsigned NumTemplateArgs);
+
+  ArrayRef<TemplateArgument> getTemplateArguments() const {
+    return ArrayRef<TemplateArgument>(getTrailingObjects<TemplateArgument>(),
+                                      NumTemplateArgs);
+  }
+
+  /// \brief Set new template arguments for this concept specialization.
+  void setTemplateArguments(ArrayRef<TemplateArgument> Converted);
+
+  /// \brief Whether or not the concept with the given arguments was satisfied
+  /// when the expression was created.
+  /// The expression must not be dependent.
+  bool isSatisfied() const {
+    assert(!isValueDependent()
+           && "isSatisfied called on a dependent ConceptSpecializationExpr");
+    return Satisfaction->IsSatisfied;
+  }
+
+  /// \brief Get elaborated satisfaction info about the template arguments'
+  /// satisfaction of the named concept.
+  /// The expression must not be dependent.
+  const ASTConstraintSatisfaction &getSatisfaction() const {
+    assert(!isValueDependent()
+           && "getSatisfaction called on dependent ConceptSpecializationExpr");
+    return *Satisfaction;
+  }
+
+  static bool classof(const Stmt *T) {
+    return T->getStmtClass() == ConceptSpecializationExprClass;
+  }
+
+  SourceLocation getBeginLoc() const LLVM_READONLY {
+    return ConceptName.getBeginLoc();
+  }
+
+  SourceLocation getEndLoc() const LLVM_READONLY {
+    return ArgsAsWritten->RAngleLoc;
+  }
+
+  // Iterators
+  child_range children() {
+    return child_range(child_iterator(), child_iterator());
+  }
+  const_child_range children() const {
+    return const_child_range(const_child_iterator(), const_child_iterator());
+  }
+};
+
+namespace concepts {
+
+/// \brief A static requirement that can be used in a requires-expression to
+/// check properties of types and expression.
+class Requirement {
+public:
+  // Note - simple and compound requirements are both represented by the same
+  // class (ExprRequirement).
+  enum RequirementKind { RK_Type, RK_Simple, RK_Compound, RK_Nested };
+private:
+  const RequirementKind Kind;
+  bool Dependent : 1;
+  bool ContainsUnexpandedParameterPack : 1;
+  bool Satisfied : 1;
+public:
+  struct SubstitutionDiagnostic {
+    StringRef SubstitutedEntity;
+    // FIXME: Store diagnostics semantically and not as prerendered strings.
+    //  Fixing this probably requires serialization of PartialDiagnostic
+    //  objects.
+    SourceLocation DiagLoc;
+    StringRef DiagMessage;
+  };
+
+  Requirement(RequirementKind Kind, bool IsDependent,
+              bool ContainsUnexpandedParameterPack, bool IsSatisfied = true) :
+      Kind(Kind), Dependent(IsDependent),
+      ContainsUnexpandedParameterPack(ContainsUnexpandedParameterPack),
+      Satisfied(IsSatisfied) {}
+
+  RequirementKind getKind() const { return Kind; }
+
+  bool isSatisfied() const {
+    assert(!Dependent &&
+           "isSatisfied can only be called on non-dependent requirements.");
+    return Satisfied;
+  }
+
+  void setSatisfied(bool IsSatisfied) {
+    assert(!Dependent &&
+           "setSatisfied can only be called on non-dependent requirements.");
+    Satisfied = IsSatisfied;
+  }
+
+  void setDependent(bool IsDependent) { Dependent = IsDependent; }
+  bool isDependent() const { return Dependent; }
+
+  void setContainsUnexpandedParameterPack(bool Contains) {
+    ContainsUnexpandedParameterPack = Contains;
+  }
+  bool containsUnexpandedParameterPack() const {
+    return ContainsUnexpandedParameterPack;
+  }
+};
+
+/// \brief A requires-expression requirement which queries the existence of a
+/// type name or type template specialization ('type' requirements).
+class TypeRequirement : public Requirement {
+public:
+  enum SatisfactionStatus {
+      SS_Dependent,
+      SS_SubstitutionFailure,
+      SS_Satisfied
+  };
+private:
+  llvm::PointerUnion<SubstitutionDiagnostic *, TypeSourceInfo *> Value;
+  SatisfactionStatus Status;
+public:
+  friend ASTStmtReader;
+  friend ASTStmtWriter;
+
+  /// \brief Construct a type requirement from a type. If the given type is not
+  /// dependent, this indicates that the type exists and the requirement will be
+  /// satisfied. Otherwise, the SubstitutionDiagnostic constructor is to be
+  /// used.
+  TypeRequirement(TypeSourceInfo *T);
+
+  /// \brief Construct a type requirement when the nested name specifier is
+  /// invalid due to a bad substitution. The requirement is unsatisfied.
+  TypeRequirement(SubstitutionDiagnostic *Diagnostic) :
+      Requirement(RK_Type, false, false, false), Value(Diagnostic),
+      Status(SS_SubstitutionFailure) {}
+
+  SatisfactionStatus getSatisfactionStatus() const { return Status; }
+  void setSatisfactionStatus(SatisfactionStatus Status) {
+    this->Status = Status;
+  }
+
+  bool isSubstitutionFailure() const {
+    return Status == SS_SubstitutionFailure;
+  }
+
+  SubstitutionDiagnostic *getSubstitutionDiagnostic() const {
+    assert(Status == SS_SubstitutionFailure &&
+           "Attempted to get substitution diagnostic when there has been no "
+           "substitution failure.");
+    return Value.get<SubstitutionDiagnostic *>();
+  }
+
+  TypeSourceInfo *getType() const {
+    assert(!isSubstitutionFailure() &&
+           "Attempted to get type when there has been a substitution failure.");
+    return Value.get<TypeSourceInfo *>();
+  }
+
+  static bool classof(const Requirement *R) {
+    return R->getKind() == RK_Type;
+  }
+};
+
+/// \brief A requires-expression requirement which queries the validity and
+/// properties of an expression ('simple' and 'compound' requirements).
+class ExprRequirement : public Requirement {
+public:
+  enum SatisfactionStatus {
+      SS_Dependent,
+      SS_ExprSubstitutionFailure,
+      SS_NoexceptNotMet,
+      SS_TypeRequirementSubstitutionFailure,
+      SS_ConstraintsNotSatisfied,
+      SS_Satisfied
+  };
+  class ReturnTypeRequirement {
+      llvm::PointerIntPair<
+          llvm::PointerUnion<TemplateParameterList *, SubstitutionDiagnostic *>,
+          1, bool>
+          TypeConstraintInfo;
+  public:
+      friend ASTStmtReader;
+      friend ASTStmtWriter;
+
+      /// \brief No return type requirement was specified.
+      ReturnTypeRequirement() : TypeConstraintInfo(nullptr, 0) {}
+
+      /// \brief A return type requirement was specified but it was a
+      /// substitution failure.
+      ReturnTypeRequirement(SubstitutionDiagnostic *SubstDiag) :
+          TypeConstraintInfo(SubstDiag, 0) {}
+
+      /// \brief A 'type constraint' style return type requirement.
+      /// \param TPL an invented template parameter list containing a single
+      /// type parameter with a type-constraint.
+      // TODO: Can we maybe not save the whole template parameter list and just
+      //  the type constraint? Saving the whole TPL makes it easier to handle in
+      //  serialization but is less elegant.
+      ReturnTypeRequirement(TemplateParameterList *TPL);
+
+      bool isDependent() const {
+        return TypeConstraintInfo.getInt();
+      }
+
+      bool containsUnexpandedParameterPack() const {
+        if (!isTypeConstraint())
+          return false;
+        return getTypeConstraintTemplateParameterList()
+                ->containsUnexpandedParameterPack();
+      }
+
+      bool isEmpty() const {
+        return TypeConstraintInfo.getPointer().isNull();
+      }
+
+      bool isSubstitutionFailure() const {
+        return !isEmpty() &&
+            TypeConstraintInfo.getPointer().is<SubstitutionDiagnostic *>();
+      }
+
+      bool isTypeConstraint() const {
+        return !isEmpty() &&
+            TypeConstraintInfo.getPointer().is<TemplateParameterList *>();
+      }
+
+      SubstitutionDiagnostic *getSubstitutionDiagnostic() const {
+        assert(isSubstitutionFailure());
+        return TypeConstraintInfo.getPointer().get<SubstitutionDiagnostic *>();
+      }
+
+      const TypeConstraint *getTypeConstraint() const;
+
+      TemplateParameterList *getTypeConstraintTemplateParameterList() const {
+        assert(isTypeConstraint());
+        return TypeConstraintInfo.getPointer().get<TemplateParameterList *>();
+      }
+  };
+private:
+  llvm::PointerUnion<Expr *, SubstitutionDiagnostic *> Value;
+  SourceLocation NoexceptLoc; // May be empty if noexcept wasn't specified.
+  ReturnTypeRequirement TypeReq;
+  ConceptSpecializationExpr *SubstitutedConstraintExpr;
+  SatisfactionStatus Status;
+public:
+  friend ASTStmtReader;
+  friend ASTStmtWriter;
+
+  /// \brief Construct a compound requirement.
+  /// \param E the expression which is checked by this requirement.
+  /// \param IsSimple whether this was a simple requirement in source.
+  /// \param NoexceptLoc the location of the noexcept keyword, if it was
+  /// specified, otherwise an empty location.
+  /// \param Req the requirement for the type of the checked expression.
+  /// \param Status the satisfaction status of this requirement.
+  ExprRequirement(
+      Expr *E, bool IsSimple, SourceLocation NoexceptLoc,
+      ReturnTypeRequirement Req, SatisfactionStatus Status,
+      ConceptSpecializationExpr *SubstitutedConstraintExpr = nullptr);
+
+  /// \brief Construct a compound requirement whose expression was a
+  /// substitution failure. The requirement is not satisfied.
+  /// \param E the diagnostic emitted while instantiating the original
+  /// expression.
+  /// \param IsSimple whether this was a simple requirement in source.
+  /// \param NoexceptLoc the location of the noexcept keyword, if it was
+  /// specified, otherwise an empty location.
+  /// \param Req the requirement for the type of the checked expression (omit
+  /// if no requirement was specified).
+  ExprRequirement(SubstitutionDiagnostic *E, bool IsSimple,
+                  SourceLocation NoexceptLoc, ReturnTypeRequirement Req = {});
+
+  bool isSimple() const { return getKind() == RK_Simple; }
+  bool isCompound() const { return getKind() == RK_Compound; }
+
+  bool hasNoexceptRequirement() const { return NoexceptLoc.isValid(); }
+  SourceLocation getNoexceptLoc() const { return NoexceptLoc; }
+
+  SatisfactionStatus getSatisfactionStatus() const { return Status; }
+
+  bool isExprSubstitutionFailure() const {
+    return Status == SS_ExprSubstitutionFailure;
+  }
+
+  const ReturnTypeRequirement &getReturnTypeRequirement() const {
+    return TypeReq;
+  }
+
+  ConceptSpecializationExpr *
+  getReturnTypeRequirementSubstitutedConstraintExpr() const {
+    assert(Status >= SS_TypeRequirementSubstitutionFailure);
+    return SubstitutedConstraintExpr;
+  }
+
+  SubstitutionDiagnostic *getExprSubstitutionDiagnostic() const {
+    assert(isExprSubstitutionFailure() &&
+           "Attempted to get expression substitution diagnostic when there has "
+           "been no expression substitution failure");
+    return Value.get<SubstitutionDiagnostic *>();
+  }
+
+  Expr *getExpr() const {
+    assert(!isExprSubstitutionFailure() &&
+           "ExprRequirement has no expression because there has been a "
+           "substitution failure.");
+    return Value.get<Expr *>();
+  }
+
+  static bool classof(const Requirement *R) {
+    return R->getKind() == RK_Compound || R->getKind() == RK_Simple;
+  }
+};
+
+/// \brief A requires-expression requirement which is satisfied when a general
+/// constraint expression is satisfied ('nested' requirements).
+class NestedRequirement : public Requirement {
+  llvm::PointerUnion<Expr *, SubstitutionDiagnostic *> Value;
+  const ASTConstraintSatisfaction *Satisfaction = nullptr;
+
+public:
+  friend ASTStmtReader;
+  friend ASTStmtWriter;
+
+  NestedRequirement(SubstitutionDiagnostic *SubstDiag) :
+      Requirement(RK_Nested, /*Dependent=*/false,
+                  /*ContainsUnexpandedParameterPack*/false,
+                  /*Satisfied=*/false), Value(SubstDiag) {}
+
+  NestedRequirement(Expr *Constraint) :
+      Requirement(RK_Nested, /*Dependent=*/true,
+                  Constraint->containsUnexpandedParameterPack()),
+      Value(Constraint) {
+    assert(Constraint->isInstantiationDependent() &&
+           "Nested requirement with non-dependent constraint must be "
+           "constructed with a ConstraintSatisfaction object");
+  }
+
+  NestedRequirement(ASTContext &C, Expr *Constraint,
+                    const ConstraintSatisfaction &Satisfaction) :
+      Requirement(RK_Nested, Constraint->isInstantiationDependent(),
+                  Constraint->containsUnexpandedParameterPack(),
+                  Satisfaction.IsSatisfied),
+      Value(Constraint),
+      Satisfaction(ASTConstraintSatisfaction::Create(C, Satisfaction)) {}
+
+  bool isSubstitutionFailure() const {
+    return Value.is<SubstitutionDiagnostic *>();
+  }
+
+  SubstitutionDiagnostic *getSubstitutionDiagnostic() const {
+    assert(isSubstitutionFailure() &&
+           "getSubstitutionDiagnostic() may not be called when there was no "
+           "substitution failure.");
+    return Value.get<SubstitutionDiagnostic *>();
+  }
+
+  Expr *getConstraintExpr() const {
+    assert(!isSubstitutionFailure() && "getConstraintExpr() may not be called "
+                                       "on nested requirements with "
+                                       "substitution failures.");
+    return Value.get<Expr *>();
+  }
+
+  const ASTConstraintSatisfaction &getConstraintSatisfaction() const {
+    assert(!isSubstitutionFailure() && "getConstraintSatisfaction() may not be "
+                                       "called on nested requirements with "
+                                       "substitution failures.");
+    return *Satisfaction;
+  }
+
+  static bool classof(const Requirement *R) {
+    return R->getKind() == RK_Nested;
+  }
+};
+
+} // namespace concepts
+
+/// C++2a [expr.prim.req]:
+///     A requires-expression provides a concise way to express requirements on
+///     template arguments. A requirement is one that can be checked by name
+///     lookup (6.4) or by checking properties of types and expressions.
+///     [...]
+///     A requires-expression is a prvalue of type bool [...]
+class RequiresExpr final : public Expr,
+    llvm::TrailingObjects<RequiresExpr, ParmVarDecl *,
+                          concepts::Requirement *> {
+  friend TrailingObjects;
+  friend class ASTStmtReader;
+
+  unsigned NumLocalParameters;
+  unsigned NumRequirements;
+  RequiresExprBodyDecl *Body;
+  SourceLocation RBraceLoc;
+
+  unsigned numTrailingObjects(OverloadToken<ParmVarDecl *>) const {
+    return NumLocalParameters;
+  }
+
+  unsigned numTrailingObjects(OverloadToken<concepts::Requirement *>) const {
+    return NumRequirements;
+  }
+
+  RequiresExpr(ASTContext &C, SourceLocation RequiresKWLoc,
+               RequiresExprBodyDecl *Body,
+               ArrayRef<ParmVarDecl *> LocalParameters,
+               ArrayRef<concepts::Requirement *> Requirements,
+               SourceLocation RBraceLoc);
+  RequiresExpr(ASTContext &C, EmptyShell Empty, unsigned NumLocalParameters,
+               unsigned NumRequirements);
+
+public:
+  static RequiresExpr *
+  Create(ASTContext &C, SourceLocation RequiresKWLoc,
+         RequiresExprBodyDecl *Body, ArrayRef<ParmVarDecl *> LocalParameters,
+         ArrayRef<concepts::Requirement *> Requirements,
+         SourceLocation RBraceLoc);
+  static RequiresExpr *
+  Create(ASTContext &C, EmptyShell Empty, unsigned NumLocalParameters,
+         unsigned NumRequirements);
+
+  ArrayRef<ParmVarDecl *> getLocalParameters() const {
+    return {getTrailingObjects<ParmVarDecl *>(), NumLocalParameters};
+  }
+
+  RequiresExprBodyDecl *getBody() const { return Body; }
+
+  ArrayRef<concepts::Requirement *> getRequirements() const {
+    return {getTrailingObjects<concepts::Requirement *>(), NumRequirements};
+  }
+
+  /// \brief Whether or not the requires clause is satisfied.
+  /// The expression must not be dependent.
+  bool isSatisfied() const {
+    assert(!isValueDependent()
+           && "isSatisfied called on a dependent RequiresExpr");
+    return RequiresExprBits.IsSatisfied;
+  }
+
+  SourceLocation getRequiresKWLoc() const {
+    return RequiresExprBits.RequiresKWLoc;
+  }
+
+  SourceLocation getRBraceLoc() const { return RBraceLoc; }
+
+  static bool classof(const Stmt *T) {
+    return T->getStmtClass() == RequiresExprClass;
+  }
+
+  SourceLocation getBeginLoc() const LLVM_READONLY {
+    return RequiresExprBits.RequiresKWLoc;
+  }
+  SourceLocation getEndLoc() const LLVM_READONLY {
+    return RBraceLoc;
+  }
+
+  // Iterators
+  child_range children() {
+    return child_range(child_iterator(), child_iterator());
+  }
+  const_child_range children() const {
+    return const_child_range(const_child_iterator(), const_child_iterator());
+  }
+};
+
+} // namespace clang
+
+#endif // LLVM_CLANG_AST_EXPRCONCEPTS_H
\ No newline at end of file
diff --git a/clang/include/clang/AST/PropertiesBase.td b/clang/include/clang/AST/PropertiesBase.td
index 9aacdb9fee36..ba0f237a3bc3 100644
--- a/clang/include/clang/AST/PropertiesBase.td
+++ b/clang/include/clang/AST/PropertiesBase.td
@@ -99,6 +99,8 @@ def DeclRef : RefPropertyType<"Decl"> { let ConstWhenWriting = 1; }
     SubclassPropertyType<"TagDecl", DeclRef>;
   def TemplateDeclRef :
     SubclassPropertyType<"TemplateDecl", DeclRef>;
+  def ConceptDeclRef :
+    SubclassPropertyType<"ConceptDecl", DeclRef>;
   def TemplateTypeParmDeclRef :
     SubclassPropertyType<"TemplateTypeParmDecl", DeclRef>;
   def TemplateTemplateParmDeclRef :
diff --git a/clang/include/clang/AST/RecursiveASTVisitor.h b/clang/include/clang/AST/RecursiveASTVisitor.h
index f8ab8e451d8c..86521d82c6ff 100644
--- a/clang/include/clang/AST/RecursiveASTVisitor.h
+++ b/clang/include/clang/AST/RecursiveASTVisitor.h
@@ -23,6 +23,7 @@
 #include "clang/AST/DeclOpenMP.h"
 #include "clang/AST/DeclTemplate.h"
 #include "clang/AST/Expr.h"
+#include "clang/AST/ExprConcepts.h"
 #include "clang/AST/ExprCXX.h"
 #include "clang/AST/ExprObjC.h"
 #include "clang/AST/ExprOpenMP.h"
@@ -1039,7 +1040,13 @@ DEF_TRAVERSE_TYPE(UnaryTransformType, {
   TRY_TO(TraverseType(T->getUnderlyingType()));
 })
 
-DEF_TRAVERSE_TYPE(AutoType, { TRY_TO(TraverseType(T->getDeducedType())); })
+DEF_TRAVERSE_TYPE(AutoType, {
+  TRY_TO(TraverseType(T->getDeducedType()));
+  if (T->isConstrained()) {
+    TRY_TO(TraverseDecl(T->getTypeConstraintConcept()));
+    TRY_TO(TraverseTemplateArguments(T->getArgs(), T->getNumArgs()));
+  }
+})
 DEF_TRAVERSE_TYPE(DeducedTemplateSpecializationType, {
   TRY_TO(TraverseTemplateName(T->getTemplateName()));
   TRY_TO(TraverseType(T->getDeducedType()));
@@ -1286,6 +1293,12 @@ DEF_TRAVERSE_TYPELOC(UnaryTransformType, {
 
 DEF_TRAVERSE_TYPELOC(AutoType, {
   TRY_TO(TraverseType(TL.getTypePtr()->getDeducedType()));
+  if (TL.isConstrained()) {
+    TRY_TO(TraverseNestedNameSpecifierLoc(TL.getNestedNameSpecifierLoc()));
+    TRY_TO(TraverseDeclarationNameInfo(TL.getConceptNameInfo()));
+    for (unsigned I = 0, E = TL.getNumArgs(); I != E; ++I)
+      TRY_TO(TraverseTemplateArgumentLoc(TL.getArgLoc(I)));
+  }
 })
 
 DEF_TRAVERSE_TYPELOC(DeducedTemplateSpecializationType, {
@@ -2138,6 +2151,8 @@ DEF_TRAVERSE_DECL(ParmVarDecl, {
     TRY_TO(TraverseStmt(D->getDefaultArg()));
 })
 
+DEF_TRAVERSE_DECL(RequiresExprBodyDecl, {})
+
 #undef DEF_TRAVERSE_DECL
 
 // ----------------- Stmt traversal -----------------
@@ -2709,6 +2724,28 @@ DEF_TRAVERSE_STMT(ConceptSpecializationExpr, {
   TRY_TO(TraverseConceptReference(*S));
 })
 
+DEF_TRAVERSE_STMT(RequiresExpr, {
+  TRY_TO(TraverseDecl(S->getBody()));
+  for (ParmVarDecl *Parm : S->getLocalParameters())
+    TRY_TO(TraverseDecl(Parm));
+  for (concepts::Requirement *Req : S->getRequirements())
+    if (auto *TypeReq = dyn_cast<concepts::TypeRequirement>(Req)) {
+      if (!TypeReq->isSubstitutionFailure())
+        TRY_TO(TraverseTypeLoc(TypeReq->getType()->getTypeLoc()));
+    } else if (auto *ExprReq = dyn_cast<concepts::ExprRequirement>(Req)) {
+      if (!ExprReq->isExprSubstitutionFailure())
+        TRY_TO(TraverseStmt(ExprReq->getExpr()));
+      auto &RetReq = ExprReq->getReturnTypeRequirement();
+      if (RetReq.isTypeConstraint())
+        TRY_TO(TraverseTemplateParameterListHelper(
+                   RetReq.getTypeConstraintTemplateParameterList()));
+    } else {
+      auto *NestedReq = cast<concepts::NestedRequirement>(Req);
+      if (!NestedReq->isSubstitutionFailure())
+        TRY_TO(TraverseStmt(NestedReq->getConstraintExpr()));
+    }
+})
+
 // These literals (all of them) do not need any action.
 DEF_TRAVERSE_STMT(IntegerLiteral, {})
 DEF_TRAVERSE_STMT(FixedPointLiteral, {})
diff --git a/clang/include/clang/AST/Stmt.h b/clang/include/clang/AST/Stmt.h
index eaacb1a5b252..253b76941991 100644
--- a/clang/include/clang/AST/Stmt.h
+++ b/clang/include/clang/AST/Stmt.h
@@ -910,6 +910,17 @@ protected:
     SourceLocation NameLoc;
   };
 
+  class RequiresExprBitfields {
+    friend class ASTStmtReader;
+    friend class ASTStmtWriter;
+    friend class RequiresExpr;
+
+    unsigned : NumExprBits;
+
+    unsigned IsSatisfied : 1;
+    SourceLocation RequiresKWLoc;
+  };
+
   //===--- C++ Coroutines TS bitfields classes ---===//
 
   class CoawaitExprBitfields {
@@ -1008,6 +1019,7 @@ protected:
     UnresolvedMemberExprBitfields UnresolvedMemberExprBits;
     CXXNoexceptExprBitfields CXXNoexceptExprBits;
     SubstNonTypeTemplateParmExprBitfields SubstNonTypeTemplateParmExprBits;
+    RequiresExprBitfields RequiresExprBits;
 
     // C++ Coroutines TS expressions
     CoawaitExprBitfields CoawaitBits;
diff --git a/clang/include/clang/AST/StmtVisitor.h b/clang/include/clang/AST/StmtVisitor.h
index d3be93d228cc..3e5155199eac 100644
--- a/clang/include/clang/AST/StmtVisitor.h
+++ b/clang/include/clang/AST/StmtVisitor.h
@@ -13,6 +13,7 @@
 #ifndef LLVM_CLANG_AST_STMTVISITOR_H
 #define LLVM_CLANG_AST_STMTVISITOR_H
 
+#include "clang/AST/ExprConcepts.h"
 #include "clang/AST/ExprCXX.h"
 #include "clang/AST/ExprObjC.h"
 #include "clang/AST/ExprOpenMP.h"
diff --git a/clang/include/clang/AST/TemplateBase.h b/clang/include/clang/AST/TemplateBase.h
index 058a5bc0a067..93f7b62b8aea 100644
--- a/clang/include/clang/AST/TemplateBase.h
+++ b/clang/include/clang/AST/TemplateBase.h
@@ -637,7 +637,7 @@ public:
   }
 
   static const ASTTemplateArgumentListInfo *
-  Create(ASTContext &C, const TemplateArgumentListInfo &List);
+  Create(const ASTContext &C, const TemplateArgumentListInfo &List);
 };
 
 /// Represents an explicit template argument list in C++, e.g.,
@@ -702,6 +702,11 @@ inline const TemplateArgument &
   return getArgs()[Idx];
 }
 
+inline const TemplateArgument &AutoType::getArg(unsigned Idx) const {
+  assert(Idx < getNumArgs() && "Template argument out of range");
+  return getArgs()[Idx];
+}
+
 } // namespace clang
 
 #endif // LLVM_CLANG_AST_TEMPLATEBASE_H
diff --git a/clang/include/clang/AST/Type.h b/clang/include/clang/AST/Type.h
index f5955c45fafc..abc8136653fa 100644
--- a/clang/include/clang/AST/Type.h
+++ b/clang/include/clang/AST/Type.h
@@ -58,6 +58,7 @@ namespace clang {
 
 class ExtQuals;
 class QualType;
+class ConceptDecl;
 class TagDecl;
 class Type;
 
@@ -1683,6 +1684,15 @@ protected:
     /// Was this placeholder type spelled as 'auto', 'decltype(auto)',
     /// or '__auto_type'?  AutoTypeKeyword value.
     unsigned Keyword : 2;
+
+    /// The number of template arguments in the type-constraints, which is
+    /// expected to be able to hold at least 1024 according to [implimits].
+    /// However as this limit is somewhat easy to hit with template
+    /// metaprogramming we'd prefer to keep it as large as possible.
+    /// At the moment it has been left as a non-bitfield since this type
+    /// safely fits in 64 bits as an unsigned, so there is no reason to
+    /// introduce the performance impact of a bitfield.
+    unsigned NumArgs;
   };
 
   class SubstTemplateTypeParmPackTypeBitfields {
@@ -4814,8 +4824,7 @@ public:
 
 /// Common base class for placeholders for types that get replaced by
 /// placeholder type deduction: C++11 auto, C++14 decltype(auto), C++17 deduced
-/// class template types, and (eventually) constrained type names from the C++
-/// Concepts TS.
+/// class template types, and constrained type names.
 ///
 /// These types are usually a placeholder for a deduced type. However, before
 /// the initializer is attached, or (usually) if the initializer is
@@ -4860,18 +4869,50 @@ public:
   }
 };
 
-/// Represents a C++11 auto or C++14 decltype(auto) type.
-class AutoType : public DeducedType, public llvm::FoldingSetNode {
+/// Represents a C++11 auto or C++14 decltype(auto) type, possibly constrained
+/// by a type-constraint.
+class alignas(8) AutoType : public DeducedType, public llvm::FoldingSetNode {
   friend class ASTContext; // ASTContext creates these
 
+  ConceptDecl *TypeConstraintConcept;
+
   AutoType(QualType DeducedAsType, AutoTypeKeyword Keyword,
-           bool IsDeducedAsDependent, bool IsDeducedAsPack)
-      : DeducedType(Auto, DeducedAsType, IsDeducedAsDependent,
-                    IsDeducedAsDependent, IsDeducedAsPack) {
-    AutoTypeBits.Keyword = (unsigned)Keyword;
+           bool IsDeducedAsDependent, bool IsDeducedAsPack, ConceptDecl *CD,
+           ArrayRef<TemplateArgument> TypeConstraintArgs);
+
+  const TemplateArgument *getArgBuffer() const {
+    return reinterpret_cast<const TemplateArgument*>(this+1);
+  }
+
+  TemplateArgument *getArgBuffer() {
+    return reinterpret_cast<TemplateArgument*>(this+1);
   }
 
 public:
+  /// Retrieve the template arguments.
+  const TemplateArgument *getArgs() const {
+    return getArgBuffer();
+  }
+
+  /// Retrieve the number of template arguments.
+  unsigned getNumArgs() const {
+    return AutoTypeBits.NumArgs;
+  }
+
+  const TemplateArgument &getArg(unsigned Idx) const; // in TemplateBase.h
+
+  ArrayRef<TemplateArgument> getTypeConstraintArguments() const {
+    return {getArgs(), getNumArgs()};
+  }
+
+  ConceptDecl *getTypeConstraintConcept() const {
+    return TypeConstraintConcept;
+  }
+
+  bool isConstrained() const {
+    return TypeConstraintConcept != nullptr;
+  }
+
   bool isDecltypeAuto() const {
     return getKeyword() == AutoTypeKeyword::DecltypeAuto;
   }
@@ -4880,18 +4921,15 @@ public:
     return (AutoTypeKeyword)AutoTypeBits.Keyword;
   }
 
-  void Profile(llvm::FoldingSetNodeID &ID) {
-    Profile(ID, getDeducedType(), getKeyword(), isDependentType(),
-            containsUnexpandedParameterPack());
+  void Profile(llvm::FoldingSetNodeID &ID, const ASTContext &Context) {
+    Profile(ID, Context, getDeducedType(), getKeyword(), isDependentType(),
+            getTypeConstraintConcept(), getTypeConstraintArguments());
   }
 
-  static void Profile(llvm::FoldingSetNodeID &ID, QualType Deduced,
-                      AutoTypeKeyword Keyword, bool IsDependent, bool IsPack) {
-    ID.AddPointer(Deduced.getAsOpaquePtr());
-    ID.AddInteger((unsigned)Keyword);
-    ID.AddBoolean(IsDependent);
-    ID.AddBoolean(IsPack);
-  }
+  static void Profile(llvm::FoldingSetNodeID &ID, const ASTContext &Context,
+                      QualType Deduced, AutoTypeKeyword Keyword,
+                      bool IsDependent, ConceptDecl *CD,
+                      ArrayRef<TemplateArgument> Arguments);
 
   static bool classof(const Type *T) {
     return T->getTypeClass() == Auto;
diff --git a/clang/include/clang/AST/TypeLoc.h b/clang/include/clang/AST/TypeLoc.h
index c3baaa3e4174..3fc53d823c37 100644
--- a/clang/include/clang/AST/TypeLoc.h
+++ b/clang/include/clang/AST/TypeLoc.h
@@ -14,6 +14,7 @@
 #ifndef LLVM_CLANG_AST_TYPELOC_H
 #define LLVM_CLANG_AST_TYPELOC_H
 
+#include "clang/AST/DeclarationName.h"
 #include "clang/AST/NestedNameSpecifier.h"
 #include "clang/AST/TemplateBase.h"
 #include "clang/AST/Type.h"
@@ -34,6 +35,7 @@ namespace clang {
 class Attr;
 class ASTContext;
 class CXXRecordDecl;
+class ConceptDecl;
 class Expr;
 class ObjCInterfaceDecl;
 class ObjCProtocolDecl;
@@ -181,6 +183,11 @@ public:
   ///   AttributedTypeLoc, for those type attributes that behave as qualifiers
   TypeLoc findExplicitQualifierLoc() const;
 
+  /// Get the typeloc of an AutoType whose type will be deduced for a variable
+  /// with an initializer of this type. This looks through declarators like
+  /// pointer types, but not through decltype or typedefs.
+  AutoTypeLoc getContainedAutoTypeLoc() const;
+
   /// Initializes this to state that every location in this
   /// type is the given location.
   ///
@@ -1923,8 +1930,137 @@ class DeducedTypeLoc
     : public InheritingConcreteTypeLoc<TypeSpecTypeLoc, DeducedTypeLoc,
                                        DeducedType> {};
 
+struct AutoTypeLocInfo : TypeSpecLocInfo {
+  NestedNameSpecifierLoc NestedNameSpec;
+  SourceLocation TemplateKWLoc;
+  SourceLocation ConceptNameLoc;
+  NamedDecl *FoundDecl;
+  SourceLocation LAngleLoc;
+  SourceLocation RAngleLoc;
+};
+
 class AutoTypeLoc
-    : public InheritingConcreteTypeLoc<DeducedTypeLoc, AutoTypeLoc, AutoType> {
+    : public ConcreteTypeLoc<DeducedTypeLoc,
+                             AutoTypeLoc,
+                             AutoType,
+                             AutoTypeLocInfo> {
+public:
+  AutoTypeKeyword getAutoKeyword() const {
+    return getTypePtr()->getKeyword();
+  }
+
+  bool isConstrained() const {
+    return getTypePtr()->isConstrained();
+  }
+
+  const NestedNameSpecifierLoc &getNestedNameSpecifierLoc() const {
+    return getLocalData()->NestedNameSpec;
+  }
+
+  void setNestedNameSpecifierLoc(NestedNameSpecifierLoc NNS) {
+    getLocalData()->NestedNameSpec = NNS;
+  }
+
+  SourceLocation getTemplateKWLoc() const {
+    return getLocalData()->TemplateKWLoc;
+  }
+
+  void setTemplateKWLoc(SourceLocation Loc) {
+    getLocalData()->TemplateKWLoc = Loc;
+  }
+
+  SourceLocation getConceptNameLoc() const {
+    return getLocalData()->ConceptNameLoc;
+  }
+
+  void setConceptNameLoc(SourceLocation Loc) {
+    getLocalData()->ConceptNameLoc = Loc;
+  }
+
+  NamedDecl *getFoundDecl() const {
+    return getLocalData()->FoundDecl;
+  }
+
+  void setFoundDecl(NamedDecl *D) {
+    getLocalData()->FoundDecl = D;
+  }
+
+  ConceptDecl *getNamedConcept() const {
+    return getTypePtr()->getTypeConstraintConcept();
+  }
+
+  DeclarationNameInfo getConceptNameInfo() const;
+
+  bool hasExplicitTemplateArgs() const {
+    return getLocalData()->LAngleLoc.isValid();
+  }
+
+  SourceLocation getLAngleLoc() const {
+    return this->getLocalData()->LAngleLoc;
+  }
+
+  void setLAngleLoc(SourceLocation Loc) {
+    this->getLocalData()->LAngleLoc = Loc;
+  }
+
+  SourceLocation getRAngleLoc() const {
+    return this->getLocalData()->RAngleLoc;
+  }
+
+  void setRAngleLoc(SourceLocation Loc) {
+    this->getLocalData()->RAngleLoc = Loc;
+  }
+
+  unsigned getNumArgs() const {
+    return getTypePtr()->getNumArgs();
+  }
+
+  void setArgLocInfo(unsigned i, TemplateArgumentLocInfo AI) {
+    getArgInfos()[i] = AI;
+  }
+
+  TemplateArgumentLocInfo getArgLocInfo(unsigned i) const {
+    return getArgInfos()[i];
+  }
+
+  TemplateArgumentLoc getArgLoc(unsigned i) const {
+    return TemplateArgumentLoc(getTypePtr()->getTypeConstraintArguments()[i],
+                               getArgLocInfo(i));
+  }
+
+  SourceRange getLocalSourceRange() const {
+    return{
+        isConstrained()
+          ? (getNestedNameSpecifierLoc()
+               ? getNestedNameSpecifierLoc().getBeginLoc()
+               : (getTemplateKWLoc().isValid()
+                  ? getTemplateKWLoc()
+                  : getConceptNameLoc()))
+          : getNameLoc(),
+        getNameLoc()
+    };
+  }
+
+  void copy(AutoTypeLoc Loc) {
+    unsigned size = getFullDataSize();
+    assert(size == Loc.getFullDataSize());
+    memcpy(Data, Loc.Data, size);
+  }
+
+  void initializeLocal(ASTContext &Context, SourceLocation Loc);
+
+  unsigned getExtraLocalDataSize() const {
+    return getNumArgs() * sizeof(TemplateArgumentLocInfo);
+  }
+
+  unsigned getExtraLocalDataAlignment() const {
+    return alignof(TemplateArgumentLocInfo);
+  }
+
+private:
+  TemplateArgumentLocInfo *getArgInfos() const {
+    return static_cast<TemplateArgumentLocInfo*>(getExtraLocalData());
+  }
 };
 
 class DeducedTemplateSpecializationTypeLoc
diff --git a/clang/include/clang/AST/TypeProperties.td b/clang/include/clang/AST/TypeProperties.td
index 4df2e2f77e2b..3cf56e5a5629 100644
--- a/clang/include/clang/AST/TypeProperties.td
+++ b/clang/include/clang/AST/TypeProperties.td
@@ -395,6 +395,13 @@ let Class = AutoType in {
   def : Property<"keyword", AutoTypeKeyword> {
     let Read = [{ node->getKeyword() }];
   }
+  def : Property<"typeConstraintConcept", Optional<ConceptDeclRef>> {
+    let Read = [{ makeOptionalFromPointer(
+        const_cast<const ConceptDecl*>(node->getTypeConstraintConcept())) }];
+  }
+  def : Property<"typeConstraintArguments", Array<TemplateArgument>> {
+    let Read = [{ node->getTypeConstraintArguments() }];
+  }
   // FIXME: better enumerated value
   // Only really required when the deduced type is null
   def : Property<"dependence", UInt32> {
@@ -406,7 +413,9 @@ let Class = AutoType in {
   def : Creator<[{
     return ctx.getAutoType(makeNullableFromOptional(deducedType), keyword,
                            /*isDependentWithoutDeducedType*/ dependence > 0,
-                           /*isPackWithoutDeducedType*/ dependence > 1);
+                           /*isPackWithoutDeducedType*/ dependence > 1,
+                           makePointerFromOptional(typeConstraintConcept),
+                           typeConstraintArguments);
   }]>;
 }
 
diff --git a/clang/include/clang/Basic/AttrDocs.td b/clang/include/clang/Basic/AttrDocs.td
index 03d36ae7ab32..7976d08a5258 100644
--- a/clang/include/clang/Basic/AttrDocs.td
+++ b/clang/include/clang/Basic/AttrDocs.td
@@ -3995,8 +3995,6 @@ def PatchableFunctionEntryDocs : Documentation {
 before the function entry and N-M NOPs after the function entry. This attribute
 takes precedence over the command line option ``-fpatchable-function-entry=N,M``.
 ``M`` defaults to 0 if omitted.
-
-Currently, only M=0 is supported.
 }];
 }
 
diff --git a/clang/include/clang/Basic/Builtins.def b/clang/include/clang/Basic/Builtins.def
index d388afe7fae6..1a6c85ce2dd3 100644
--- a/clang/include/clang/Basic/Builtins.def
+++ b/clang/include/clang/Basic/Builtins.def
@@ -788,6 +788,9 @@ BUILTIN(__builtin_abort, "v", "Fnr")
 BUILTIN(__builtin_index, "c*cC*i", "Fn")
 BUILTIN(__builtin_rindex, "c*cC*i", "Fn")
 
+// ignored glibc builtin, see https://sourceware.org/bugzilla/show_bug.cgi?id=25399
+BUILTIN(__warn_memset_zero_len, "v", "nU")
+
 // Microsoft builtins.  These are only active with -fms-extensions.
 LANGBUILTIN(_alloca,          "v*z", "n", ALL_MS_LANGUAGES)
 LANGBUILTIN(__annotation,     "wC*.","n", ALL_MS_LANGUAGES)
diff --git a/clang/include/clang/Basic/CodeGenOptions.def b/clang/include/clang/Basic/CodeGenOptions.def
index 50fc1836282f..1ecae98b13b1 100644
--- a/clang/include/clang/Basic/CodeGenOptions.def
+++ b/clang/include/clang/Basic/CodeGenOptions.def
@@ -111,6 +111,7 @@ CODEGENOPT(XRayAlwaysEmitTypedEvents , 1, 0)
 VALUE_CODEGENOPT(XRayInstructionThreshold , 32, 200)
 
 VALUE_CODEGENOPT(PatchableFunctionEntryCount , 32, 0) ///< Number of NOPs at function entry
+VALUE_CODEGENOPT(PatchableFunctionEntryOffset , 32, 0)
 
 CODEGENOPT(InstrumentForProfiling , 1, 0) ///< Set when -pg is enabled.
 CODEGENOPT(CallFEntry , 1, 0) ///< Set when -mfentry is enabled.
diff --git a/clang/include/clang/Basic/DeclNodes.td b/clang/include/clang/Basic/DeclNodes.td
index c2c23237285b..d5bbc604819f 100644
--- a/clang/include/clang/Basic/DeclNodes.td
+++ b/clang/include/clang/Basic/DeclNodes.td
@@ -100,5 +100,6 @@ def OMPThreadPrivate : DeclNode<Decl>;
 def OMPAllocate : DeclNode<Decl>;
 def OMPRequires : DeclNode<Decl>;
 def Empty : DeclNode<Decl>;
+def RequiresExprBody : DeclNode<Decl>, DeclContext;
 def LifetimeExtendedTemporary : DeclNode<Decl>;
 
diff --git a/clang/include/clang/Basic/DiagnosticDriverKinds.td b/clang/include/clang/Basic/DiagnosticDriverKinds.td
index 0fe14e4e05be..2da41bef2669 100644
--- a/clang/include/clang/Basic/DiagnosticDriverKinds.td
+++ b/clang/include/clang/Basic/DiagnosticDriverKinds.td
@@ -408,7 +408,7 @@ def err_drv_unsupported_indirect_jump_opt : Error<
 def err_drv_unknown_indirect_jump_opt : Error<
   "unknown '-mindirect-jump=' option '%0'">;
 def err_drv_unsupported_fpatchable_function_entry_argument : Error<
-  "the second argument of '-fpatchable-function-entry' must be 0 or omitted">;
+  "the second argument of '-fpatchable-function-entry' must be smaller than the first argument">;
 
 def warn_drv_unable_to_find_directory_expected : Warning<
   "unable to find %0 directory, expected to be in '%1'">,
diff --git a/clang/include/clang/Basic/DiagnosticFrontendKinds.td b/clang/include/clang/Basic/DiagnosticFrontendKinds.td
index a798b498d4e9..ed2092fb4c84 100644
--- a/clang/include/clang/Basic/DiagnosticFrontendKinds.td
+++ b/clang/include/clang/Basic/DiagnosticFrontendKinds.td
@@ -105,6 +105,9 @@ def err_fe_invalid_wchar_type
     : Error<"invalid wchar_t type '%0'; must be one of 'char', 'short', 'int'">;
 def err_fe_invalid_exception_model
    : Error<"invalid exception model '%0' for target '%1'">;
+def warn_fe_concepts_ts_flag : Warning<
+  "-fconcepts-ts is deprecated - use '-std=c++2a' for Concepts support">,
+  InGroup<Deprecated>;
 
 def warn_fe_serialized_diag_merge_failure : Warning<
     "unable to merge a subprocess's serialized diagnostics">,
diff --git a/clang/include/clang/Basic/DiagnosticParseKinds.td b/clang/include/clang/Basic/DiagnosticParseKinds.td
index cc6a74ac3e6d..04b103e3087a 100644
--- a/clang/include/clang/Basic/DiagnosticParseKinds.td
+++ b/clang/include/clang/Basic/DiagnosticParseKinds.td
@@ -33,10 +33,6 @@ def err_asm_goto_cannot_have_output : Error<
 
 let CategoryName = "Parse Issue" in {
 
-def warn_cxx2a_compat_explicit_bool : Warning<
-  "this expression will be parsed as explicit(bool) in C++2a">,
-  InGroup<CXX2aCompat>, DefaultIgnore;
-
 def ext_empty_translation_unit : Extension<
   "ISO C requires a translation unit to contain at least one declaration">,
   InGroup<DiagGroup<"empty-translation-unit">>;
@@ -684,6 +680,15 @@ def err_ms_property_expected_comma_or_rparen : Error<
 def err_ms_property_initializer : Error<
   "property declaration cannot have an in-class initializer">;
 
+def warn_cxx2a_compat_explicit_bool : Warning<
+  "this expression will be parsed as explicit(bool) in C++2a">,
+  InGroup<CXX2aCompat>, DefaultIgnore;
+def warn_cxx17_compat_explicit_bool : Warning<
+  "explicit(bool) is incompatible with C++ standards before C++2a">,
+  InGroup<CXXPre2aCompat>, DefaultIgnore;
+def ext_explicit_bool : ExtWarn<"explicit(bool) is a C++2a extension">,
+  InGroup<CXX2a>;
+
 /// C++ Templates
 def err_expected_template : Error<"expected template">;
 def err_unknown_template_name : Error<
@@ -739,6 +744,33 @@ def err_friend_explicit_instantiation : Error<
 def err_explicit_instantiation_enum : Error<
   "enumerations cannot be explicitly instantiated">;
 def err_expected_template_parameter : Error<"expected template parameter">;
+def note_ill_formed_requires_expression_outside_template : Note<
+  "requires expression outside a template declaration may not contain invalid "
+  "types or expressions">;
+def err_empty_requires_expr : Error<
+  "a requires expression must contain at least one requirement">;
+def err_requires_expr_parameter_list_ellipsis : Error<
+  "varargs not allowed in requires expression">;
+def err_requires_expr_type_req_illegal_identifier : Error<
+  "expected identifier or template-id in type requirement">;
+def err_requires_expr_type_req_template_args_on_non_template : Error<
+  "template arguments provided for non-template '%0'">;
+def err_expected_semi_requirement : Error<
+  "expected ';' at end of requirement">;
+def err_requires_expr_missing_arrow : Error<
+  "expected '->' before expression type requirement">;
+def err_requires_expr_expected_type_constraint : Error<
+  "expected concept name with optional arguments">;
+def err_requires_expr_simple_requirement_noexcept : Error<
+  "'noexcept' can only be used in a compound requirement (with '{' '}' around "
+  "the expression)">;
+def err_requires_expr_simple_requirement_unexpected_tok : Error<
+  "unexpected %0 after expression; did you intend to use a compound "
+  "requirement (with '{' '}' around the expression)?">;
+def warn_requires_expr_in_simple_requirement : Warning<
+  "this requires expression will only be checked for syntactic validity; did "
+  "you intend to place it in a nested requirement? (add another 'requires' "
+  "before the expression)">, InGroup<DiagGroup<"requires-expression">>;
 
 def err_missing_dependent_template_keyword : Error<
   "use 'template' keyword to treat '%0' as a dependent template name">;
@@ -1339,6 +1371,8 @@ def err_concept_definition_not_identifier : Error<
 def ext_concept_legacy_bool_keyword : ExtWarn<
   "ISO C++2a does not permit the 'bool' keyword after 'concept'">,
   InGroup<DiagGroup<"concepts-ts-compat">>;
+def err_placeholder_expected_auto_or_decltype_auto : Error<
+  "expected 'auto' or 'decltype(auto)' after concept name">;
 }
 
 } // end of Parser diagnostics
diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index 7d8231d140e4..7636d04a34c3 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -2102,12 +2102,18 @@ def err_auto_not_allowed : Error<
   "|in template argument|in typedef|in type alias|in function return type"
   "|in conversion function type|here|in lambda parameter"
   "|in type allocated by 'new'|in K&R-style function parameter"
-  "|in template parameter|in friend declaration}1">;
+  "|in template parameter|in friend declaration|in function prototype that is "
+  "not a function declaration|in requires expression parameter}1">;
 def err_dependent_deduced_tst : Error<
   "typename specifier refers to "
   "%select{class template|function template|variable template|alias template|"
   "template template parameter|template}0 member in %1; "
   "argument deduction not allowed here">;
+def err_deduced_tst : Error<
+  "typename specifier refers to "
+  "%select{class template|function template|variable template|alias template|"
+  "template template parameter|template}0; argument deduction not allowed "
+  "here">;
 def err_auto_not_allowed_var_inst : Error<
   "'auto' variable template instantiation is not allowed">;
 def err_auto_var_requires_init : Error<
@@ -2590,25 +2596,65 @@ def note_constraints_not_satisfied : Note<
 def note_substituted_constraint_expr_is_ill_formed : Note<
   "because substituted constraint expression is ill-formed%0">;
 def note_atomic_constraint_evaluated_to_false : Note<
-  "%select{and |because }0'%1' evaluated to false">;
+  "%select{and|because}0 '%1' evaluated to false">;
 def note_concept_specialization_constraint_evaluated_to_false : Note<
-  "%select{and |because }0'%1' evaluated to false">;
+  "%select{and|because}0 '%1' evaluated to false">;
 def note_single_arg_concept_specialization_constraint_evaluated_to_false : Note<
-  "%select{and |because }0%1 does not satisfy %2">;
+  "%select{and|because}0 %1 does not satisfy %2">;
 def note_atomic_constraint_evaluated_to_false_elaborated : Note<
-  "%select{and |because }0'%1' (%2 %3 %4) evaluated to false">;
+  "%select{and|because}0 '%1' (%2 %3 %4) evaluated to false">;
 def err_constrained_virtual_method : Error<
   "virtual function cannot have a requires clause">;
 def err_trailing_requires_clause_on_deduction_guide : Error<
   "deduction guide cannot have a requires clause">;
 def err_reference_to_function_with_unsatisfied_constraints : Error<
   "invalid reference to function %0: constraints not satisfied">;
+def note_requires_expr_ill_formed_expr : Note<
+  "expression is invalid: %0">;
+def note_requires_expr_no_implicit_conversion : Note<
+  "no implicit conversion exists between expression type %0 and expected type "
+  "%1">;
+def err_requires_expr_local_parameter_default_argument : Error<
+  "default arguments not allowed for parameters of a requires expression">;
+def err_requires_expr_parameter_referenced_in_evaluated_context : Error<
+  "constraint variable %0 cannot be used in an evaluated context">;
+def note_expr_requirement_expr_substitution_error : Note<
+  "%select{and|because}0 '%1' would be invalid: %2">;
+def note_expr_requirement_expr_unknown_substitution_error : Note<
+  "%select{and|because}0 '%1' would be invalid">;
+def note_expr_requirement_noexcept_not_met : Note<
+  "%select{and|because}0 '%1' may throw an exception">;
+def note_expr_requirement_type_requirement_substitution_error : Note<
+  "%select{and|because}0 '%1' would be invalid: %2">;
+def note_expr_requirement_type_requirement_unknown_substitution_error : Note<
+  "%select{and|because}0 '%1' would be invalid">;
+def note_expr_requirement_constraints_not_satisfied : Note<
+  "%select{and|because}0 type constraint '%1' was not satisfied:">;
+def note_expr_requirement_constraints_not_satisfied_simple : Note<
+  "%select{and|because}0 %1 does not satisfy %2:">;
+def note_type_requirement_substitution_error : Note<
+  "%select{and|because}0 '%1' would be invalid: %2">;
+def note_type_requirement_unknown_substitution_error : Note<
+  "%select{and|because}0 '%1' would be invalid">;
+def err_type_requirement_non_type_template : Error<
+  "'%0' refers to a %select{class template|function template|"
+  "variable template|alias template|template template parameter|template}1, "
+  "not a type template">;
+def err_type_requirement_no_such_type : Error<
+  "'%0' does not name a type">;
+def note_nested_requirement_substitution_error : Note<
+  "%select{and|because}0 '%1' would be invalid: %2">;
+def note_nested_requirement_unknown_substitution_error : Note<
+  "%select{and|because}0 '%1' would be invalid">;
 def note_ambiguous_atomic_constraints : Note<
   "similar constraint expressions not considered equivalent; constraint "
   "expressions cannot be considered equivalent unless they originate from the "
   "same concept">;
 def note_ambiguous_atomic_constraints_similar_expression : Note<
   "similar constraint expression here">;
+def err_unsupported_placeholder_constraint : Error<
+  "constrained placeholder types other than simple 'auto' on non-type template "
+  "parameters not supported yet">;
 
 def err_template_different_requires_clause : Error<
   "requires clause differs in template redeclaration">;
@@ -2623,6 +2669,8 @@ def err_type_constraint_non_type_concept : Error<
 def err_type_constraint_missing_arguments : Error<
   "%0 requires more than 1 template argument; provide the remaining arguments "
   "explicitly to use it here">;
+def err_placeholder_constraints_not_satisfied : Error<
+  "deduced type %0 does not satisfy %1">;
 
 // C++11 char16_t/char32_t
 def warn_cxx98_compat_unicode_type : Warning<
@@ -4588,6 +4636,8 @@ def note_template_type_alias_instantiation_here : Note<
   "in instantiation of template type alias %0 requested here">;
 def note_template_exception_spec_instantiation_here : Note<
   "in instantiation of exception specification for %0 requested here">;
+def note_template_requirement_instantiation_here : Note<
+  "in instantiation of requirement here">;
 def warn_var_template_missing : Warning<"instantiation of variable %q0 "
   "required here, but no definition is available">,
   InGroup<UndefinedVarTemplate>;
@@ -4623,6 +4673,8 @@ def note_template_default_arg_checking : Note<
   "while checking a default template argument used here">;
 def note_concept_specialization_here : Note<
   "while checking the satisfaction of concept '%0' requested here">;
+def note_nested_requirement_here : Note<
+  "while checking the satisfaction of nested requirement requested here">;
 def note_checking_constraints_for_template_id_here : Note<
   "while checking constraint satisfaction for template '%0' required here">;
 def note_checking_constraints_for_var_spec_id_here : Note<
@@ -4756,8 +4808,12 @@ def err_typename_nested_not_found_requirement : Error<
   "declaration">;
 def err_typename_nested_not_type : Error<
     "typename specifier refers to non-type member %0 in %1">;
-def note_typename_refers_here : Note<
+def err_typename_not_type : Error<
+    "typename specifier refers to non-type %0">;
+def note_typename_member_refers_here : Note<
     "referenced member %0 is declared here">;
+def note_typename_refers_here : Note<
+    "referenced %0 is declared here">;
 def err_typename_missing : Error<
   "missing 'typename' prior to dependent type name '%0%1'">;
 def err_typename_missing_template : Error<
diff --git a/clang/include/clang/Basic/LangOptions.def b/clang/include/clang/Basic/LangOptions.def
index 068f206f4484..3319a3123976 100644
--- a/clang/include/clang/Basic/LangOptions.def
+++ b/clang/include/clang/Basic/LangOptions.def
@@ -237,7 +237,7 @@ LANGOPT(SizedDeallocation , 1, 0, "sized deallocation")
 LANGOPT(AlignedAllocation , 1, 0, "aligned allocation")
 LANGOPT(AlignedAllocationUnavailable, 1, 0, "aligned allocation functions are unavailable")
 LANGOPT(NewAlignOverride  , 32, 0, "maximum alignment guaranteed by '::operator new(size_t)'")
-LANGOPT(ConceptsTS , 1, 0, "enable C++ Extensions for Concepts")
+LANGOPT(ConceptSatisfactionCaching , 1, 1, "enable satisfaction caching for C++2a Concepts")
 BENIGN_LANGOPT(ModulesCodegen , 1, 0, "Modules code generation")
 BENIGN_LANGOPT(ModulesDebugInfo , 1, 0, "Modules debug info")
 BENIGN_LANGOPT(ElideConstructors , 1, 1, "C++ copy constructor elision")
diff --git a/clang/include/clang/Basic/StmtNodes.td b/clang/include/clang/Basic/StmtNodes.td
index 294993298a18..41923cddc493 100644
--- a/clang/include/clang/Basic/StmtNodes.td
+++ b/clang/include/clang/Basic/StmtNodes.td
@@ -164,6 +164,7 @@ def CoyieldExpr : StmtNode<CoroutineSuspendExpr>;
 
 // C++2a Concepts expressions
 def ConceptSpecializationExpr : StmtNode<Expr>;
+def RequiresExpr : StmtNode<Expr>;
 
 // Obj-C Expressions.
 def ObjCStringLiteral : StmtNode<Expr>;
diff --git a/clang/include/clang/Basic/TokenKinds.def b/clang/include/clang/Basic/TokenKinds.def
index d58b0d987d71..fec029ae995e 100644
--- a/clang/include/clang/Basic/TokenKinds.def
+++ b/clang/include/clang/Basic/TokenKinds.def
@@ -373,7 +373,7 @@ CXX11_KEYWORD(nullptr               , 0)
 CXX11_KEYWORD(static_assert         , KEYMSCOMPAT)
 CXX11_KEYWORD(thread_local          , 0)
 
-// C++2a / concepts TS keywords
+// C++2a keywords
 CONCEPTS_KEYWORD(concept)
 CONCEPTS_KEYWORD(requires)
 
diff --git a/clang/include/clang/Driver/CC1Options.td b/clang/include/clang/Driver/CC1Options.td
index 1d550eb15ea8..9387285518de 100644
--- a/clang/include/clang/Driver/CC1Options.td
+++ b/clang/include/clang/Driver/CC1Options.td
@@ -372,6 +372,9 @@ def fsanitize_coverage_no_prune
 def fsanitize_coverage_stack_depth
     : Flag<["-"], "fsanitize-coverage-stack-depth">,
       HelpText<"Enable max stack depth tracing">;
+def fpatchable_function_entry_offset_EQ
+    : Joined<["-"], "fpatchable-function-entry-offset=">, MetaVarName<"<M>">,
+      HelpText<"Generate M NOPs before function entry">;
 def fprofile_instrument_EQ : Joined<["-"], "fprofile-instrument=">,
     HelpText<"Enable PGO instrumentation. The accepted value is clang, llvm, "
              "or none">, Values<"none,clang,llvm">;
@@ -553,7 +556,10 @@ def ftest_module_file_extension_EQ :
   HelpText<"introduce a module file extension for testing purposes. "
            "The argument is parsed as blockname:major:minor:hashed:user info">;
 def fconcepts_ts : Flag<["-"], "fconcepts-ts">,
-  HelpText<"Enable C++ Extensions for Concepts.">;
+  HelpText<"Enable C++ Extensions for Concepts. (deprecated - use -std=c++2a)">;
+def fno_concept_satisfaction_caching : Flag<["-"],
+                                            "fno-concept-satisfaction-caching">,
+  HelpText<"Disable satisfaction caching for C++2a Concepts.">;
 
 let Group = Action_Group in {
 
diff --git a/clang/include/clang/Driver/Driver.h b/clang/include/clang/Driver/Driver.h
index fd25663bd358..6c3feaba0568 100644
--- a/clang/include/clang/Driver/Driver.h
+++ b/clang/include/clang/Driver/Driver.h
@@ -208,7 +208,7 @@ public:
   /// When the clangDriver lib is used through clang.exe, this provides a
   /// shortcut for executing the -cc1 command-line directly, in the same
   /// process.
-  typedef int (*CC1ToolFunc)(ArrayRef<const char *> argv);
+  typedef int (*CC1ToolFunc)(SmallVectorImpl<const char *> &ArgV);
   CC1ToolFunc CC1Main = nullptr;
 
 private:
diff --git a/clang/include/clang/Driver/Options.td b/clang/include/clang/Driver/Options.td
index abfa767afea8..0a60873443fc 100644
--- a/clang/include/clang/Driver/Options.td
+++ b/clang/include/clang/Driver/Options.td
@@ -1707,7 +1707,7 @@ def fno_max_type_align : Flag<["-"], "fno-max-type-align">, Group<f_Group>;
 def fpascal_strings : Flag<["-"], "fpascal-strings">, Group<f_Group>, Flags<[CC1Option]>,
   HelpText<"Recognize and construct Pascal-style string literals">;
 def fpatchable_function_entry_EQ : Joined<["-"], "fpatchable-function-entry=">, Group<f_Group>, Flags<[CC1Option]>,
-  HelpText<"Generate N NOPs at function entry">;
+  MetaVarName<"<N,M>">, HelpText<"Generate M NOPs before function entry and N-M NOPs after function entry">;
 def fpcc_struct_return : Flag<["-"], "fpcc-struct-return">, Group<f_Group>, Flags<[CC1Option]>,
   HelpText<"Override the default ABI to return all structs on the stack">;
 def fpch_preprocess : Flag<["-"], "fpch-preprocess">, Group<f_Group>;
diff --git a/clang/include/clang/Parse/Parser.h b/clang/include/clang/Parse/Parser.h
index e320c9647818..41f46861d089 100644
--- a/clang/include/clang/Parse/Parser.h
+++ b/clang/include/clang/Parse/Parser.h
@@ -806,6 +806,16 @@ public:
                                                  bool IsNewScope);
   bool TryAnnotateCXXScopeToken(bool EnteringContext = false);
 
+  bool MightBeCXXScopeToken() {
+    return Tok.is(tok::identifier) || Tok.is(tok::coloncolon) ||
+           (Tok.is(tok::annot_template_id) &&
+            NextToken().is(tok::coloncolon)) ||
+           Tok.is(tok::kw_decltype) || Tok.is(tok::kw___super);
+  }
+  bool TryAnnotateOptionalCXXScopeToken(bool EnteringContext = false) {
+    return MightBeCXXScopeToken() && TryAnnotateCXXScopeToken(EnteringContext);
+  }
+
 private:
   enum AnnotatedNameKind {
     /// Annotation has failed and emitted an error.
@@ -1923,6 +1933,7 @@ private:
   //===--------------------------------------------------------------------===//
   // C++ Concepts
 
+  ExprResult ParseRequiresExpression();
   void ParseTrailingRequiresClause(Declarator &D);
 
   //===--------------------------------------------------------------------===//
@@ -2395,6 +2406,11 @@ private:
   /// rather than a less-than expression.
   TPResult isTemplateArgumentList(unsigned TokensToSkip);
 
+  /// Determine whether an '(' after an 'explicit' keyword is part of a C++20
+  /// 'explicit(bool)' declaration, in earlier language modes where that is an
+  /// extension.
+  TPResult isExplicitBool();
+
   /// Determine whether an identifier has been tentatively declared as a
   /// non-type. Such tentative declarations should not be found to name a type
   /// during a tentative parse, but also should not be annotated as a non-type.
@@ -2756,7 +2772,7 @@ private:
          Declarator &D,
          SmallVectorImpl<DeclaratorChunk::ParamInfo> &ParamInfo);
   void ParseParameterDeclarationClause(
-         Declarator &D,
+         DeclaratorContext DeclaratorContext,
          ParsedAttributes &attrs,
          SmallVectorImpl<DeclaratorChunk::ParamInfo> &ParamInfo,
          SourceLocation &EllipsisLoc);
@@ -3064,13 +3080,13 @@ private:
                                SourceLocation &RAngleLoc);
   bool ParseTemplateParameterList(unsigned Depth,
                                   SmallVectorImpl<NamedDecl*> &TemplateParams);
-  bool isStartOfTemplateTypeParameter(bool &ScopeError);
+  TPResult isStartOfTemplateTypeParameter();
   NamedDecl *ParseTemplateParameter(unsigned Depth, unsigned Position);
   NamedDecl *ParseTypeParameter(unsigned Depth, unsigned Position);
   NamedDecl *ParseTemplateTemplateParameter(unsigned Depth, unsigned Position);
   NamedDecl *ParseNonTypeTemplateParameter(unsigned Depth, unsigned Position);
   bool isTypeConstraintAnnotation();
-  bool TryAnnotateTypeConstraint(CXXScopeSpec &SS);
+  bool TryAnnotateTypeConstraint();
   NamedDecl *
   ParseConstrainedTemplateTypeParameter(unsigned Depth, unsigned Position);
   void DiagnoseMisplacedEllipsis(SourceLocation EllipsisLoc,
@@ -3096,7 +3112,8 @@ private:
                                UnqualifiedId &TemplateName,
                                bool AllowTypeAnnotation = true,
                                bool TypeConstraint = false);
-  void AnnotateTemplateIdTokenAsType(bool IsClassName = false);
+  void AnnotateTemplateIdTokenAsType(CXXScopeSpec &SS,
+                                     bool IsClassName = false);
   bool ParseTemplateArgumentList(TemplateArgList &TemplateArgs);
   ParsedTemplateArgument ParseTemplateTemplateArgument();
   ParsedTemplateArgument ParseTemplateArgument();
diff --git a/clang/include/clang/Sema/DeclSpec.h b/clang/include/clang/Sema/DeclSpec.h
index aceec9cbe1c9..1559b51ea77f 100644
--- a/clang/include/clang/Sema/DeclSpec.h
+++ b/clang/include/clang/Sema/DeclSpec.h
@@ -349,6 +349,7 @@ private:
   unsigned TypeSpecOwned : 1;
   unsigned TypeSpecPipe : 1;
   unsigned TypeSpecSat : 1;
+  unsigned ConstrainedAuto : 1;
 
   // type-qualifiers
   unsigned TypeQualifiers : 5;  // Bitwise OR of TQ.
@@ -369,6 +370,7 @@ private:
     UnionParsedType TypeRep;
     Decl *DeclRep;
     Expr *ExprRep;
+    TemplateIdAnnotation *TemplateIdRep;
   };
 
   /// ExplicitSpecifier - Store information about explicit spicifer.
@@ -413,6 +415,9 @@ private:
   static bool isExprRep(TST T) {
     return (T == TST_typeofExpr || T == TST_decltype);
   }
+  static bool isTemplateIdRep(TST T) {
+    return (T == TST_auto || T == TST_decltype_auto);
+  }
 
   DeclSpec(const DeclSpec &) = delete;
   void operator=(const DeclSpec &) = delete;
@@ -430,7 +435,8 @@ public:
         TypeSpecComplex(TSC_unspecified), TypeSpecSign(TSS_unspecified),
         TypeSpecType(TST_unspecified), TypeAltiVecVector(false),
         TypeAltiVecPixel(false), TypeAltiVecBool(false), TypeSpecOwned(false),
-        TypeSpecPipe(false), TypeSpecSat(false), TypeQualifiers(TQ_unspecified),
+        TypeSpecPipe(false), TypeSpecSat(false), ConstrainedAuto(false),
+        TypeQualifiers(TQ_unspecified),
         FS_inline_specified(false), FS_forceinline_specified(false),
         FS_virtual_specified(false), FS_noreturn_specified(false),
         Friend_specified(false), ConstexprSpecifier(CSK_unspecified),
@@ -478,6 +484,7 @@ public:
   bool isTypeRep() const { return isTypeRep((TST) TypeSpecType); }
   bool isTypeSpecPipe() const { return TypeSpecPipe; }
   bool isTypeSpecSat() const { return TypeSpecSat; }
+  bool isConstrainedAuto() const { return ConstrainedAuto; }
 
   ParsedType getRepAsType() const {
     assert(isTypeRep((TST) TypeSpecType) && "DeclSpec does not store a type");
@@ -491,6 +498,11 @@ public:
     assert(isExprRep((TST) TypeSpecType) && "DeclSpec does not store an expr");
     return ExprRep;
   }
+  TemplateIdAnnotation *getRepAsTemplateId() const {
+    assert(isTemplateIdRep((TST) TypeSpecType) &&
+           "DeclSpec does not store a template id");
+    return TemplateIdRep;
+  }
   CXXScopeSpec &getTypeSpecScope() { return TypeScope; }
   const CXXScopeSpec &getTypeSpecScope() const { return TypeScope; }
 
@@ -666,6 +678,9 @@ public:
                        SourceLocation TagNameLoc, const char *&PrevSpec,
                        unsigned &DiagID, Decl *Rep, bool Owned,
                        const PrintingPolicy &Policy);
+  bool SetTypeSpecType(TST T, SourceLocation Loc, const char *&PrevSpec,
+                       unsigned &DiagID, TemplateIdAnnotation *Rep,
+                       const PrintingPolicy &Policy);
 
   bool SetTypeSpecType(TST T, SourceLocation Loc, const char *&PrevSpec,
                        unsigned &DiagID, Expr *Rep,
@@ -1757,7 +1772,8 @@ enum class DeclaratorContext {
     TemplateArgContext,  // Any template argument (in template argument list).
     TemplateTypeArgContext, // Template type argument (in default argument).
     AliasDeclContext,    // C++11 alias-declaration.
-    AliasTemplateContext // C++11 alias-declaration template.
+    AliasTemplateContext, // C++11 alias-declaration template.
+    RequiresExprContext   // C++2a requires-expression.
 };
 
 
@@ -1830,6 +1846,14 @@ private:
   /// requires-clause, or null if no such clause was specified.
   Expr *TrailingRequiresClause;
 
+  /// If this declarator declares a template, its template parameter lists.
+  ArrayRef<TemplateParameterList *> TemplateParameterLists;
+
+  /// If the declarator declares an abbreviated function template, the innermost
+  /// template parameter list containing the invented and explicit template
+  /// parameters (if any).
+  TemplateParameterList *InventedTemplateParameterList;
+
 #ifndef _MSC_VER
   union {
 #endif
@@ -1860,7 +1884,8 @@ public:
         Redeclaration(false), Extension(false), ObjCIvar(false),
         ObjCWeakProperty(false), InlineStorageUsed(false),
         Attrs(ds.getAttributePool().getFactory()), AsmLabel(nullptr),
-        TrailingRequiresClause(nullptr) {}
+        TrailingRequiresClause(nullptr),
+        InventedTemplateParameterList(nullptr) {}
 
   ~Declarator() {
     clear();
@@ -1981,6 +2006,7 @@ public:
     case DeclaratorContext::TemplateTypeArgContext:
     case DeclaratorContext::TrailingReturnContext:
     case DeclaratorContext::TrailingReturnVarContext:
+    case DeclaratorContext::RequiresExprContext:
       return true;
     }
     llvm_unreachable("unknown context kind!");
@@ -2003,6 +2029,7 @@ public:
     case DeclaratorContext::TemplateParamContext:
     case DeclaratorContext::CXXCatchContext:
     case DeclaratorContext::ObjCCatchContext:
+    case DeclaratorContext::RequiresExprContext:
       return true;
 
     case DeclaratorContext::TypeNameContext:
@@ -2039,6 +2066,7 @@ public:
     case DeclaratorContext::MemberContext:
     case DeclaratorContext::PrototypeContext:
     case DeclaratorContext::TemplateParamContext:
+    case DeclaratorContext::RequiresExprContext:
       // Maybe one day...
       return false;
 
@@ -2116,6 +2144,7 @@ public:
     case DeclaratorContext::TemplateArgContext:
     case DeclaratorContext::TemplateTypeArgContext:
     case DeclaratorContext::TrailingReturnContext:
+    case DeclaratorContext::RequiresExprContext:
       return false;
     }
     llvm_unreachable("unknown context kind!");
@@ -2337,6 +2366,7 @@ public:
     case DeclaratorContext::TemplateTypeArgContext:
     case DeclaratorContext::TrailingReturnContext:
     case DeclaratorContext::TrailingReturnVarContext:
+    case DeclaratorContext::RequiresExprContext:
       return false;
     }
     llvm_unreachable("unknown context kind!");
@@ -2370,6 +2400,7 @@ public:
     case DeclaratorContext::TrailingReturnContext:
     case DeclaratorContext::TrailingReturnVarContext:
     case DeclaratorContext::TemplateTypeArgContext:
+    case DeclaratorContext::RequiresExprContext:
       return false;
 
     case DeclaratorContext::BlockContext:
@@ -2422,6 +2453,30 @@ public:
     return TrailingRequiresClause != nullptr;
   }
 
+  /// Sets the template parameter lists that preceded the declarator.
+  void setTemplateParameterLists(ArrayRef<TemplateParameterList *> TPLs) {
+    TemplateParameterLists = TPLs;
+  }
+
+  /// The template parameter lists that preceded the declarator.
+  ArrayRef<TemplateParameterList *> getTemplateParameterLists() const {
+    return TemplateParameterLists;
+  }
+
+  /// Sets the template parameter list generated from the explicit template
+  /// parameters along with any invented template parameters from
+  /// placeholder-typed parameters.
+  void setInventedTemplateParameterList(TemplateParameterList *Invented) {
+    InventedTemplateParameterList = Invented;
+  }
+
+  /// The template parameter list generated from the explicit template
+  /// parameters along with any invented template parameters from
+  /// placeholder-typed parameters, if there were any such parameters.
+  TemplateParameterList * getInventedTemplateParameterList() const {
+    return InventedTemplateParameterList;
+  }
+
   /// takeAttributes - Takes attributes from the given parsed-attributes
   /// set and add them to this declarator.
   ///
@@ -2622,6 +2677,26 @@ struct LambdaIntroducer {
   }
 };
 
+struct InventedTemplateParameterInfo {
+  /// The number of parameters in the template parameter list that were
+  /// explicitly specified by the user, as opposed to being invented by use
+  /// of an auto parameter.
+  unsigned NumExplicitTemplateParams = 0;
+
+  /// If this is a generic lambda or abbreviated function template, use this
+  /// as the depth of each 'auto' parameter, during initial AST construction.
+  unsigned AutoTemplateParameterDepth = 0;
+
+  /// Store the list of the template parameters for a generic lambda or an
+  /// abbreviated function template.
+  /// If this is a generic lambda or abbreviated function template, this holds
+  /// the explicit template parameters followed by the auto parameters
+  /// converted into TemplateTypeParmDecls.
+  /// It can be used to construct the generic lambda or abbreviated template's
+  /// template parameter list during initial AST construction.
+  SmallVector<NamedDecl*, 4> TemplateParams;
+};
+
 } // end namespace clang
 
 #endif // LLVM_CLANG_SEMA_DECLSPEC_H
diff --git a/clang/include/clang/Sema/ParsedTemplate.h b/clang/include/clang/Sema/ParsedTemplate.h
index 0874905b38a5..82d00494b0d6 100644
--- a/clang/include/clang/Sema/ParsedTemplate.h
+++ b/clang/include/clang/Sema/ParsedTemplate.h
@@ -139,9 +139,8 @@ namespace clang {
   /// Information about a template-id annotation
   /// token.
   ///
-  /// A template-id annotation token contains the template declaration,
-  /// template arguments, whether those template arguments were types,
-  /// expressions, or template names, and the source locations for important
+  /// A template-id annotation token contains the template name,
+  /// template arguments, and the source locations for important
   /// tokens. All of the information about template arguments is allocated
   /// directly after this structure.
   /// A template-id annotation token can also be generated by a type-constraint
@@ -152,9 +151,6 @@ namespace clang {
       : private llvm::TrailingObjects<TemplateIdAnnotation,
                                       ParsedTemplateArgument> {
     friend TrailingObjects;
-    /// The nested-name-specifier that precedes the template name.
-    CXXScopeSpec SS;
-
     /// TemplateKWLoc - The location of the template keyword.
     /// For e.g. typename T::template Y<U>
     SourceLocation TemplateKWLoc;
@@ -195,16 +191,15 @@ namespace clang {
     /// Creates a new TemplateIdAnnotation with NumArgs arguments and
     /// appends it to List.
     static TemplateIdAnnotation *
-    Create(CXXScopeSpec SS, SourceLocation TemplateKWLoc,
-           SourceLocation TemplateNameLoc, IdentifierInfo *Name,
-           OverloadedOperatorKind OperatorKind,
+    Create(SourceLocation TemplateKWLoc, SourceLocation TemplateNameLoc,
+           IdentifierInfo *Name, OverloadedOperatorKind OperatorKind,
            ParsedTemplateTy OpaqueTemplateName, TemplateNameKind TemplateKind,
            SourceLocation LAngleLoc, SourceLocation RAngleLoc,
            ArrayRef<ParsedTemplateArgument> TemplateArgs,
            SmallVectorImpl<TemplateIdAnnotation *> &CleanupList) {
       TemplateIdAnnotation *TemplateId = new (llvm::safe_malloc(
           totalSizeToAlloc<ParsedTemplateArgument>(TemplateArgs.size())))
-          TemplateIdAnnotation(SS, TemplateKWLoc, TemplateNameLoc, Name,
+          TemplateIdAnnotation(TemplateKWLoc, TemplateNameLoc, Name,
                                OperatorKind, OpaqueTemplateName, TemplateKind,
                                LAngleLoc, RAngleLoc, TemplateArgs);
       CleanupList.push_back(TemplateId);
@@ -221,17 +216,16 @@ namespace clang {
   private:
     TemplateIdAnnotation(const TemplateIdAnnotation &) = delete;
 
-    TemplateIdAnnotation(CXXScopeSpec SS, SourceLocation TemplateKWLoc,
+    TemplateIdAnnotation(SourceLocation TemplateKWLoc,
                          SourceLocation TemplateNameLoc, IdentifierInfo *Name,
                          OverloadedOperatorKind OperatorKind,
                          ParsedTemplateTy OpaqueTemplateName,
                          TemplateNameKind TemplateKind,
                          SourceLocation LAngleLoc, SourceLocation RAngleLoc,
                          ArrayRef<ParsedTemplateArgument> TemplateArgs) noexcept
-        : SS(SS), TemplateKWLoc(TemplateKWLoc),
-          TemplateNameLoc(TemplateNameLoc), Name(Name), Operator(OperatorKind),
-          Template(OpaqueTemplateName), Kind(TemplateKind),
-          LAngleLoc(LAngleLoc), RAngleLoc(RAngleLoc),
+        : TemplateKWLoc(TemplateKWLoc), TemplateNameLoc(TemplateNameLoc),
+          Name(Name), Operator(OperatorKind), Template(OpaqueTemplateName),
+          Kind(TemplateKind), LAngleLoc(LAngleLoc), RAngleLoc(RAngleLoc),
           NumArgs(TemplateArgs.size()) {
 
       std::uninitialized_copy(TemplateArgs.begin(), TemplateArgs.end(),
diff --git a/clang/include/clang/Sema/Scope.h b/clang/include/clang/Sema/Scope.h
index 7848df8f70d9..6133425a42a6 100644
--- a/clang/include/clang/Sema/Scope.h
+++ b/clang/include/clang/Sema/Scope.h
@@ -385,6 +385,12 @@ public:
     return getFlags() & Scope::FunctionPrototypeScope;
   }
 
+  /// isFunctionDeclarationScope - Return true if this scope is a
+  /// function prototype scope.
+  bool isFunctionDeclarationScope() const {
+    return getFlags() & Scope::FunctionDeclarationScope;
+  }
+
   /// isAtCatchScope - Return true if this scope is \@catch.
   bool isAtCatchScope() const {
     return getFlags() & Scope::AtCatchScope;
diff --git a/clang/include/clang/Sema/ScopeInfo.h b/clang/include/clang/Sema/ScopeInfo.h
index 4f7534f9ef1a..3c4847a2932c 100644
--- a/clang/include/clang/Sema/ScopeInfo.h
+++ b/clang/include/clang/Sema/ScopeInfo.h
@@ -22,6 +22,7 @@
 #include "clang/Basic/PartialDiagnostic.h"
 #include "clang/Basic/SourceLocation.h"
 #include "clang/Sema/CleanupInfo.h"
+#include "clang/Sema/DeclSpec.h"
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/DenseMapInfo.h"
 #include "llvm/ADT/MapVector.h"
@@ -789,7 +790,8 @@ public:
   }
 };
 
-class LambdaScopeInfo final : public CapturingScopeInfo {
+class LambdaScopeInfo final :
+    public CapturingScopeInfo, public InventedTemplateParameterInfo {
 public:
   /// The class that describes the lambda.
   CXXRecordDecl *Lambda = nullptr;
@@ -823,25 +825,9 @@ public:
   /// Packs introduced by this lambda, if any.
   SmallVector<NamedDecl*, 4> LocalPacks;
 
-  /// If this is a generic lambda, use this as the depth of
-  /// each 'auto' parameter, during initial AST construction.
-  unsigned AutoTemplateParameterDepth = 0;
-
-  /// The number of parameters in the template parameter list that were
-  /// explicitly specified by the user, as opposed to being invented by use
-  /// of an auto parameter.
-  unsigned NumExplicitTemplateParams = 0;
-
   /// Source range covering the explicit template parameter list (if it exists).
   SourceRange ExplicitTemplateParamsRange;
 
-  /// Store the list of the template parameters for a generic lambda.
-  /// If this is a generic lambda, this holds the explicit template parameters
-  /// followed by the auto parameters converted into TemplateTypeParmDecls.
-  /// It can be used to construct the generic lambda's template parameter list
-  /// during initial AST construction.
-  SmallVector<NamedDecl*, 4> TemplateParams;
-
   /// If this is a generic lambda, and the template parameter
   /// list has been created (from the TemplateParams) then store
   /// a reference to it (cache it to avoid reconstructing it).
diff --git a/clang/include/clang/Sema/Sema.h b/clang/include/clang/Sema/Sema.h
index 47a055f696b1..a88dd2814487 100644
--- a/clang/include/clang/Sema/Sema.h
+++ b/clang/include/clang/Sema/Sema.h
@@ -21,6 +21,7 @@
 #include "clang/AST/DeclTemplate.h"
 #include "clang/AST/DeclarationName.h"
 #include "clang/AST/Expr.h"
+#include "clang/AST/ExprConcepts.h"
 #include "clang/AST/ExprCXX.h"
 #include "clang/AST/ExprObjC.h"
 #include "clang/AST/ExternalASTSource.h"
@@ -619,6 +620,13 @@ public:
   /// function, block, and method scopes that are currently active.
   SmallVector<sema::FunctionScopeInfo *, 4> FunctionScopes;
 
+  /// Stack containing information needed when in C++2a an 'auto' is encountered
+  /// in a function declaration parameter type specifier in order to invent a
+  /// corresponding template parameter in the enclosing abbreviated function
+  /// template. This information is also present in LambdaScopeInfo, stored in
+  /// the FunctionScopes stack.
+  SmallVector<InventedTemplateParameterInfo, 4> InventedParameterInfos;
+
   typedef LazyVector<TypedefNameDecl *, ExternalSemaSource,
                      &ExternalSemaSource::ReadExtVectorDecls, 2, 2>
     ExtVectorDeclsType;
@@ -1424,6 +1432,11 @@ public:
   /// Retrieve the module loader associated with the preprocessor.
   ModuleLoader &getModuleLoader() const;
 
+  /// Invent a new identifier for parameters of abbreviated templates.
+  IdentifierInfo *
+  InventAbbreviatedTemplateParameterTypeName(IdentifierInfo *ParamName,
+                                             unsigned Index);
+
   void emitAndClearUnusedLocalTypedefWarnings();
 
   enum TUFragmentKind {
@@ -1518,6 +1531,15 @@ public:
   /// WeakTopLevelDeclDecls - access to \#pragma weak-generated Decls
   SmallVectorImpl<Decl *> &WeakTopLevelDecls() { return WeakTopLevelDecl; }
 
+  /// Called before parsing a function declarator belonging to a function
+  /// declaration.
+  void ActOnStartFunctionDeclarationDeclarator(Declarator &D,
+                                               unsigned TemplateParameterDepth);
+
+  /// Called after parsing a function declarator belonging to a function
+  /// declaration.
+  void ActOnFinishFunctionDeclarationDeclarator(Declarator &D);
+
   void ActOnComment(SourceRange Comment);
 
   //===--------------------------------------------------------------------===//
@@ -1921,6 +1943,8 @@ public:
     NC_FunctionTemplate,
     /// The name was classified as an ADL-only function template name.
     NC_UndeclaredTemplate,
+    /// The name was classified as a concept name.
+    NC_Concept,
   };
 
   class NameClassification {
@@ -1985,6 +2009,12 @@ public:
       return Result;
     }
 
+    static NameClassification Concept(TemplateName Name) {
+      NameClassification Result(NC_Concept);
+      Result.Template = Name;
+      return Result;
+    }
+
     static NameClassification UndeclaredTemplate(TemplateName Name) {
       NameClassification Result(NC_UndeclaredTemplate);
       Result.Template = Name;
@@ -2010,7 +2040,8 @@ public:
 
     TemplateName getTemplateName() const {
       assert(Kind == NC_TypeTemplate || Kind == NC_FunctionTemplate ||
-             Kind == NC_VarTemplate || Kind == NC_UndeclaredTemplate);
+             Kind == NC_VarTemplate || Kind == NC_Concept ||
+             Kind == NC_UndeclaredTemplate);
       return Template;
     }
 
@@ -2022,6 +2053,8 @@ public:
         return TNK_Function_template;
       case NC_VarTemplate:
         return TNK_Var_template;
+      case NC_Concept:
+        return TNK_Concept_template;
       case NC_UndeclaredTemplate:
         return TNK_Undeclared_template;
       default:
@@ -6199,6 +6232,9 @@ private:
   llvm::DenseMap<NamedDecl *, NormalizedConstraint *>
       NormalizationCache;
 
+  llvm::ContextualFoldingSet<ConstraintSatisfaction, const ASTContext &>
+      SatisfactionCache;
+
 public:
   const NormalizedConstraint *
   getNormalizedAssociatedConstraints(
@@ -6225,6 +6261,8 @@ public:
 
   /// \brief Check whether the given list of constraint expressions are
   /// satisfied (as if in a 'conjunction') given template arguments.
+  /// \param Template the template-like entity that triggered the constraints
+  /// check (either a concept or a constrained entity).
   /// \param ConstraintExprs a list of constraint expressions, treated as if
   /// they were 'AND'ed together.
   /// \param TemplateArgs the list of template arguments to substitute into the
@@ -6236,23 +6274,10 @@ public:
   /// expression.
   /// \returns true if an error occurred and satisfaction could not be checked,
   /// false otherwise.
-  bool CheckConstraintSatisfaction(TemplateDecl *Template,
-                                   ArrayRef<const Expr *> ConstraintExprs,
-                                   ArrayRef<TemplateArgument> TemplateArgs,
-                                   SourceRange TemplateIDRange,
-                                   ConstraintSatisfaction &Satisfaction);
-
-  bool CheckConstraintSatisfaction(ClassTemplatePartialSpecializationDecl *TD,
-                                   ArrayRef<const Expr *> ConstraintExprs,
-                                   ArrayRef<TemplateArgument> TemplateArgs,
-                                   SourceRange TemplateIDRange,
-                                   ConstraintSatisfaction &Satisfaction);
-
-  bool CheckConstraintSatisfaction(VarTemplatePartialSpecializationDecl *TD,
-                                   ArrayRef<const Expr *> ConstraintExprs,
-                                   ArrayRef<TemplateArgument> TemplateArgs,
-                                   SourceRange TemplateIDRange,
-                                   ConstraintSatisfaction &Satisfaction);
+  bool CheckConstraintSatisfaction(
+      NamedDecl *Template, ArrayRef<const Expr *> ConstraintExprs,
+      ArrayRef<TemplateArgument> TemplateArgs,
+      SourceRange TemplateIDRange, ConstraintSatisfaction &Satisfaction);
 
   /// \brief Check whether the given non-dependent constraint expression is
   /// satisfied. Returns false and updates Satisfaction with the satisfaction
@@ -6282,13 +6307,17 @@ public:
 
   /// \brief Emit diagnostics explaining why a constraint expression was deemed
   /// unsatisfied.
+  /// \param First whether this is the first time an unsatisfied constraint is
+  /// diagnosed for this error.
   void
-  DiagnoseUnsatisfiedConstraint(const ConstraintSatisfaction& Satisfaction);
+  DiagnoseUnsatisfiedConstraint(const ConstraintSatisfaction &Satisfaction,
+                                bool First = true);
 
   /// \brief Emit diagnostics explaining why a constraint expression was deemed
   /// unsatisfied.
   void
-  DiagnoseUnsatisfiedConstraint(const ASTConstraintSatisfaction& Satisfaction);
+  DiagnoseUnsatisfiedConstraint(const ASTConstraintSatisfaction &Satisfaction,
+                                bool First = true);
 
   /// \brief Emit diagnostics explaining why a constraint expression was deemed
   /// unsatisfied because it was ill-formed.
@@ -6873,7 +6902,8 @@ public:
                                 SourceLocation EqualLoc,
                                 ParsedType DefaultArg, bool HasTypeConstraint);
 
-  bool ActOnTypeConstraint(TemplateIdAnnotation *TypeConstraint,
+  bool ActOnTypeConstraint(const CXXScopeSpec &SS,
+                           TemplateIdAnnotation *TypeConstraint,
                            TemplateTypeParmDecl *ConstrainedParameter,
                            SourceLocation EllipsisLoc);
 
@@ -6884,6 +6914,10 @@ public:
                             TemplateTypeParmDecl *ConstrainedParameter,
                             SourceLocation EllipsisLoc);
 
+  bool AttachTypeConstraint(AutoTypeLoc TL,
+                            NonTypeTemplateParmDecl *ConstrainedParameter,
+                            SourceLocation EllipsisLoc);
+
   QualType CheckNonTypeTemplateParameterType(TypeSourceInfo *&TSI,
                                              SourceLocation Loc);
   QualType CheckNonTypeTemplateParameterType(QualType T, SourceLocation Loc);
@@ -6933,7 +6967,8 @@ public:
       SourceLocation DeclStartLoc, SourceLocation DeclLoc,
       const CXXScopeSpec &SS, TemplateIdAnnotation *TemplateId,
       ArrayRef<TemplateParameterList *> ParamLists,
-      bool IsFriend, bool &IsMemberSpecialization, bool &Invalid);
+      bool IsFriend, bool &IsMemberSpecialization, bool &Invalid,
+      bool SuppressDiagnostic = false);
 
   DeclResult CheckClassTemplate(
       Scope *S, unsigned TagSpec, TagUseKind TUK, SourceLocation KWLoc,
@@ -7028,8 +7063,8 @@ public:
 
   DeclResult ActOnClassTemplateSpecialization(
       Scope *S, unsigned TagSpec, TagUseKind TUK, SourceLocation KWLoc,
-      SourceLocation ModulePrivateLoc, TemplateIdAnnotation &TemplateId,
-      const ParsedAttributesView &Attr,
+      SourceLocation ModulePrivateLoc, CXXScopeSpec &SS,
+      TemplateIdAnnotation &TemplateId, const ParsedAttributesView &Attr,
       MultiTemplateParamsArg TemplateParameterLists,
       SkipBodyInfo *SkipBody = nullptr);
 
@@ -7261,7 +7296,17 @@ public:
                              SourceLocation KeywordLoc,
                              NestedNameSpecifierLoc QualifierLoc,
                              const IdentifierInfo &II,
-                             SourceLocation IILoc);
+                             SourceLocation IILoc,
+                             TypeSourceInfo **TSI,
+                             bool DeducedTSTContext);
+
+  QualType CheckTypenameType(ElaboratedTypeKeyword Keyword,
+                             SourceLocation KeywordLoc,
+                             NestedNameSpecifierLoc QualifierLoc,
+                             const IdentifierInfo &II,
+                             SourceLocation IILoc,
+                             bool DeducedTSTContext = true);
+
 
   TypeSourceInfo *RebuildTypeInCurrentInstantiation(TypeSourceInfo *T,
                                                     SourceLocation Loc,
@@ -7281,11 +7326,52 @@ public:
                                   const TemplateArgument *Args,
                                   unsigned NumArgs);
 
-  // Concepts
+  //===--------------------------------------------------------------------===//
+  // C++ Concepts
+  //===--------------------------------------------------------------------===//
   Decl *ActOnConceptDefinition(
       Scope *S, MultiTemplateParamsArg TemplateParameterLists,
       IdentifierInfo *Name, SourceLocation NameLoc, Expr *ConstraintExpr);
 
+  RequiresExprBodyDecl *
+  ActOnStartRequiresExpr(SourceLocation RequiresKWLoc,
+                         ArrayRef<ParmVarDecl *> LocalParameters,
+                         Scope *BodyScope);
+  void ActOnFinishRequiresExpr();
+  concepts::Requirement *ActOnSimpleRequirement(Expr *E);
+  concepts::Requirement *ActOnTypeRequirement(
+      SourceLocation TypenameKWLoc, CXXScopeSpec &SS, SourceLocation NameLoc,
+      IdentifierInfo *TypeName, TemplateIdAnnotation *TemplateId);
+  concepts::Requirement *ActOnCompoundRequirement(Expr *E,
+                                                  SourceLocation NoexceptLoc);
+  concepts::Requirement *
+  ActOnCompoundRequirement(
+      Expr *E, SourceLocation NoexceptLoc, CXXScopeSpec &SS,
+      TemplateIdAnnotation *TypeConstraint, unsigned Depth);
+  concepts::Requirement *ActOnNestedRequirement(Expr *Constraint);
+  concepts::ExprRequirement *
+  BuildExprRequirement(
+      Expr *E, bool IsSatisfied, SourceLocation NoexceptLoc,
+      concepts::ExprRequirement::ReturnTypeRequirement ReturnTypeRequirement);
+  concepts::ExprRequirement *
+  BuildExprRequirement(
+      concepts::Requirement::SubstitutionDiagnostic *ExprSubstDiag,
+      bool IsSatisfied, SourceLocation NoexceptLoc,
+      concepts::ExprRequirement::ReturnTypeRequirement ReturnTypeRequirement);
+  concepts::TypeRequirement *BuildTypeRequirement(TypeSourceInfo *Type);
+  concepts::TypeRequirement *
+  BuildTypeRequirement(
+      concepts::Requirement::SubstitutionDiagnostic *SubstDiag);
+  concepts::NestedRequirement *BuildNestedRequirement(Expr *E);
+  concepts::NestedRequirement *
+  BuildNestedRequirement(
+      concepts::Requirement::SubstitutionDiagnostic *SubstDiag);
+  ExprResult ActOnRequiresExpr(SourceLocation RequiresKWLoc,
+                               RequiresExprBodyDecl *Body,
+                               ArrayRef<ParmVarDecl *> LocalParameters,
+                               ArrayRef<concepts::Requirement *> Requirements,
+                               SourceLocation ClosingBraceLoc);
+
   //===--------------------------------------------------------------------===//
   // C++ Variadic Templates (C++0x [temp.variadic])
   //===--------------------------------------------------------------------===//
@@ -7794,10 +7880,12 @@ public:
 
   DeduceAutoResult
   DeduceAutoType(TypeSourceInfo *AutoType, Expr *&Initializer, QualType &Result,
-                 Optional<unsigned> DependentDeductionDepth = None);
+                 Optional<unsigned> DependentDeductionDepth = None,
+                 bool IgnoreConstraints = false);
   DeduceAutoResult
   DeduceAutoType(TypeLoc AutoTypeLoc, Expr *&Initializer, QualType &Result,
-                 Optional<unsigned> DependentDeductionDepth = None);
+                 Optional<unsigned> DependentDeductionDepth = None,
+                 bool IgnoreConstraints = false);
   void DiagnoseAutoDeductionFailure(VarDecl *VDecl, Expr *Init);
   bool DeduceReturnType(FunctionDecl *FD, SourceLocation Loc,
                         bool Diagnose = true);
@@ -7932,6 +8020,13 @@ public:
       /// template which was deferred until it was needed.
       ExceptionSpecInstantiation,
 
+      /// We are instantiating a requirement of a requires expression.
+      RequirementInstantiation,
+
+      /// We are checking the satisfaction of a nested requirement of a requires
+      /// expression.
+      NestedRequirementConstraintsCheck,
+
       /// We are declaring an implicit special member function.
       DeclaringSpecialMember,
 
@@ -8253,6 +8348,19 @@ public:
                           ParameterMappingSubstitution, NamedDecl *Template,
                           SourceRange InstantiationRange);
 
+    /// \brief Note that we are substituting template arguments into a part of
+    /// a requirement of a requires expression.
+    InstantiatingTemplate(Sema &SemaRef, SourceLocation PointOfInstantiation,
+                          concepts::Requirement *Req,
+                          sema::TemplateDeductionInfo &DeductionInfo,
+                          SourceRange InstantiationRange = SourceRange());
+
+    /// \brief Note that we are checking the satisfaction of the constraint
+    /// expression inside of a nested requirement.
+    InstantiatingTemplate(Sema &SemaRef, SourceLocation PointOfInstantiation,
+                          concepts::NestedRequirement *Req, ConstraintsCheck,
+                          SourceRange InstantiationRange = SourceRange());
+
     /// Note that we have finished instantiating this template.
     void Clear();
 
diff --git a/clang/include/clang/Sema/SemaConcept.h b/clang/include/clang/Sema/SemaConcept.h
index acd1e604211a..7fc42a4816ec 100644
--- a/clang/include/clang/Sema/SemaConcept.h
+++ b/clang/include/clang/Sema/SemaConcept.h
@@ -13,10 +13,17 @@
 
 #ifndef LLVM_CLANG_SEMA_SEMACONCEPT_H
 #define LLVM_CLANG_SEMA_SEMACONCEPT_H
+#include "clang/AST/ASTConcept.h"
 #include "clang/AST/ASTContext.h"
 #include "clang/AST/Expr.h"
+#include "clang/AST/DeclTemplate.h"
+#include "clang/Basic/SourceLocation.h"
+#include "llvm/ADT/PointerUnion.h"
 #include "llvm/ADT/Optional.h"
 #include "llvm/ADT/SmallVector.h"
+#include <string>
+#include <utility>
+
 namespace clang {
 class Sema;
 
diff --git a/clang/include/clang/Sema/TemplateDeduction.h b/clang/include/clang/Sema/TemplateDeduction.h
index b60939c97872..f787c2689d85 100644
--- a/clang/include/clang/Sema/TemplateDeduction.h
+++ b/clang/include/clang/Sema/TemplateDeduction.h
@@ -15,6 +15,7 @@
 #define LLVM_CLANG_SEMA_TEMPLATEDEDUCTION_H
 
 #include "clang/Sema/Ownership.h"
+#include "clang/Sema/SemaConcept.h"
 #include "clang/AST/ASTConcept.h"
 #include "clang/AST/DeclAccessPair.h"
 #include "clang/AST/DeclTemplate.h"
diff --git a/clang/include/clang/Serialization/ASTBitCodes.h b/clang/include/clang/Serialization/ASTBitCodes.h
index 1bfcbda8c9f1..44a12c875da7 100644
--- a/clang/include/clang/Serialization/ASTBitCodes.h
+++ b/clang/include/clang/Serialization/ASTBitCodes.h
@@ -1403,6 +1403,9 @@ namespace serialization {
       /// An LifetimeExtendedTemporaryDecl record.
       DECL_LIFETIME_EXTENDED_TEMPORARY,
 
+      /// A RequiresExprBodyDecl record.
+      DECL_REQUIRES_EXPR_BODY,
+
       /// An ObjCTypeParamDecl record.
       DECL_OBJC_TYPE_PARAM,
 
@@ -1785,6 +1788,7 @@ namespace serialization {
       EXPR_MATERIALIZE_TEMPORARY, // MaterializeTemporaryExpr
       EXPR_CXX_FOLD,              // CXXFoldExpr
       EXPR_CONCEPT_SPECIALIZATION,// ConceptSpecializationExpr
+      EXPR_REQUIRES,              // RequiresExpr
 
       // CUDA
       EXPR_CUDA_KERNEL_CALL,       // CUDAKernelCallExpr
diff --git a/clang/lib/AST/ASTConcept.cpp b/clang/lib/AST/ASTConcept.cpp
index fc32e768d92f..c28a06bdf0b2 100644
--- a/clang/lib/AST/ASTConcept.cpp
+++ b/clang/lib/AST/ASTConcept.cpp
@@ -14,6 +14,10 @@
 
 #include "clang/AST/ASTConcept.h"
 #include "clang/AST/ASTContext.h"
+#include "clang/AST/Decl.h"
+#include "clang/AST/TemplateBase.h"
+#include "llvm/ADT/ArrayRef.h"
+#include "llvm/ADT/FoldingSet.h"
 using namespace clang;
 
 ASTConstraintSatisfaction::ASTConstraintSatisfaction(const ASTContext &C,
@@ -53,3 +57,12 @@ ASTConstraintSatisfaction::Create(const ASTContext &C,
   void *Mem = C.Allocate(size, alignof(ASTConstraintSatisfaction));
   return new (Mem) ASTConstraintSatisfaction(C, Satisfaction);
 }
+
+void ConstraintSatisfaction::Profile(
+    llvm::FoldingSetNodeID &ID, const ASTContext &C, NamedDecl *ConstraintOwner,
+    ArrayRef<TemplateArgument> TemplateArgs) {
+  ID.AddPointer(ConstraintOwner);
+  ID.AddInteger(TemplateArgs.size());
+  for (auto &Arg : TemplateArgs)
+    Arg.Profile(ID, C);
+}
diff --git a/clang/lib/AST/ASTContext.cpp b/clang/lib/AST/ASTContext.cpp
index a51429264dbe..6d1db38e36cc 100644
--- a/clang/lib/AST/ASTContext.cpp
+++ b/clang/lib/AST/ASTContext.cpp
@@ -716,6 +716,61 @@ ASTContext::CanonicalTemplateTemplateParm::Profile(llvm::FoldingSetNodeID &ID,
     RequiresClause->Profile(ID, C, /*Canonical=*/true);
 }
 
+static Expr *
+canonicalizeImmediatelyDeclaredConstraint(const ASTContext &C, Expr *IDC,
+                                          QualType ConstrainedType) {
+  // This is a bit ugly - we need to form a new immediately-declared
+  // constraint that references the new parameter; this would ideally
+  // require semantic analysis (e.g. template<C T> struct S {}; - the
+  // converted arguments of C<T> could be an argument pack if C is
+  // declared as template<typename... T> concept C = ...).
+  // We don't have semantic analysis here so we dig deep into the
+  // ready-made constraint expr and change the thing manually.
+  ConceptSpecializationExpr *CSE;
+  if (const auto *Fold = dyn_cast<CXXFoldExpr>(IDC))
+    CSE = cast<ConceptSpecializationExpr>(Fold->getLHS());
+  else
+    CSE = cast<ConceptSpecializationExpr>(IDC);
+  ArrayRef<TemplateArgument> OldConverted = CSE->getTemplateArguments();
+  SmallVector<TemplateArgument, 3> NewConverted;
+  NewConverted.reserve(OldConverted.size());
+  if (OldConverted.front().getKind() == TemplateArgument::Pack) {
+    // The case:
+    // template<typename... T> concept C = true;
+    // template<C<int> T> struct S; -> constraint is C<{T, int}>
+    NewConverted.push_back(ConstrainedType);
+    for (auto &Arg : OldConverted.front().pack_elements().drop_front(1))
+      NewConverted.push_back(Arg);
+    TemplateArgument NewPack(NewConverted);
+
+    NewConverted.clear();
+    NewConverted.push_back(NewPack);
+    assert(OldConverted.size() == 1 &&
+           "Template parameter pack should be the last parameter");
+  } else {
+    assert(OldConverted.front().getKind() == TemplateArgument::Type &&
+           "Unexpected first argument kind for immediately-declared "
+           "constraint");
+    NewConverted.push_back(ConstrainedType);
+    for (auto &Arg : OldConverted.drop_front(1))
+      NewConverted.push_back(Arg);
+  }
+  Expr *NewIDC = ConceptSpecializationExpr::Create(
+      C, NestedNameSpecifierLoc(), /*TemplateKWLoc=*/SourceLocation(),
+      CSE->getConceptNameInfo(), /*FoundDecl=*/CSE->getNamedConcept(),
+      CSE->getNamedConcept(),
+      // Actually canonicalizing a TemplateArgumentLoc is difficult so we
+      // simply omit the ArgsAsWritten
+      /*ArgsAsWritten=*/nullptr, NewConverted, nullptr);
+
+  if (auto *OrigFold = dyn_cast<CXXFoldExpr>(IDC))
+    NewIDC = new (C) CXXFoldExpr(OrigFold->getType(), SourceLocation(), NewIDC,
+                                 BinaryOperatorKind::BO_LAnd,
+                                 SourceLocation(), /*RHS=*/nullptr,
+                                 SourceLocation(), /*NumExpansions=*/None);
+  return NewIDC;
+}
+
 TemplateTemplateParmDecl *
 ASTContext::getCanonicalTemplateTemplateParmDecl(
                                           TemplateTemplateParmDecl *TTP) const {
@@ -743,68 +798,23 @@ ASTContext::getCanonicalTemplateTemplateParmDecl(
           TTP->isExpandedParameterPack() ?
           llvm::Optional<unsigned>(TTP->getNumExpansionParameters()) : None);
       if (const auto *TC = TTP->getTypeConstraint()) {
-        // This is a bit ugly - we need to form a new immediately-declared
-        // constraint that references the new parameter; this would ideally
-        // require semantic analysis (e.g. template<C T> struct S {}; - the
-        // converted arguments of C<T> could be an argument pack if C is
-        // declared as template<typename... T> concept C = ...).
-        // We don't have semantic analysis here so we dig deep into the
-        // ready-made constraint expr and change the thing manually.
-        Expr *IDC = TC->getImmediatelyDeclaredConstraint();
-        ConceptSpecializationExpr *CSE;
-        if (const auto *Fold = dyn_cast<CXXFoldExpr>(IDC))
-          CSE = cast<ConceptSpecializationExpr>(Fold->getLHS());
-        else
-          CSE = cast<ConceptSpecializationExpr>(IDC);
-        ArrayRef<TemplateArgument> OldConverted = CSE->getTemplateArguments();
-        SmallVector<TemplateArgument, 3> NewConverted;
-        NewConverted.reserve(OldConverted.size());
-
         QualType ParamAsArgument(NewTTP->getTypeForDecl(), 0);
-        if (OldConverted.front().getKind() == TemplateArgument::Pack) {
-          // The case:
-          // template<typename... T> concept C = true;
-          // template<C<int> T> struct S; -> constraint is C<{T, int}>
-          NewConverted.push_back(ParamAsArgument);
-          for (auto &Arg : OldConverted.front().pack_elements().drop_front(1))
-            NewConverted.push_back(Arg);
-          TemplateArgument NewPack(NewConverted);
-
-          NewConverted.clear();
-          NewConverted.push_back(NewPack);
-          assert(OldConverted.size() == 1 &&
-                 "Template parameter pack should be the last parameter");
-        } else {
-          assert(OldConverted.front().getKind() == TemplateArgument::Type &&
-                 "Unexpected first argument kind for immediately-declared "
-                 "constraint");
-          NewConverted.push_back(ParamAsArgument);
-          for (auto &Arg : OldConverted.drop_front(1))
-            NewConverted.push_back(Arg);
-        }
-        Expr *NewIDC = ConceptSpecializationExpr::Create(*this,
-            NestedNameSpecifierLoc(), /*TemplateKWLoc=*/SourceLocation(),
-            CSE->getConceptNameInfo(), /*FoundDecl=*/CSE->getNamedConcept(),
-            CSE->getNamedConcept(),
-            // Actually canonicalizing a TemplateArgumentLoc is difficult so we
-            // simply omit the ArgsAsWritten
-            /*ArgsAsWritten=*/nullptr, NewConverted, nullptr);
-
-        if (auto *OrigFold = dyn_cast<CXXFoldExpr>(IDC))
-          NewIDC = new (*this) CXXFoldExpr(OrigFold->getType(),
-                                           SourceLocation(), NewIDC,
-                                           BinaryOperatorKind::BO_LAnd,
-                                           SourceLocation(), /*RHS=*/nullptr,
-                                           SourceLocation(),
-                                           /*NumExpansions=*/None);
-
+        Expr *NewIDC = canonicalizeImmediatelyDeclaredConstraint(
+                *this, TC->getImmediatelyDeclaredConstraint(),
+                ParamAsArgument);
+        TemplateArgumentListInfo CanonArgsAsWritten;
+        if (auto *Args = TC->getTemplateArgsAsWritten())
+          for (const auto &ArgLoc : Args->arguments())
+            CanonArgsAsWritten.addArgument(
+                TemplateArgumentLoc(ArgLoc.getArgument(),
+                                    TemplateArgumentLocInfo()));
         NewTTP->setTypeConstraint(
             NestedNameSpecifierLoc(),
             DeclarationNameInfo(TC->getNamedConcept()->getDeclName(),
                                 SourceLocation()), /*FoundDecl=*/nullptr,
             // Actually canonicalizing a TemplateArgumentLoc is difficult so we
             // simply omit the ArgsAsWritten
-            CSE->getNamedConcept(), /*ArgsAsWritten=*/nullptr, NewIDC);
+            TC->getNamedConcept(), /*ArgsAsWritten=*/nullptr, NewIDC);
       }
       CanonParams.push_back(NewTTP);
     } else if (const auto *NTTP = dyn_cast<NonTypeTemplateParmDecl>(*P)) {
@@ -839,6 +849,13 @@ ASTContext::getCanonicalTemplateTemplateParmDecl(
                                                 NTTP->isParameterPack(),
                                                 TInfo);
       }
+      if (AutoType *AT = T->getContainedAutoType()) {
+        if (AT->isConstrained()) {
+          Param->setPlaceholderTypeConstraint(
+              canonicalizeImmediatelyDeclaredConstraint(
+                  *this, NTTP->getPlaceholderTypeConstraint(), T));
+        }
+      }
       CanonParams.push_back(Param);
 
     } else
@@ -943,7 +960,7 @@ ASTContext::ASTContext(LangOptions &LOpts, SourceManager &SM,
                        Builtin::Context &builtins)
     : ConstantArrayTypes(this_()), FunctionProtoTypes(this_()),
       TemplateSpecializationTypes(this_()),
-      DependentTemplateSpecializationTypes(this_()),
+      DependentTemplateSpecializationTypes(this_()), AutoTypes(this_()),
       SubstTemplateTemplateParmPacks(this_()),
       CanonTemplateTemplateParms(this_()), SourceMgr(SM), LangOpts(LOpts),
       SanitizerBL(new SanitizerBlacklist(LangOpts.SanitizerBlacklistFiles, SM)),
@@ -5124,21 +5141,29 @@ QualType ASTContext::getUnaryTransformType(QualType BaseType,
 /// getAutoType - Return the uniqued reference to the 'auto' type which has been
 /// deduced to the given type, or to the canonical undeduced 'auto' type, or the
 /// canonical deduced-but-dependent 'auto' type.
-QualType ASTContext::getAutoType(QualType DeducedType, AutoTypeKeyword Keyword,
-                                 bool IsDependent, bool IsPack) const {
+QualType
+ASTContext::getAutoType(QualType DeducedType, AutoTypeKeyword Keyword,
+                        bool IsDependent, bool IsPack,
+                        ConceptDecl *TypeConstraintConcept,
+                        ArrayRef<TemplateArgument> TypeConstraintArgs) const {
   assert((!IsPack || IsDependent) && "only use IsPack for a dependent pack");
-  if (DeducedType.isNull() && Keyword == AutoTypeKeyword::Auto && !IsDependent)
+  if (DeducedType.isNull() && Keyword == AutoTypeKeyword::Auto &&
+      !TypeConstraintConcept && !IsDependent)
     return getAutoDeductType();
 
   // Look in the folding set for an existing type.
   void *InsertPos = nullptr;
   llvm::FoldingSetNodeID ID;
-  AutoType::Profile(ID, DeducedType, Keyword, IsDependent, IsPack);
+  AutoType::Profile(ID, *this, DeducedType, Keyword, IsDependent,
+                    TypeConstraintConcept, TypeConstraintArgs);
   if (AutoType *AT = AutoTypes.FindNodeOrInsertPos(ID, InsertPos))
     return QualType(AT, 0);
 
-  auto *AT = new (*this, TypeAlignment)
-      AutoType(DeducedType, Keyword, IsDependent, IsPack);
+  void *Mem = Allocate(sizeof(AutoType) +
+                       sizeof(TemplateArgument) * TypeConstraintArgs.size(),
+                       TypeAlignment);
+  auto *AT = new (Mem) AutoType(DeducedType, Keyword, IsDependent, IsPack,
+                                TypeConstraintConcept, TypeConstraintArgs);
   Types.push_back(AT);
   if (InsertPos)
     AutoTypes.InsertNode(AT, InsertPos);
@@ -5200,7 +5225,8 @@ QualType ASTContext::getAutoDeductType() const {
   if (AutoDeductTy.isNull())
     AutoDeductTy = QualType(
       new (*this, TypeAlignment) AutoType(QualType(), AutoTypeKeyword::Auto,
-                                          /*dependent*/false, /*pack*/false),
+                                          /*dependent*/false, /*pack*/false,
+                                          /*concept*/nullptr, /*args*/{}),
       0);
   return AutoDeductTy;
 }
diff --git a/clang/lib/AST/ASTImporter.cpp b/clang/lib/AST/ASTImporter.cpp
index 22fb67478c96..1f2ce30398c9 100644
--- a/clang/lib/AST/ASTImporter.cpp
+++ b/clang/lib/AST/ASTImporter.cpp
@@ -1366,9 +1366,21 @@ ExpectedType ASTNodeImporter::VisitAutoType(const AutoType *T) {
   if (!ToDeducedTypeOrErr)
     return ToDeducedTypeOrErr.takeError();
 
-  return Importer.getToContext().getAutoType(*ToDeducedTypeOrErr,
-                                             T->getKeyword(),
-                                             /*IsDependent*/false);
+  ExpectedDecl ToTypeConstraintConcept = import(T->getTypeConstraintConcept());
+  if (!ToTypeConstraintConcept)
+    return ToTypeConstraintConcept.takeError();
+
+  SmallVector<TemplateArgument, 2> ToTemplateArgs;
+  ArrayRef<TemplateArgument> FromTemplateArgs = T->getTypeConstraintArguments();
+  if (Error Err = ImportTemplateArguments(FromTemplateArgs.data(),
+                                          FromTemplateArgs.size(),
+                                          ToTemplateArgs))
+    return std::move(Err);
+
+  return Importer.getToContext().getAutoType(
+      *ToDeducedTypeOrErr, T->getKeyword(), /*IsDependent*/false,
+      /*IsPack=*/false, cast_or_null<ConceptDecl>(*ToTypeConstraintConcept),
+      ToTemplateArgs);
 }
 
 ExpectedType ASTNodeImporter::VisitInjectedClassNameType(
diff --git a/clang/lib/AST/ASTStructuralEquivalence.cpp b/clang/lib/AST/ASTStructuralEquivalence.cpp
index db48405055cd..91a2f3a8391b 100644
--- a/clang/lib/AST/ASTStructuralEquivalence.cpp
+++ b/clang/lib/AST/ASTStructuralEquivalence.cpp
@@ -729,11 +729,31 @@ static bool IsStructurallyEquivalent(StructuralEquivalenceContext &Context,
       return false;
     break;
 
-  case Type::Auto:
-    if (!IsStructurallyEquivalent(Context, cast<AutoType>(T1)->getDeducedType(),
-                                  cast<AutoType>(T2)->getDeducedType()))
+  case Type::Auto: {
+    auto *Auto1 = cast<AutoType>(T1);
+    auto *Auto2 = cast<AutoType>(T2);
+    if (!IsStructurallyEquivalent(Context, Auto1->getDeducedType(),
+                                  Auto2->getDeducedType()))
       return false;
+    if (Auto1->isConstrained() != Auto2->isConstrained())
+      return false;
+    if (Auto1->isConstrained()) {
+      if (Auto1->getTypeConstraintConcept() !=
+          Auto2->getTypeConstraintConcept())
+        return false;
+      ArrayRef<TemplateArgument> Auto1Args =
+          Auto1->getTypeConstraintArguments();
+      ArrayRef<TemplateArgument> Auto2Args =
+          Auto2->getTypeConstraintArguments();
+      if (Auto1Args.size() != Auto2Args.size())
+        return false;
+      for (unsigned I = 0, N = Auto1Args.size(); I != N; ++I) {
+        if (!IsStructurallyEquivalent(Context, Auto1Args[I], Auto2Args[I]))
+          return false;
+      }
+    }
     break;
+  }
 
   case Type::DeducedTemplateSpecialization: {
     const auto *DT1 = cast<DeducedTemplateSpecializationType>(T1);
diff --git a/clang/lib/AST/DeclBase.cpp b/clang/lib/AST/DeclBase.cpp
index 6ee767ccecf7..cb4d61cac2c7 100644
--- a/clang/lib/AST/DeclBase.cpp
+++ b/clang/lib/AST/DeclBase.cpp
@@ -804,6 +804,7 @@ unsigned Decl::getIdentifierNamespaceForKind(Kind DeclKind) {
     case OMPCapturedExpr:
     case Empty:
     case LifetimeExtendedTemporary:
+    case RequiresExprBody:
       // Never looked up by name.
       return 0;
   }
@@ -1177,6 +1178,7 @@ DeclContext *DeclContext::getPrimaryContext() {
   case Decl::Captured:
   case Decl::OMPDeclareReduction:
   case Decl::OMPDeclareMapper:
+  case Decl::RequiresExprBody:
     // There is only one DeclContext for these entities.
     return this;
 
diff --git a/clang/lib/AST/DeclCXX.cpp b/clang/lib/AST/DeclCXX.cpp
index 2ead1e70ea0d..48e310e858b2 100644
--- a/clang/lib/AST/DeclCXX.cpp
+++ b/clang/lib/AST/DeclCXX.cpp
@@ -1968,6 +1968,16 @@ CXXDeductionGuideDecl *CXXDeductionGuideDecl::CreateDeserialized(ASTContext &C,
       QualType(), nullptr, SourceLocation());
 }
 
+RequiresExprBodyDecl *RequiresExprBodyDecl::Create(
+    ASTContext &C, DeclContext *DC, SourceLocation StartLoc) {
+  return new (C, DC) RequiresExprBodyDecl(C, DC, StartLoc);
+}
+
+RequiresExprBodyDecl *RequiresExprBodyDecl::CreateDeserialized(ASTContext &C,
+                                                               unsigned ID) {
+  return new (C, ID) RequiresExprBodyDecl(C, nullptr, SourceLocation());
+}
+
 void CXXMethodDecl::anchor() {}
 
 bool CXXMethodDecl::isStatic() const {
diff --git a/clang/lib/AST/DeclTemplate.cpp b/clang/lib/AST/DeclTemplate.cpp
index 95a2e26e0df8..b5e4ec2d7f43 100644
--- a/clang/lib/AST/DeclTemplate.cpp
+++ b/clang/lib/AST/DeclTemplate.cpp
@@ -164,10 +164,15 @@ static void AdoptTemplateParameterList(TemplateParameterList *Params,
 void TemplateParameterList::
 getAssociatedConstraints(llvm::SmallVectorImpl<const Expr *> &AC) const {
   if (HasConstrainedParameters)
-    for (const NamedDecl *Param : *this)
-      if (const auto *TTP = dyn_cast<TemplateTypeParmDecl>(Param))
+    for (const NamedDecl *Param : *this) {
+      if (const auto *TTP = dyn_cast<TemplateTypeParmDecl>(Param)) {
         if (const auto *TC = TTP->getTypeConstraint())
           AC.push_back(TC->getImmediatelyDeclaredConstraint());
+      } else if (const auto *NTTP = dyn_cast<NonTypeTemplateParmDecl>(Param)) {
+        if (const Expr *E = NTTP->getPlaceholderTypeConstraint())
+          AC.push_back(E);
+      }
+    }
   if (HasRequiresClause)
     AC.push_back(getRequiresClause());
 }
@@ -483,7 +488,10 @@ static void ProfileTemplateParameterList(ASTContext &C,
     if (const auto *TTP = dyn_cast<TemplateTypeParmDecl>(D)) {
       ID.AddInteger(1);
       ID.AddBoolean(TTP->isParameterPack());
-      // TODO: Concepts: profile type-constraints.
+      ID.AddBoolean(TTP->hasTypeConstraint());
+      if (const TypeConstraint *TC = TTP->getTypeConstraint())
+        TC->getImmediatelyDeclaredConstraint()->Profile(ID, C,
+                                                        /*Canonical=*/true);
       continue;
     }
     const auto *TTP = cast<TemplateTemplateParmDecl>(D);
@@ -684,8 +692,14 @@ NonTypeTemplateParmDecl::Create(const ASTContext &C, DeclContext *DC,
                                 unsigned D, unsigned P, IdentifierInfo *Id,
                                 QualType T, bool ParameterPack,
                                 TypeSourceInfo *TInfo) {
-  return new (C, DC) NonTypeTemplateParmDecl(DC, StartLoc, IdLoc, D, P, Id,
-                                             T, ParameterPack, TInfo);
+  AutoType *AT =
+      C.getLangOpts().CPlusPlus2a ? T->getContainedAutoType() : nullptr;
+  return new (C, DC,
+              additionalSizeToAlloc<std::pair<QualType, TypeSourceInfo *>,
+                                    Expr *>(0,
+                                            AT && AT->isConstrained() ? 1 : 0))
+      NonTypeTemplateParmDecl(DC, StartLoc, IdLoc, D, P, Id, T, ParameterPack,
+                              TInfo);
 }
 
 NonTypeTemplateParmDecl *NonTypeTemplateParmDecl::Create(
@@ -693,26 +707,34 @@ NonTypeTemplateParmDecl *NonTypeTemplateParmDecl::Create(
     SourceLocation IdLoc, unsigned D, unsigned P, IdentifierInfo *Id,
     QualType T, TypeSourceInfo *TInfo, ArrayRef<QualType> ExpandedTypes,
     ArrayRef<TypeSourceInfo *> ExpandedTInfos) {
+  AutoType *AT = TInfo->getType()->getContainedAutoType();
   return new (C, DC,
-              additionalSizeToAlloc<std::pair<QualType, TypeSourceInfo *>>(
-                  ExpandedTypes.size()))
+              additionalSizeToAlloc<std::pair<QualType, TypeSourceInfo *>,
+                                    Expr *>(
+                  ExpandedTypes.size(), AT && AT->isConstrained() ? 1 : 0))
       NonTypeTemplateParmDecl(DC, StartLoc, IdLoc, D, P, Id, T, TInfo,
                               ExpandedTypes, ExpandedTInfos);
 }
 
 NonTypeTemplateParmDecl *
-NonTypeTemplateParmDecl::CreateDeserialized(ASTContext &C, unsigned ID) {
-  return new (C, ID) NonTypeTemplateParmDecl(nullptr, SourceLocation(),
-                                             SourceLocation(), 0, 0, nullptr,
-                                             QualType(), false, nullptr);
+NonTypeTemplateParmDecl::CreateDeserialized(ASTContext &C, unsigned ID,
+                                            bool HasTypeConstraint) {
+  return new (C, ID, additionalSizeToAlloc<std::pair<QualType,
+                                                     TypeSourceInfo *>,
+                                           Expr *>(0,
+                                                   HasTypeConstraint ? 1 : 0))
+          NonTypeTemplateParmDecl(nullptr, SourceLocation(), SourceLocation(),
+                                  0, 0, nullptr, QualType(), false, nullptr);
 }
 
 NonTypeTemplateParmDecl *
 NonTypeTemplateParmDecl::CreateDeserialized(ASTContext &C, unsigned ID,
-                                            unsigned NumExpandedTypes) {
+                                            unsigned NumExpandedTypes,
+                                            bool HasTypeConstraint) {
   auto *NTTP =
-      new (C, ID, additionalSizeToAlloc<std::pair<QualType, TypeSourceInfo *>>(
-                      NumExpandedTypes))
+      new (C, ID, additionalSizeToAlloc<std::pair<QualType, TypeSourceInfo *>,
+                                        Expr *>(
+                      NumExpandedTypes, HasTypeConstraint ? 1 : 0))
           NonTypeTemplateParmDecl(nullptr, SourceLocation(), SourceLocation(),
                                   0, 0, nullptr, QualType(), nullptr, None,
                                   None);
diff --git a/clang/lib/AST/Expr.cpp b/clang/lib/AST/Expr.cpp
index 73ddbc62482d..835198958766 100644
--- a/clang/lib/AST/Expr.cpp
+++ b/clang/lib/AST/Expr.cpp
@@ -3457,6 +3457,7 @@ bool Expr::HasSideEffects(const ASTContext &Ctx,
   case OpaqueValueExprClass:
   case SourceLocExprClass:
   case ConceptSpecializationExprClass:
+  case RequiresExprClass:
     // These never have a side-effect.
     return false;
 
diff --git a/clang/lib/AST/ExprCXX.cpp b/clang/lib/AST/ExprCXX.cpp
index 422227d787b1..e4bd218ae2d3 100644
--- a/clang/lib/AST/ExprCXX.cpp
+++ b/clang/lib/AST/ExprCXX.cpp
@@ -17,6 +17,7 @@
 #include "clang/AST/DeclAccessPair.h"
 #include "clang/AST/DeclBase.h"
 #include "clang/AST/DeclCXX.h"
+#include "clang/AST/DeclTemplate.h"
 #include "clang/AST/DeclarationName.h"
 #include "clang/AST/Expr.h"
 #include "clang/AST/LambdaCapture.h"
@@ -1764,81 +1765,3 @@ CUDAKernelCallExpr *CUDAKernelCallExpr::CreateEmpty(const ASTContext &Ctx,
                            alignof(CUDAKernelCallExpr));
   return new (Mem) CUDAKernelCallExpr(NumArgs, Empty);
 }
-
-ConceptSpecializationExpr::ConceptSpecializationExpr(const ASTContext &C,
-    NestedNameSpecifierLoc NNS, SourceLocation TemplateKWLoc,
-    DeclarationNameInfo ConceptNameInfo, NamedDecl *FoundDecl,
-    ConceptDecl *NamedConcept, const ASTTemplateArgumentListInfo *ArgsAsWritten,
-    ArrayRef<TemplateArgument> ConvertedArgs,
-    const ConstraintSatisfaction *Satisfaction)
-    : Expr(ConceptSpecializationExprClass, C.BoolTy, VK_RValue, OK_Ordinary,
-           /*TypeDependent=*/false,
-           // All the flags below are set in setTemplateArguments.
-           /*ValueDependent=*/!Satisfaction, /*InstantiationDependent=*/false,
-           /*ContainsUnexpandedParameterPacks=*/false),
-      ConceptReference(NNS, TemplateKWLoc, ConceptNameInfo, FoundDecl,
-                       NamedConcept, ArgsAsWritten),
-      NumTemplateArgs(ConvertedArgs.size()),
-      Satisfaction(Satisfaction ?
-                   ASTConstraintSatisfaction::Create(C, *Satisfaction) :
-                   nullptr) {
-  setTemplateArguments(ConvertedArgs);
-}
-
-ConceptSpecializationExpr::ConceptSpecializationExpr(EmptyShell Empty,
-    unsigned NumTemplateArgs)
-    : Expr(ConceptSpecializationExprClass, Empty), ConceptReference(),
-      NumTemplateArgs(NumTemplateArgs) { }
-
-void ConceptSpecializationExpr::setTemplateArguments(
-    ArrayRef<TemplateArgument> Converted) {
-  assert(Converted.size() == NumTemplateArgs);
-  std::uninitialized_copy(Converted.begin(), Converted.end(),
-                          getTrailingObjects<TemplateArgument>());
-  bool IsInstantiationDependent = false;
-  bool ContainsUnexpandedParameterPack = false;
-  for (const TemplateArgument& Arg : Converted) {
-    if (Arg.isInstantiationDependent())
-      IsInstantiationDependent = true;
-    if (Arg.containsUnexpandedParameterPack())
-      ContainsUnexpandedParameterPack = true;
-    if (ContainsUnexpandedParameterPack && IsInstantiationDependent)
-      break;
-  }
-
-  // Currently guaranteed by the fact concepts can only be at namespace-scope.
-  assert(!NestedNameSpec ||
-         (!NestedNameSpec.getNestedNameSpecifier()->isInstantiationDependent() &&
-          !NestedNameSpec.getNestedNameSpecifier()
-              ->containsUnexpandedParameterPack()));
-  setInstantiationDependent(IsInstantiationDependent);
-  setContainsUnexpandedParameterPack(ContainsUnexpandedParameterPack);
-  assert((!isValueDependent() || isInstantiationDependent()) &&
-         "should not be value-dependent");
-}
-
-ConceptSpecializationExpr *
-ConceptSpecializationExpr::Create(const ASTContext &C,
-                                  NestedNameSpecifierLoc NNS,
-                                  SourceLocation TemplateKWLoc,
-                                  DeclarationNameInfo ConceptNameInfo,
-                                  NamedDecl *FoundDecl,
-                                  ConceptDecl *NamedConcept,
-                               const ASTTemplateArgumentListInfo *ArgsAsWritten,
-                                  ArrayRef<TemplateArgument> ConvertedArgs,
-                                  const ConstraintSatisfaction *Satisfaction) {
-  void *Buffer = C.Allocate(totalSizeToAlloc<TemplateArgument>(
-                                ConvertedArgs.size()));
-  return new (Buffer) ConceptSpecializationExpr(C, NNS, TemplateKWLoc,
-                                                ConceptNameInfo, FoundDecl,
-                                                NamedConcept, ArgsAsWritten,
-                                                ConvertedArgs, Satisfaction);
-}
-
-ConceptSpecializationExpr *
-ConceptSpecializationExpr::Create(ASTContext &C, EmptyShell Empty,
-                                  unsigned NumTemplateArgs) {
-  void *Buffer = C.Allocate(totalSizeToAlloc<TemplateArgument>(
-                                NumTemplateArgs));
-  return new (Buffer) ConceptSpecializationExpr(Empty, NumTemplateArgs);
-}
diff --git a/clang/lib/AST/ExprClassification.cpp b/clang/lib/AST/ExprClassification.cpp
index 9dbf6fe9e0f0..d201af31f521 100644
--- a/clang/lib/AST/ExprClassification.cpp
+++ b/clang/lib/AST/ExprClassification.cpp
@@ -193,6 +193,7 @@ static Cl::Kinds ClassifyInternal(ASTContext &Ctx, const Expr *E) {
   case Expr::DesignatedInitUpdateExprClass:
   case Expr::SourceLocExprClass:
   case Expr::ConceptSpecializationExprClass:
+  case Expr::RequiresExprClass:
     return Cl::CL_PRValue;
 
   case Expr::ConstantExprClass:
diff --git a/clang/lib/AST/ExprConcepts.cpp b/clang/lib/AST/ExprConcepts.cpp
new file mode 100644
index 000000000000..76d57ed5d5b1
--- /dev/null
+++ b/clang/lib/AST/ExprConcepts.cpp
@@ -0,0 +1,185 @@
+//===- ExprCXX.cpp - (C++) Expression AST Node Implementation -------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements the subclesses of Expr class declared in ExprCXX.h
+//
+//===----------------------------------------------------------------------===//
+
+#include "clang/AST/ExprConcepts.h"
+#include "clang/AST/ASTContext.h"
+#include "clang/AST/ASTConcept.h"
+#include "clang/AST/Decl.h"
+#include "clang/AST/DeclarationName.h"
+#include "clang/AST/DeclTemplate.h"
+#include "clang/AST/Expr.h"
+#include "clang/AST/NestedNameSpecifier.h"
+#include "clang/AST/TemplateBase.h"
+#include "clang/AST/Type.h"
+#include "clang/Basic/SourceLocation.h"
+#include "llvm/Support/TrailingObjects.h"
+#include <algorithm>
+#include <utility>
+#include <string>
+
+using namespace clang;
+
+ConceptSpecializationExpr::ConceptSpecializationExpr(const ASTContext &C,
+    NestedNameSpecifierLoc NNS, SourceLocation TemplateKWLoc,
+    DeclarationNameInfo ConceptNameInfo, NamedDecl *FoundDecl,
+    ConceptDecl *NamedConcept, const ASTTemplateArgumentListInfo *ArgsAsWritten,
+    ArrayRef<TemplateArgument> ConvertedArgs,
+    const ConstraintSatisfaction *Satisfaction)
+    : Expr(ConceptSpecializationExprClass, C.BoolTy, VK_RValue, OK_Ordinary,
+           /*TypeDependent=*/false,
+           // All the flags below are set in setTemplateArguments.
+           /*ValueDependent=*/!Satisfaction, /*InstantiationDependent=*/false,
+           /*ContainsUnexpandedParameterPacks=*/false),
+      ConceptReference(NNS, TemplateKWLoc, ConceptNameInfo, FoundDecl,
+                       NamedConcept, ArgsAsWritten),
+      NumTemplateArgs(ConvertedArgs.size()),
+      Satisfaction(Satisfaction ?
+                   ASTConstraintSatisfaction::Create(C, *Satisfaction) :
+                   nullptr) {
+  setTemplateArguments(ConvertedArgs);
+}
+
+ConceptSpecializationExpr::ConceptSpecializationExpr(EmptyShell Empty,
+    unsigned NumTemplateArgs)
+    : Expr(ConceptSpecializationExprClass, Empty), ConceptReference(),
+      NumTemplateArgs(NumTemplateArgs) { }
+
+void ConceptSpecializationExpr::setTemplateArguments(
+    ArrayRef<TemplateArgument> Converted) {
+  assert(Converted.size() == NumTemplateArgs);
+  std::uninitialized_copy(Converted.begin(), Converted.end(),
+                          getTrailingObjects<TemplateArgument>());
+  bool IsInstantiationDependent = false;
+  bool ContainsUnexpandedParameterPack = false;
+  for (const TemplateArgument& Arg : Converted) {
+    if (Arg.isInstantiationDependent())
+      IsInstantiationDependent = true;
+    if (Arg.containsUnexpandedParameterPack())
+      ContainsUnexpandedParameterPack = true;
+    if (ContainsUnexpandedParameterPack && IsInstantiationDependent)
+      break;
+  }
+
+  // Currently guaranteed by the fact concepts can only be at namespace-scope.
+  assert(!NestedNameSpec ||
+         (!NestedNameSpec.getNestedNameSpecifier()->isInstantiationDependent() &&
+          !NestedNameSpec.getNestedNameSpecifier()
+              ->containsUnexpandedParameterPack()));
+  setInstantiationDependent(IsInstantiationDependent);
+  setContainsUnexpandedParameterPack(ContainsUnexpandedParameterPack);
+  assert((!isValueDependent() || isInstantiationDependent()) &&
+         "should not be value-dependent");
+}
+
+ConceptSpecializationExpr *
+ConceptSpecializationExpr::Create(const ASTContext &C,
+                                  NestedNameSpecifierLoc NNS,
+                                  SourceLocation TemplateKWLoc,
+                                  DeclarationNameInfo ConceptNameInfo,
+                                  NamedDecl *FoundDecl,
+                                  ConceptDecl *NamedConcept,
+                               const ASTTemplateArgumentListInfo *ArgsAsWritten,
+                                  ArrayRef<TemplateArgument> ConvertedArgs,
+                                  const ConstraintSatisfaction *Satisfaction) {
+  void *Buffer = C.Allocate(totalSizeToAlloc<TemplateArgument>(
+                                ConvertedArgs.size()));
+  return new (Buffer) ConceptSpecializationExpr(C, NNS, TemplateKWLoc,
+                                                ConceptNameInfo, FoundDecl,
+                                                NamedConcept, ArgsAsWritten,
+                                                ConvertedArgs, Satisfaction);
+}
+
+ConceptSpecializationExpr *
+ConceptSpecializationExpr::Create(ASTContext &C, EmptyShell Empty,
+                                  unsigned NumTemplateArgs) {
+  void *Buffer = C.Allocate(totalSizeToAlloc<TemplateArgument>(
+                                NumTemplateArgs));
+  return new (Buffer) ConceptSpecializationExpr(Empty, NumTemplateArgs);
+}
+
+const TypeConstraint *
+concepts::ExprRequirement::ReturnTypeRequirement::getTypeConstraint() const {
+  assert(isTypeConstraint());
+  auto TPL =
+      TypeConstraintInfo.getPointer().get<TemplateParameterList *>();
+  return cast<TemplateTypeParmDecl>(TPL->getParam(0))
+      ->getTypeConstraint();
+}
+
+RequiresExpr::RequiresExpr(ASTContext &C, SourceLocation RequiresKWLoc,
+                           RequiresExprBodyDecl *Body,
+                           ArrayRef<ParmVarDecl *> LocalParameters,
+                           ArrayRef<concepts::Requirement *> Requirements,
+                           SourceLocation RBraceLoc)
+  : Expr(RequiresExprClass, C.BoolTy, VK_RValue, OK_Ordinary,
+         /*TD=*/false, /*VD=*/false, /*ID=*/false,
+         /*ContainsUnexpandedParameterPack=*/false),
+    NumLocalParameters(LocalParameters.size()),
+    NumRequirements(Requirements.size()), Body(Body), RBraceLoc(RBraceLoc) {
+  RequiresExprBits.IsSatisfied = false;
+  RequiresExprBits.RequiresKWLoc = RequiresKWLoc;
+  bool Dependent = false;
+  bool ContainsUnexpandedParameterPack = false;
+  for (ParmVarDecl *P : LocalParameters) {
+    Dependent |= P->getType()->isInstantiationDependentType();
+    ContainsUnexpandedParameterPack |=
+        P->getType()->containsUnexpandedParameterPack();
+  }
+  RequiresExprBits.IsSatisfied = true;
+  for (concepts::Requirement *R : Requirements) {
+    Dependent |= R->isDependent();
+    ContainsUnexpandedParameterPack |= R->containsUnexpandedParameterPack();
+    if (!Dependent) {
+      RequiresExprBits.IsSatisfied = R->isSatisfied();
+      if (!RequiresExprBits.IsSatisfied)
+        break;
+    }
+  }
+  std::copy(LocalParameters.begin(), LocalParameters.end(),
+            getTrailingObjects<ParmVarDecl *>());
+  std::copy(Requirements.begin(), Requirements.end(),
+            getTrailingObjects<concepts::Requirement *>());
+  RequiresExprBits.IsSatisfied |= Dependent;
+  setValueDependent(Dependent);
+  setInstantiationDependent(Dependent);
+  setContainsUnexpandedParameterPack(ContainsUnexpandedParameterPack);
+}
+
+RequiresExpr::RequiresExpr(ASTContext &C, EmptyShell Empty,
+                           unsigned NumLocalParameters,
+                           unsigned NumRequirements)
+  : Expr(RequiresExprClass, Empty), NumLocalParameters(NumLocalParameters),
+    NumRequirements(NumRequirements) { }
+
+RequiresExpr *
+RequiresExpr::Create(ASTContext &C, SourceLocation RequiresKWLoc,
+                     RequiresExprBodyDecl *Body,
+                     ArrayRef<ParmVarDecl *> LocalParameters,
+                     ArrayRef<concepts::Requirement *> Requirements,
+                     SourceLocation RBraceLoc) {
+  void *Mem =
+      C.Allocate(totalSizeToAlloc<ParmVarDecl *, concepts::Requirement *>(
+                     LocalParameters.size(), Requirements.size()),
+                 alignof(RequiresExpr));
+  return new (Mem) RequiresExpr(C, RequiresKWLoc, Body, LocalParameters,
+                                Requirements, RBraceLoc);
+}
+
+RequiresExpr *
+RequiresExpr::Create(ASTContext &C, EmptyShell Empty,
+                     unsigned NumLocalParameters, unsigned NumRequirements) {
+  void *Mem =
+      C.Allocate(totalSizeToAlloc<ParmVarDecl *, concepts::Requirement *>(
+                     NumLocalParameters, NumRequirements),
+                 alignof(RequiresExpr));
+  return new (Mem) RequiresExpr(C, Empty, NumLocalParameters, NumRequirements);
+}
diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index c4b27b5d1daa..c79973507323 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -9912,6 +9912,7 @@ public:
   bool VisitSizeOfPackExpr(const SizeOfPackExpr *E);
   bool VisitSourceLocExpr(const SourceLocExpr *E);
   bool VisitConceptSpecializationExpr(const ConceptSpecializationExpr *E);
+  bool VisitRequiresExpr(const RequiresExpr *E);
   // FIXME: Missing: array subscript of vector, member of vector
 };
 
@@ -12524,6 +12525,9 @@ bool IntExprEvaluator::VisitConceptSpecializationExpr(
   return Success(E->isSatisfied(), E);
 }
 
+bool IntExprEvaluator::VisitRequiresExpr(const RequiresExpr *E) {
+  return Success(E->isSatisfied(), E);
+}
 
 bool FixedPointExprEvaluator::VisitUnaryOperator(const UnaryOperator *E) {
   switch (E->getOpcode()) {
@@ -14182,6 +14186,7 @@ static ICEDiag CheckICE(const Expr* E, const ASTContext &Ctx) {
   case Expr::CXXScalarValueInitExprClass:
   case Expr::TypeTraitExprClass:
   case Expr::ConceptSpecializationExprClass:
+  case Expr::RequiresExprClass:
   case Expr::ArrayTypeTraitExprClass:
   case Expr::ExpressionTraitExprClass:
   case Expr::CXXNoexceptExprClass:
diff --git a/clang/lib/AST/ItaniumMangle.cpp b/clang/lib/AST/ItaniumMangle.cpp
index 0d567edac521..5d485e000750 100644
--- a/clang/lib/AST/ItaniumMangle.cpp
+++ b/clang/lib/AST/ItaniumMangle.cpp
@@ -22,6 +22,7 @@
 #include "clang/AST/DeclOpenMP.h"
 #include "clang/AST/DeclTemplate.h"
 #include "clang/AST/Expr.h"
+#include "clang/AST/ExprConcepts.h"
 #include "clang/AST/ExprCXX.h"
 #include "clang/AST/ExprObjC.h"
 #include "clang/AST/TypeLoc.h"
@@ -3668,6 +3669,7 @@ recurse:
   case Expr::ConvertVectorExprClass:
   case Expr::StmtExprClass:
   case Expr::TypeTraitExprClass:
+  case Expr::RequiresExprClass:
   case Expr::ArrayTypeTraitExprClass:
   case Expr::ExpressionTraitExprClass:
   case Expr::VAArgExprClass:
diff --git a/clang/lib/AST/ODRHash.cpp b/clang/lib/AST/ODRHash.cpp
index 27fdca1c4b9c..1f9ff9e407dc 100644
--- a/clang/lib/AST/ODRHash.cpp
+++ b/clang/lib/AST/ODRHash.cpp
@@ -857,6 +857,13 @@ public:
 
   void VisitAutoType(const AutoType *T) {
     ID.AddInteger((unsigned)T->getKeyword());
+    ID.AddInteger(T->isConstrained());
+    if (T->isConstrained()) {
+      AddDecl(T->getTypeConstraintConcept());
+      ID.AddInteger(T->getNumArgs());
+      for (const auto &TA : T->getTypeConstraintArguments())
+        Hash.AddTemplateArgument(TA);
+    }
     VisitDeducedType(T);
   }
 
diff --git a/clang/lib/AST/Stmt.cpp b/clang/lib/AST/Stmt.cpp
index b6e4d8aff21e..7409ae7ddc9e 100644
--- a/clang/lib/AST/Stmt.cpp
+++ b/clang/lib/AST/Stmt.cpp
@@ -16,6 +16,7 @@
 #include "clang/AST/Decl.h"
 #include "clang/AST/DeclGroup.h"
 #include "clang/AST/Expr.h"
+#include "clang/AST/ExprConcepts.h"
 #include "clang/AST/ExprCXX.h"
 #include "clang/AST/ExprObjC.h"
 #include "clang/AST/ExprOpenMP.h"
diff --git a/clang/lib/AST/StmtPrinter.cpp b/clang/lib/AST/StmtPrinter.cpp
index c14bb886bb11..45fd8ceae8d3 100644
--- a/clang/lib/AST/StmtPrinter.cpp
+++ b/clang/lib/AST/StmtPrinter.cpp
@@ -2269,6 +2269,60 @@ void StmtPrinter::VisitConceptSpecializationExpr(ConceptSpecializationExpr *E) {
                             Policy);
 }
 
+void StmtPrinter::VisitRequiresExpr(RequiresExpr *E) {
+  OS << "requires ";
+  auto LocalParameters = E->getLocalParameters();
+  if (!LocalParameters.empty()) {
+    OS << "(";
+    for (ParmVarDecl *LocalParam : LocalParameters) {
+      PrintRawDecl(LocalParam);
+      if (LocalParam != LocalParameters.back())
+        OS << ", ";
+    }
+
+    OS << ") ";
+  }
+  OS << "{ ";
+  auto Requirements = E->getRequirements();
+  for (concepts::Requirement *Req : Requirements) {
+    if (auto *TypeReq = dyn_cast<concepts::TypeRequirement>(Req)) {
+      if (TypeReq->isSubstitutionFailure())
+        OS << "<<error-type>>";
+      else
+        TypeReq->getType()->getType().print(OS, Policy);
+    } else if (auto *ExprReq = dyn_cast<concepts::ExprRequirement>(Req)) {
+      if (ExprReq->isCompound())
+        OS << "{ ";
+      if (ExprReq->isExprSubstitutionFailure())
+        OS << "<<error-expression>>";
+      else
+        PrintExpr(ExprReq->getExpr());
+      if (ExprReq->isCompound()) {
+        OS << " }";
+        if (ExprReq->getNoexceptLoc().isValid())
+          OS << " noexcept";
+        const auto &RetReq = ExprReq->getReturnTypeRequirement();
+        if (!RetReq.isEmpty()) {
+          OS << " -> ";
+          if (RetReq.isSubstitutionFailure())
+            OS << "<<error-type>>";
+          else if (RetReq.isTypeConstraint())
+            RetReq.getTypeConstraint()->print(OS, Policy);
+        }
+      }
+    } else {
+      auto *NestedReq = cast<concepts::NestedRequirement>(Req);
+      OS << "requires ";
+      if (NestedReq->isSubstitutionFailure())
+        OS << "<<error-expression>>";
+      else
+        PrintExpr(NestedReq->getConstraintExpr());
+    }
+    OS << "; ";
+  }
+  OS << "}";
+}
+
 // C++ Coroutines TS
 
 void StmtPrinter::VisitCoroutineBodyStmt(CoroutineBodyStmt *S) {
diff --git a/clang/lib/AST/StmtProfile.cpp b/clang/lib/AST/StmtProfile.cpp
index 2aa5106e90fa..382ea5c8d7ef 100644
--- a/clang/lib/AST/StmtProfile.cpp
+++ b/clang/lib/AST/StmtProfile.cpp
@@ -1335,9 +1335,52 @@ void StmtProfiler::VisitAtomicExpr(const AtomicExpr *S) {
 void StmtProfiler::VisitConceptSpecializationExpr(
                                            const ConceptSpecializationExpr *S) {
   VisitExpr(S);
-  VisitDecl(S->getFoundDecl());
-  VisitTemplateArguments(S->getTemplateArgsAsWritten()->getTemplateArgs(),
-                         S->getTemplateArgsAsWritten()->NumTemplateArgs);
+  VisitDecl(S->getNamedConcept());
+  for (const TemplateArgument &Arg : S->getTemplateArguments())
+    VisitTemplateArgument(Arg);
+}
+
+void StmtProfiler::VisitRequiresExpr(const RequiresExpr *S) {
+  VisitExpr(S);
+  ID.AddInteger(S->getLocalParameters().size());
+  for (ParmVarDecl *LocalParam : S->getLocalParameters())
+    VisitDecl(LocalParam);
+  ID.AddInteger(S->getRequirements().size());
+  for (concepts::Requirement *Req : S->getRequirements()) {
+    if (auto *TypeReq = dyn_cast<concepts::TypeRequirement>(Req)) {
+      ID.AddInteger(concepts::Requirement::RK_Type);
+      ID.AddBoolean(TypeReq->isSubstitutionFailure());
+      if (!TypeReq->isSubstitutionFailure())
+        VisitType(TypeReq->getType()->getType());
+    } else if (auto *ExprReq = dyn_cast<concepts::ExprRequirement>(Req)) {
+      ID.AddInteger(concepts::Requirement::RK_Compound);
+      ID.AddBoolean(ExprReq->isExprSubstitutionFailure());
+      if (!ExprReq->isExprSubstitutionFailure())
+        Visit(ExprReq->getExpr());
+      // C++2a [expr.prim.req.compound]p1 Example:
+      //    [...] The compound-requirement in C1 requires that x++ is a valid
+      //    expression. It is equivalent to the simple-requirement x++; [...]
+      // We therefore do not profile isSimple() here.
+      ID.AddBoolean(ExprReq->getNoexceptLoc().isValid());
+      const concepts::ExprRequirement::ReturnTypeRequirement &RetReq =
+          ExprReq->getReturnTypeRequirement();
+      if (RetReq.isEmpty()) {
+        ID.AddInteger(0);
+      } else if (RetReq.isTypeConstraint()) {
+        ID.AddInteger(1);
+        Visit(RetReq.getTypeConstraint()->getImmediatelyDeclaredConstraint());
+      } else {
+        assert(RetReq.isSubstitutionFailure());
+        ID.AddInteger(2);
+      }
+    } else {
+      ID.AddInteger(concepts::Requirement::RK_Nested);
+      auto *NestedReq = cast<concepts::NestedRequirement>(Req);
+      ID.AddBoolean(NestedReq->isSubstitutionFailure());
+      if (!NestedReq->isSubstitutionFailure())  
+        Visit(NestedReq->getConstraintExpr());
+    }
+  }
 }
 
 static Stmt::StmtClass DecodeOperatorCall(const CXXOperatorCallExpr *S,
diff --git a/clang/lib/AST/TemplateBase.cpp b/clang/lib/AST/TemplateBase.cpp
index db16c2a06b64..6f0ebf232e77 100644
--- a/clang/lib/AST/TemplateBase.cpp
+++ b/clang/lib/AST/TemplateBase.cpp
@@ -561,7 +561,7 @@ const DiagnosticBuilder &clang::operator<<(const DiagnosticBuilder &DB,
 }
 
 const ASTTemplateArgumentListInfo *
-ASTTemplateArgumentListInfo::Create(ASTContext &C,
+ASTTemplateArgumentListInfo::Create(const ASTContext &C,
                                     const TemplateArgumentListInfo &List) {
   std::size_t size = totalSizeToAlloc<TemplateArgumentLoc>(List.size());
   void *Mem = C.Allocate(size, alignof(ASTTemplateArgumentListInfo));
diff --git a/clang/lib/AST/TextNodeDumper.cpp b/clang/lib/AST/TextNodeDumper.cpp
index 965ad17fcfa5..c9b571862c19 100644
--- a/clang/lib/AST/TextNodeDumper.cpp
+++ b/clang/lib/AST/TextNodeDumper.cpp
@@ -1201,6 +1201,11 @@ void TextNodeDumper::VisitAutoType(const AutoType *T) {
     OS << " decltype(auto)";
   if (!T->isDeduced())
     OS << " undeduced";
+  if (T->isConstrained()) {
+    dumpDeclRef(T->getTypeConstraintConcept());
+    for (const auto &Arg : T->getTypeConstraintArguments())
+      VisitTemplateArgument(Arg);
+  }
 }
 
 void TextNodeDumper::VisitTemplateSpecializationType(
diff --git a/clang/lib/AST/Type.cpp b/clang/lib/AST/Type.cpp
index c5ad711d872e..5099494da5fd 100644
--- a/clang/lib/AST/Type.cpp
+++ b/clang/lib/AST/Type.cpp
@@ -1114,7 +1114,9 @@ public:
       return QualType(T, 0);
 
     return Ctx.getAutoType(deducedType, T->getKeyword(),
-                           T->isDependentType());
+                           T->isDependentType(), /*IsPack=*/false,
+                           T->getTypeConstraintConcept(),
+                           T->getTypeConstraintArguments());
   }
 
   // FIXME: Non-trivial to implement, but important for C++
@@ -4158,3 +4160,35 @@ void clang::FixedPointValueToString(SmallVectorImpl<char> &Str,
                              /*HasUnsignedPadding=*/false);
   APFixedPoint(Val, FXSema).toString(Str);
 }
+
+AutoType::AutoType(QualType DeducedAsType, AutoTypeKeyword Keyword,
+                   bool IsDeducedAsDependent, bool IsDeducedAsPack,
+                   ConceptDecl *TypeConstraintConcept,
+                   ArrayRef<TemplateArgument> TypeConstraintArgs)
+    : DeducedType(Auto, DeducedAsType, IsDeducedAsDependent,
+                  IsDeducedAsDependent, IsDeducedAsPack) {
+  AutoTypeBits.Keyword = (unsigned)Keyword;
+  AutoTypeBits.NumArgs = TypeConstraintArgs.size();
+  this->TypeConstraintConcept = TypeConstraintConcept;
+  if (TypeConstraintConcept) {
+    TemplateArgument *ArgBuffer = getArgBuffer();
+    for (const TemplateArgument &Arg : TypeConstraintArgs) {
+      if (Arg.containsUnexpandedParameterPack())
+        setContainsUnexpandedParameterPack();
+
+      new (ArgBuffer++) TemplateArgument(Arg);
+    }
+  }
+}
+
+void AutoType::Profile(llvm::FoldingSetNodeID &ID, const ASTContext &Context,
+                      QualType Deduced, AutoTypeKeyword Keyword,
+                      bool IsDependent, ConceptDecl *CD,
+                      ArrayRef<TemplateArgument> Arguments) {
+  ID.AddPointer(Deduced.getAsOpaquePtr());
+  ID.AddInteger((unsigned)Keyword);
+  ID.AddBoolean(IsDependent);
+  ID.AddPointer(CD);
+  for (const TemplateArgument &Arg : Arguments)
+    Arg.Profile(ID, Context);
+}
diff --git a/clang/lib/AST/TypeLoc.cpp b/clang/lib/AST/TypeLoc.cpp
index 6e67ca8e0af7..665a86f2c143 100644
--- a/clang/lib/AST/TypeLoc.cpp
+++ b/clang/lib/AST/TypeLoc.cpp
@@ -11,6 +11,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "clang/AST/TypeLoc.h"
+#include "clang/AST/DeclTemplate.h"
 #include "clang/AST/ASTContext.h"
 #include "clang/AST/Attr.h"
 #include "clang/AST/Expr.h"
@@ -589,3 +590,97 @@ void TemplateSpecializationTypeLoc::initializeArgLocs(ASTContext &Context,
     }
   }
 }
+
+DeclarationNameInfo AutoTypeLoc::getConceptNameInfo() const {
+  return DeclarationNameInfo(getNamedConcept()->getDeclName(),
+                             getLocalData()->ConceptNameLoc);
+}
+
+void AutoTypeLoc::initializeLocal(ASTContext &Context, SourceLocation Loc) {
+  setNestedNameSpecifierLoc(NestedNameSpecifierLoc());
+  setTemplateKWLoc(Loc);
+  setConceptNameLoc(Loc);
+  setFoundDecl(nullptr);
+  setRAngleLoc(Loc);
+  setLAngleLoc(Loc);
+  TemplateSpecializationTypeLoc::initializeArgLocs(Context, getNumArgs(),
+                                                   getTypePtr()->getArgs(),
+                                                   getArgInfos(), Loc);
+  setNameLoc(Loc);
+}
+
+
+namespace {
+
+  class GetContainedAutoTypeLocVisitor :
+    public TypeLocVisitor<GetContainedAutoTypeLocVisitor, TypeLoc> {
+  public:
+    using TypeLocVisitor<GetContainedAutoTypeLocVisitor, TypeLoc>::Visit;
+
+    TypeLoc VisitAutoTypeLoc(AutoTypeLoc TL) {
+      return TL;
+    }
+
+    // Only these types can contain the desired 'auto' type.
+
+    TypeLoc VisitElaboratedTypeLoc(ElaboratedTypeLoc T) {
+      return Visit(T.getNamedTypeLoc());
+    }
+
+    TypeLoc VisitQualifiedTypeLoc(QualifiedTypeLoc T) {
+      return Visit(T.getUnqualifiedLoc());
+    }
+
+    TypeLoc VisitPointerTypeLoc(PointerTypeLoc T) {
+      return Visit(T.getPointeeLoc());
+    }
+
+    TypeLoc VisitBlockPointerTypeLoc(BlockPointerTypeLoc T) {
+      return Visit(T.getPointeeLoc());
+    }
+
+    TypeLoc VisitReferenceTypeLoc(ReferenceTypeLoc T) {
+      return Visit(T.getPointeeLoc());
+    }
+
+    TypeLoc VisitMemberPointerTypeLoc(MemberPointerTypeLoc T) {
+      return Visit(T.getPointeeLoc());
+    }
+
+    TypeLoc VisitArrayTypeLoc(ArrayTypeLoc T) {
+      return Visit(T.getElementLoc());
+    }
+
+    TypeLoc VisitFunctionTypeLoc(FunctionTypeLoc T) {
+      return Visit(T.getReturnLoc());
+    }
+
+    TypeLoc VisitParenTypeLoc(ParenTypeLoc T) {
+      return Visit(T.getInnerLoc());
+    }
+
+    TypeLoc VisitAttributedTypeLoc(AttributedTypeLoc T) {
+      return Visit(T.getModifiedLoc());
+    }
+
+    TypeLoc VisitMacroQualifiedTypeLoc(MacroQualifiedTypeLoc T) {
+      return Visit(T.getInnerLoc());
+    }
+
+    TypeLoc VisitAdjustedTypeLoc(AdjustedTypeLoc T) {
+      return Visit(T.getOriginalLoc());
+    }
+
+    TypeLoc VisitPackExpansionTypeLoc(PackExpansionTypeLoc T) {
+      return Visit(T.getPatternLoc());
+    }
+  };
+
+} // namespace
+
+AutoTypeLoc TypeLoc::getContainedAutoTypeLoc() const {
+  TypeLoc Res = GetContainedAutoTypeLocVisitor().Visit(*this);
+  if (Res.isNull())
+    return AutoTypeLoc();
+  return Res.getAs<AutoTypeLoc>();
+}
diff --git a/clang/lib/AST/TypePrinter.cpp b/clang/lib/AST/TypePrinter.cpp
index c2f4baec989e..4a7e765a2bd8 100644
--- a/clang/lib/AST/TypePrinter.cpp
+++ b/clang/lib/AST/TypePrinter.cpp
@@ -1046,6 +1046,13 @@ void TypePrinter::printAutoBefore(const AutoType *T, raw_ostream &OS) {
   if (!T->getDeducedType().isNull()) {
     printBefore(T->getDeducedType(), OS);
   } else {
+    if (T->isConstrained()) {
+      OS << T->getTypeConstraintConcept()->getName();
+      auto Args = T->getTypeConstraintArguments();
+      if (!Args.empty())
+        printTemplateArgumentList(OS, Args, Policy);
+      OS << ' ';
+    }
     switch (T->getKeyword()) {
     case AutoTypeKeyword::Auto: OS << "auto"; break;
     case AutoTypeKeyword::DecltypeAuto: OS << "decltype(auto)"; break;
@@ -1234,20 +1241,18 @@ void TypePrinter::printEnumAfter(const EnumType *T, raw_ostream &OS) {}
 
 void TypePrinter::printTemplateTypeParmBefore(const TemplateTypeParmType *T,
                                               raw_ostream &OS) {
-  if (IdentifierInfo *Id = T->getIdentifier())
-    OS << Id->getName();
-  else {
-    bool IsLambdaAutoParam = false;
-    if (auto D = T->getDecl()) {
-      if (auto M = dyn_cast_or_null<CXXMethodDecl>(D->getDeclContext()))
-        IsLambdaAutoParam = D->isImplicit() && M->getParent()->isLambda();
+  TemplateTypeParmDecl *D = T->getDecl();
+  if (D && D->isImplicit()) {
+    if (auto *TC = D->getTypeConstraint()) {
+      TC->print(OS, Policy);
+      OS << ' ';
     }
+    OS << "auto";
+  } else if (IdentifierInfo *Id = T->getIdentifier())
+    OS << Id->getName();
+  else
+    OS << "type-parameter-" << T->getDepth() << '-' << T->getIndex();
 
-    if (IsLambdaAutoParam)
-      OS << "auto";
-    else
-      OS << "type-parameter-" << T->getDepth() << '-' << T->getIndex();
-  }
   spaceBeforePlaceHolder(OS);
 }
 
diff --git a/clang/lib/Basic/IdentifierTable.cpp b/clang/lib/Basic/IdentifierTable.cpp
index 4aebea19924f..ee25bd883caf 100644
--- a/clang/lib/Basic/IdentifierTable.cpp
+++ b/clang/lib/Basic/IdentifierTable.cpp
@@ -142,7 +142,7 @@ static KeywordStatus getKeywordStatus(const LangOptions &LangOpts,
   // We treat bridge casts as objective-C keywords so we can warn on them
   // in non-arc mode.
   if (LangOpts.ObjC && (Flags & KEYOBJC)) return KS_Enabled;
-  if (LangOpts.ConceptsTS && (Flags & KEYCONCEPTS)) return KS_Enabled;
+  if (LangOpts.CPlusPlus2a && (Flags & KEYCONCEPTS)) return KS_Enabled;
   if (LangOpts.Coroutines && (Flags & KEYCOROUTINES)) return KS_Enabled;
   if (LangOpts.ModulesTS && (Flags & KEYMODULES)) return KS_Enabled;
   if (LangOpts.CPlusPlus && (Flags & KEYALLCXX)) return KS_Future;
diff --git a/clang/lib/CodeGen/CGBuiltin.cpp b/clang/lib/CodeGen/CGBuiltin.cpp
index 09fd3087b494..2d20f92fbb3d 100644
--- a/clang/lib/CodeGen/CGBuiltin.cpp
+++ b/clang/lib/CodeGen/CGBuiltin.cpp
@@ -3222,6 +3222,8 @@ RValue CodeGenFunction::EmitBuiltinExpr(const GlobalDecl GD, unsigned BuiltinID,
         Builder.CreateZExt(EmitSignBit(*this, EmitScalarExpr(E->getArg(0))),
                            ConvertType(E->getType())));
   }
+  case Builtin::BI__warn_memset_zero_len:
+    return RValue::getIgnored();
   case Builtin::BI__annotation: {
     // Re-encode each wide string to UTF8 and make an MDString.
     SmallVector<Metadata *, 1> Strings;
diff --git a/clang/lib/CodeGen/CGDecl.cpp b/clang/lib/CodeGen/CGDecl.cpp
index 5aac7a8d54c7..60f1dba7c768 100644
--- a/clang/lib/CodeGen/CGDecl.cpp
+++ b/clang/lib/CodeGen/CGDecl.cpp
@@ -111,6 +111,7 @@ void CodeGenFunction::EmitDecl(const Decl &D) {
   case Decl::Empty:
   case Decl::Concept:
   case Decl::LifetimeExtendedTemporary:
+  case Decl::RequiresExprBody:
     // None of these decls require codegen support.
     return;
 
diff --git a/clang/lib/CodeGen/CGExprScalar.cpp b/clang/lib/CodeGen/CGExprScalar.cpp
index 3f23fe11e4f5..de5c3a03fb68 100644
--- a/clang/lib/CodeGen/CGExprScalar.cpp
+++ b/clang/lib/CodeGen/CGExprScalar.cpp
@@ -680,6 +680,10 @@ public:
     return Builder.getInt1(E->isSatisfied());
   }
 
+  Value *VisitRequiresExpr(const RequiresExpr *E) {
+    return Builder.getInt1(E->isSatisfied());
+  }
+
   Value *VisitArrayTypeTraitExpr(const ArrayTypeTraitExpr *E) {
     return llvm::ConstantInt::get(Builder.getInt32Ty(), E->getValue());
   }
diff --git a/clang/lib/CodeGen/CodeGenFunction.cpp b/clang/lib/CodeGen/CodeGenFunction.cpp
index 2bf94f697e01..648e6d9c214a 100644
--- a/clang/lib/CodeGen/CodeGenFunction.cpp
+++ b/clang/lib/CodeGen/CodeGenFunction.cpp
@@ -820,13 +820,18 @@ void CodeGenFunction::StartFunction(GlobalDecl GD, QualType RetTy,
             llvm::itostr(CGM.getCodeGenOpts().XRayInstructionThreshold));
     }
 
+    unsigned Count, Offset;
     if (const auto *Attr = D->getAttr<PatchableFunctionEntryAttr>()) {
-      // Attr->getStart is currently ignored.
-      Fn->addFnAttr("patchable-function-entry",
-                    std::to_string(Attr->getCount()));
-    } else if (unsigned Count = CGM.getCodeGenOpts().PatchableFunctionEntryCount) {
-      Fn->addFnAttr("patchable-function-entry",
-                    std::to_string(Count));
+      Count = Attr->getCount();
+      Offset = Attr->getOffset();
+    } else {
+      Count = CGM.getCodeGenOpts().PatchableFunctionEntryCount;
+      Offset = CGM.getCodeGenOpts().PatchableFunctionEntryOffset;
+    }
+    if (Count && Offset <= Count) {
+      Fn->addFnAttr("patchable-function-entry", std::to_string(Count - Offset));
+      if (Offset)
+        Fn->addFnAttr("patchable-function-prefix", std::to_string(Offset));
     }
   }
 
diff --git a/clang/lib/Driver/Compilation.cpp b/clang/lib/Driver/Compilation.cpp
index ba188f5c4083..25aec3690f21 100644
--- a/clang/lib/Driver/Compilation.cpp
+++ b/clang/lib/Driver/Compilation.cpp
@@ -172,7 +172,7 @@ int Compilation::ExecuteCommand(const Command &C,
     }
 
     if (getDriver().CCPrintOptions)
-      *OS << "[Logging clang options]";
+      *OS << "[Logging clang options]\n";
 
     C.Print(*OS, "\n", /*Quote=*/getDriver().CCPrintOptions);
   }
diff --git a/clang/lib/Driver/Job.cpp b/clang/lib/Driver/Job.cpp
index d57c3a1cdbb8..7dab2a022d92 100644
--- a/clang/lib/Driver/Job.cpp
+++ b/clang/lib/Driver/Job.cpp
@@ -373,7 +373,7 @@ int Command::Execute(ArrayRef<llvm::Optional<StringRef>> Redirects,
 
 void CC1Command::Print(raw_ostream &OS, const char *Terminator, bool Quote,
                        CrashReportInfo *CrashInfo) const {
-  OS << " (in-process)";
+  OS << " (in-process)\n";
   Command::Print(OS, Terminator, Quote, CrashInfo);
 }
 
diff --git a/clang/lib/Driver/ToolChain.cpp b/clang/lib/Driver/ToolChain.cpp
index 3ebbd30195b3..cab97b1a601a 100644
--- a/clang/lib/Driver/ToolChain.cpp
+++ b/clang/lib/Driver/ToolChain.cpp
@@ -68,8 +68,7 @@ static ToolChain::RTTIMode CalculateRTTIMode(const ArgList &Args,
   }
 
   // -frtti is default, except for the PS4 CPU.
-  return (Triple.isPS4CPU() || Triple.isNVPTX()) ? ToolChain::RM_Disabled
-                                                 : ToolChain::RM_Enabled;
+  return (Triple.isPS4CPU()) ? ToolChain::RM_Disabled : ToolChain::RM_Enabled;
 }
 
 ToolChain::ToolChain(const Driver &D, const llvm::Triple &T,
diff --git a/clang/lib/Driver/ToolChains/Clang.cpp b/clang/lib/Driver/ToolChains/Clang.cpp
index 9b3055413e9e..647465863d3e 100644
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -5077,20 +5077,23 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
 
   if (Arg *A = Args.getLastArg(options::OPT_fpatchable_function_entry_EQ)) {
     StringRef S0 = A->getValue(), S = S0;
-    unsigned Size, Start = 0;
+    unsigned Size, Offset = 0;
     if (!Triple.isAArch64() && Triple.getArch() != llvm::Triple::x86 &&
         Triple.getArch() != llvm::Triple::x86_64)
       D.Diag(diag::err_drv_unsupported_opt_for_target)
           << A->getAsString(Args) << TripleStr;
     else if (S.consumeInteger(10, Size) ||
              (!S.empty() && (!S.consume_front(",") ||
-                             S.consumeInteger(10, Start) || !S.empty())))
+                             S.consumeInteger(10, Offset) || !S.empty())))
       D.Diag(diag::err_drv_invalid_argument_to_option)
           << S0 << A->getOption().getName();
-    else if (Start)
+    else if (Size < Offset)
       D.Diag(diag::err_drv_unsupported_fpatchable_function_entry_argument);
-    else
+    else {
       CmdArgs.push_back(Args.MakeArgString(A->getSpelling() + Twine(Size)));
+      CmdArgs.push_back(Args.MakeArgString(
+          "-fpatchable-function-entry-offset=" + Twine(Offset)));
+    }
   }
 
   if (TC.SupportsProfiling()) {
diff --git a/clang/lib/Driver/ToolChains/HIP.cpp b/clang/lib/Driver/ToolChains/HIP.cpp
index f89e648948ab..da7004cf283f 100644
--- a/clang/lib/Driver/ToolChains/HIP.cpp
+++ b/clang/lib/Driver/ToolChains/HIP.cpp
@@ -105,9 +105,8 @@ const char *AMDGCN::Linker::constructLLVMLinkCommand(
   CmdArgs.push_back("-o");
   auto OutputFileName = getOutputFileName(C, OutputFilePrefix, "-linked", "bc");
   CmdArgs.push_back(OutputFileName);
-  SmallString<128> ExecPath(C.getDriver().Dir);
-  llvm::sys::path::append(ExecPath, "llvm-link");
-  const char *Exec = Args.MakeArgString(ExecPath);
+  const char *Exec =
+      Args.MakeArgString(getToolChain().GetProgramPath("llvm-link"));
   C.addCommand(std::make_unique<Command>(JA, *this, Exec, CmdArgs, Inputs));
   return OutputFileName;
 }
@@ -133,9 +132,8 @@ const char *AMDGCN::Linker::constructOptCommand(
   auto OutputFileName =
       getOutputFileName(C, OutputFilePrefix, "-optimized", "bc");
   OptArgs.push_back(OutputFileName);
-  SmallString<128> OptPath(C.getDriver().Dir);
-  llvm::sys::path::append(OptPath, "opt");
-  const char *OptExec = Args.MakeArgString(OptPath);
+  const char *OptExec =
+      Args.MakeArgString(getToolChain().GetProgramPath("opt"));
   C.addCommand(std::make_unique<Command>(JA, *this, OptExec, OptArgs, Inputs));
   return OutputFileName;
 }
@@ -180,9 +178,7 @@ const char *AMDGCN::Linker::constructLlcCommand(
   auto LlcOutputFile =
       getOutputFileName(C, OutputFilePrefix, "", OutputIsAsm ? "s" : "o");
   LlcArgs.push_back(LlcOutputFile);
-  SmallString<128> LlcPath(C.getDriver().Dir);
-  llvm::sys::path::append(LlcPath, "llc");
-  const char *Llc = Args.MakeArgString(LlcPath);
+  const char *Llc = Args.MakeArgString(getToolChain().GetProgramPath("llc"));
   C.addCommand(std::make_unique<Command>(JA, *this, Llc, LlcArgs, Inputs));
   return LlcOutputFile;
 }
@@ -196,9 +192,7 @@ void AMDGCN::Linker::constructLldCommand(Compilation &C, const JobAction &JA,
   // The output from ld.lld is an HSA code object file.
   ArgStringList LldArgs{
       "-flavor", "gnu", "-shared", "-o", Output.getFilename(), InputFileName};
-  SmallString<128> LldPath(C.getDriver().Dir);
-  llvm::sys::path::append(LldPath, "lld");
-  const char *Lld = Args.MakeArgString(LldPath);
+  const char *Lld = Args.MakeArgString(getToolChain().GetProgramPath("lld"));
   C.addCommand(std::make_unique<Command>(JA, *this, Lld, LldArgs, Inputs));
 }
 
@@ -230,9 +224,8 @@ void AMDGCN::constructHIPFatbinCommand(Compilation &C, const JobAction &JA,
       Args.MakeArgString(std::string("-outputs=").append(OutputFileName));
   BundlerArgs.push_back(BundlerOutputArg);
 
-  SmallString<128> BundlerPath(C.getDriver().Dir);
-  llvm::sys::path::append(BundlerPath, "clang-offload-bundler");
-  const char *Bundler = Args.MakeArgString(BundlerPath);
+  const char *Bundler = Args.MakeArgString(
+      T.getToolChain().GetProgramPath("clang-offload-bundler"));
   C.addCommand(std::make_unique<Command>(JA, T, Bundler, BundlerArgs, Inputs));
 }
 
diff --git a/clang/lib/Format/TokenAnnotator.cpp b/clang/lib/Format/TokenAnnotator.cpp
index d5d394e61926..88564e02f23e 100644
--- a/clang/lib/Format/TokenAnnotator.cpp
+++ b/clang/lib/Format/TokenAnnotator.cpp
@@ -2596,7 +2596,7 @@ bool TokenAnnotator::spaceRequiredBeforeParens(const FormatToken &Right) const {
 /// otherwise.
 static bool isKeywordWithCondition(const FormatToken &Tok) {
   return Tok.isOneOf(tok::kw_if, tok::kw_for, tok::kw_while, tok::kw_switch,
-                     tok::kw_constexpr);
+                     tok::kw_constexpr, tok::kw_catch);
 }
 
 bool TokenAnnotator::spaceRequiredBetween(const AnnotatedLine &Line,
diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
index e1e59565083b..4e5babdbaa03 100644
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -1103,6 +1103,8 @@ static bool ParseCodeGenArgs(CodeGenOptions &Opts, ArgList &Args, InputKind IK,
 
   Opts.PatchableFunctionEntryCount =
       getLastArgIntValue(Args, OPT_fpatchable_function_entry_EQ, 0, Diags);
+  Opts.PatchableFunctionEntryOffset = getLastArgIntValue(
+      Args, OPT_fpatchable_function_entry_offset_EQ, 0, Diags);
   Opts.InstrumentForProfiling = Args.hasArg(OPT_pg);
   Opts.CallFEntry = Args.hasArg(OPT_mfentry);
   Opts.MNopMCount = Args.hasArg(OPT_mnop_mcount);
@@ -2852,7 +2854,10 @@ static void ParseLangArgs(LangOptions &Opts, ArgList &Args, InputKind IK,
                                                  << A->getValue();
     Opts.NewAlignOverride = 0;
   }
-  Opts.ConceptsTS = Args.hasArg(OPT_fconcepts_ts);
+  Opts.ConceptSatisfactionCaching =
+      !Args.hasArg(OPT_fno_concept_satisfaction_caching);
+  if (Args.hasArg(OPT_fconcepts_ts))
+    Diags.Report(diag::warn_fe_concepts_ts_flag);
   Opts.HeinousExtensions = Args.hasArg(OPT_fheinous_gnu_extensions);
   Opts.AccessControl = !Args.hasArg(OPT_fno_access_control);
   Opts.ElideConstructors = !Args.hasArg(OPT_fno_elide_constructors);
diff --git a/clang/lib/Frontend/FrontendActions.cpp b/clang/lib/Frontend/FrontendActions.cpp
index 8574d0a7e813..935c64a0fa13 100644
--- a/clang/lib/Frontend/FrontendActions.cpp
+++ b/clang/lib/Frontend/FrontendActions.cpp
@@ -429,6 +429,10 @@ private:
       return "ConstraintNormalization";
     case CodeSynthesisContext::ParameterMappingSubstitution:
       return "ParameterMappingSubstitution";
+    case CodeSynthesisContext::RequirementInstantiation:
+      return "RequirementInstantiation";
+    case CodeSynthesisContext::NestedRequirementConstraintsCheck:
+      return "NestedRequirementConstraintsCheck";
     }
     return "";
   }
diff --git a/clang/lib/Frontend/InitPreprocessor.cpp b/clang/lib/Frontend/InitPreprocessor.cpp
index 2c7e3a56c043..8a0ff55e44fc 100644
--- a/clang/lib/Frontend/InitPreprocessor.cpp
+++ b/clang/lib/Frontend/InitPreprocessor.cpp
@@ -548,7 +548,7 @@ static void InitializeCPlusPlusFeatureTestMacros(const LangOptions &LangOpts,
   // C++20 features.
   if (LangOpts.CPlusPlus2a) {
     //Builder.defineMacro("__cpp_aggregate_paren_init", "201902L");
-    //Builder.defineMacro("__cpp_concepts", "201907L");
+    Builder.defineMacro("__cpp_concepts", "201907L");
     Builder.defineMacro("__cpp_conditional_explicit", "201806L");
     //Builder.defineMacro("__cpp_consteval", "201811L");
     Builder.defineMacro("__cpp_constexpr_dynamic_alloc", "201907L");
@@ -564,8 +564,6 @@ static void InitializeCPlusPlusFeatureTestMacros(const LangOptions &LangOpts,
   Builder.defineMacro("__cpp_impl_destroying_delete", "201806L");
 
   // TS features.
-  if (LangOpts.ConceptsTS)
-    Builder.defineMacro("__cpp_experimental_concepts", "1L");
   if (LangOpts.Coroutines)
     Builder.defineMacro("__cpp_coroutines", "201703L");
 }
diff --git a/clang/lib/Headers/ppc_wrappers/emmintrin.h b/clang/lib/Headers/ppc_wrappers/emmintrin.h
index 293276cc9be0..4dcb8485e2e9 100644
--- a/clang/lib/Headers/ppc_wrappers/emmintrin.h
+++ b/clang/lib/Headers/ppc_wrappers/emmintrin.h
@@ -1749,7 +1749,7 @@ _mm_sll_epi64 (__m128i __A, __m128i __B)
   lshift = vec_splat ((__v2du) __B, 0);
   shmask = vec_cmplt (lshift, shmax);
   result = vec_sl ((__v2du) __A, lshift);
-  result = vec_sel ((__v2du) shmask, result, shmask);
+  result = (__v2du)vec_sel ((__v2df) shmask, (__v2df)result, shmask);
 
   return (__m128i) result;
 }
@@ -1843,7 +1843,7 @@ _mm_srl_epi64 (__m128i __A, __m128i __B)
   rshift = vec_splat ((__v2du) __B, 0);
   shmask = vec_cmplt (rshift, shmax);
   result = vec_sr ((__v2du) __A, rshift);
-  result = vec_sel ((__v2du) shmask, result, shmask);
+  result = (__v2du)vec_sel ((__v2df) shmask, (__v2df)result, shmask);
 
   return (__m128i) result;
 }
diff --git a/clang/lib/Parse/ParseCXXInlineMethods.cpp b/clang/lib/Parse/ParseCXXInlineMethods.cpp
index f8b5fec43800..a75965784168 100644
--- a/clang/lib/Parse/ParseCXXInlineMethods.cpp
+++ b/clang/lib/Parse/ParseCXXInlineMethods.cpp
@@ -133,7 +133,9 @@ NamedDecl *Parser::ParseCXXInlineMethodDef(
 
   LexedMethod* LM = new LexedMethod(this, FnD);
   getCurrentClass().LateParsedDeclarations.push_back(LM);
-  LM->TemplateScope = getCurScope()->isTemplateParamScope();
+  LM->TemplateScope = getCurScope()->isTemplateParamScope() ||
+      (FnD && isa<FunctionTemplateDecl>(FnD) &&
+       cast<FunctionTemplateDecl>(FnD)->isAbbreviated());
   CachedTokens &Toks = LM->Toks;
 
   tok::TokenKind kind = Tok.getKind();
diff --git a/clang/lib/Parse/ParseDecl.cpp b/clang/lib/Parse/ParseDecl.cpp
index 69a3ed9cbad7..4af993c4527f 100644
--- a/clang/lib/Parse/ParseDecl.cpp
+++ b/clang/lib/Parse/ParseDecl.cpp
@@ -2962,6 +2962,7 @@ Parser::DiagnoseMissingSemiAfterTagDefinition(DeclSpec &DS, AccessSpecifier AS,
       case Sema::NC_ContextIndependentExpr:
       case Sema::NC_VarTemplate:
       case Sema::NC_FunctionTemplate:
+      case Sema::NC_Concept:
         // Might be a redeclaration of a prior entity.
         break;
       }
@@ -3177,7 +3178,7 @@ void Parser::ParseDeclarationSpecifiers(DeclSpec &DS,
              DSContext == DeclSpecContext::DSC_class) &&
             TemplateId->Name &&
             Actions.isCurrentClassName(*TemplateId->Name, getCurScope(), &SS) &&
-            isConstructorDeclarator(/*Unqualified*/ false)) {
+            isConstructorDeclarator(/*Unqualified=*/false)) {
           // The user meant this to be an out-of-line constructor
           // definition, but template arguments are not allowed
           // there.  Just allow this as a constructor; we'll
@@ -3189,7 +3190,19 @@ void Parser::ParseDeclarationSpecifiers(DeclSpec &DS,
         ConsumeAnnotationToken(); // The C++ scope.
         assert(Tok.is(tok::annot_template_id) &&
                "ParseOptionalCXXScopeSpecifier not working");
-        AnnotateTemplateIdTokenAsType();
+        AnnotateTemplateIdTokenAsType(SS);
+        continue;
+      }
+
+      if (Next.is(tok::annot_template_id) &&
+          static_cast<TemplateIdAnnotation *>(Next.getAnnotationValue())
+            ->Kind == TNK_Concept_template &&
+          GetLookAheadToken(2).isOneOf(tok::kw_auto, tok::kw_decltype)) {
+        DS.getTypeSpecScope() = SS;
+        // This is a qualified placeholder-specifier, e.g., ::C<int> auto ...
+        // Consume the scope annotation and continue to consume the template-id
+        // as a placeholder-specifier.
+        ConsumeAnnotationToken();
         continue;
       }
 
@@ -3235,6 +3248,10 @@ void Parser::ParseDeclarationSpecifiers(DeclSpec &DS,
       // C++ doesn't have implicit int.  Diagnose it as a typo w.r.t. to the
       // typename.
       if (!TypeRep) {
+        if (TryAnnotateTypeConstraint())
+          goto DoneWithDeclSpec;
+        if (isTypeConstraintAnnotation())
+          continue;
         // Eat the scope spec so the identifier is current.
         ConsumeAnnotationToken();
         ParsedAttributesWithRange Attrs(AttrFactory);
@@ -3384,6 +3401,10 @@ void Parser::ParseDeclarationSpecifiers(DeclSpec &DS,
       // If this is not a typedef name, don't parse it as part of the declspec,
       // it must be an implicit int or an error.
       if (!TypeRep) {
+        if (TryAnnotateTypeConstraint())
+          goto DoneWithDeclSpec;
+        if (isTypeConstraintAnnotation())
+          continue;
         ParsedAttributesWithRange Attrs(AttrFactory);
         if (ParseImplicitInt(DS, nullptr, TemplateInfo, AS, DSContext, Attrs)) {
           if (!Attrs.empty()) {
@@ -3433,9 +3454,51 @@ void Parser::ParseDeclarationSpecifiers(DeclSpec &DS,
       continue;
     }
 
-      // type-name
+      // type-name or placeholder-specifier
     case tok::annot_template_id: {
       TemplateIdAnnotation *TemplateId = takeTemplateIdAnnotation(Tok);
+      if (TemplateId->Kind == TNK_Concept_template) {
+        if (NextToken().is(tok::identifier)) {
+          Diag(Loc, diag::err_placeholder_expected_auto_or_decltype_auto)
+              << FixItHint::CreateInsertion(NextToken().getLocation(), "auto");
+          // Attempt to continue as if 'auto' was placed here.
+          isInvalid = DS.SetTypeSpecType(TST_auto, Loc, PrevSpec, DiagID,
+                                         TemplateId, Policy);
+          break;
+        }
+        if (!NextToken().isOneOf(tok::kw_auto, tok::kw_decltype))
+            goto DoneWithDeclSpec;
+        ConsumeAnnotationToken();
+        SourceLocation AutoLoc = Tok.getLocation();
+        if (TryConsumeToken(tok::kw_decltype)) {
+          BalancedDelimiterTracker Tracker(*this, tok::l_paren);
+          if (Tracker.consumeOpen()) {
+            // Something like `void foo(Iterator decltype i)`
+            Diag(Tok, diag::err_expected) << tok::l_paren;
+          } else {
+            if (!TryConsumeToken(tok::kw_auto)) {
+              // Something like `void foo(Iterator decltype(int) i)`
+              Tracker.skipToEnd();
+              Diag(Tok, diag::err_placeholder_expected_auto_or_decltype_auto)
+                << FixItHint::CreateReplacement(SourceRange(AutoLoc,
+                                                            Tok.getLocation()),
+                                                "auto");
+            } else {
+              Tracker.consumeClose();
+            }
+          }
+          ConsumedEnd = Tok.getLocation();
+          // Even if something went wrong above, continue as if we've seen
+          // `decltype(auto)`.
+          isInvalid = DS.SetTypeSpecType(TST_decltype_auto, Loc, PrevSpec,
+                                         DiagID, TemplateId, Policy);
+        } else {
+          isInvalid = DS.SetTypeSpecType(TST_auto, Loc, PrevSpec, DiagID,
+                                         TemplateId, Policy);
+        }
+        break;
+      }
+
       if (TemplateId->Kind != TNK_Type_template &&
           TemplateId->Kind != TNK_Undeclared_template) {
         // This template-id does not refer to a type name, so we're
@@ -3448,12 +3511,13 @@ void Parser::ParseDeclarationSpecifiers(DeclSpec &DS,
       // constructor declaration.
       if (getLangOpts().CPlusPlus && DSContext == DeclSpecContext::DSC_class &&
           Actions.isCurrentClassName(*TemplateId->Name, getCurScope()) &&
-          isConstructorDeclarator(TemplateId->SS.isEmpty()))
+          isConstructorDeclarator(/*Unqualified=*/true))
         goto DoneWithDeclSpec;
 
       // Turn the template-id annotation token into a type annotation
       // token, then try again to parse it as a type-specifier.
-      AnnotateTemplateIdTokenAsType();
+      CXXScopeSpec SS;
+      AnnotateTemplateIdTokenAsType(SS);
       continue;
     }
 
@@ -3617,7 +3681,11 @@ void Parser::ParseDeclarationSpecifiers(DeclSpec &DS,
       ConsumedEnd = ExplicitLoc;
       ConsumeToken(); // kw_explicit
       if (Tok.is(tok::l_paren)) {
-        if (getLangOpts().CPlusPlus2a) {
+        if (getLangOpts().CPlusPlus2a || isExplicitBool() == TPResult::True) {
+          Diag(Tok.getLocation(), getLangOpts().CPlusPlus2a
+                                      ? diag::warn_cxx17_compat_explicit_bool
+                                      : diag::ext_explicit_bool);
+
           ExprResult ExplicitExpr(static_cast<Expr *>(nullptr));
           BalancedDelimiterTracker Tracker(*this, tok::l_paren);
           Tracker.consumeOpen();
@@ -3630,8 +3698,9 @@ void Parser::ParseDeclarationSpecifiers(DeclSpec &DS,
                 Actions.ActOnExplicitBoolSpecifier(ExplicitExpr.get());
           } else
             Tracker.skipToEnd();
-        } else
+        } else {
           Diag(Tok.getLocation(), diag::warn_cxx2a_compat_explicit_bool);
+        }
       }
       isInvalid = DS.setFunctionSpecExplicit(ExplicitLoc, PrevSpec, DiagID,
                                              ExplicitSpec, CloseParenLoc);
@@ -6021,11 +6090,12 @@ void Parser::ParseDirectDeclarator(Declarator &D) {
 
   while (1) {
     if (Tok.is(tok::l_paren)) {
+      bool IsFunctionDeclaration = D.isFunctionDeclaratorAFunctionDeclaration();
       // Enter function-declaration scope, limiting any declarators to the
       // function prototype scope, including parameter declarators.
       ParseScope PrototypeScope(this,
                                 Scope::FunctionPrototypeScope|Scope::DeclScope|
-                                (D.isFunctionDeclaratorAFunctionDeclaration()
+                                (IsFunctionDeclaration
                                    ? Scope::FunctionDeclarationScope : 0));
 
       // The paren may be part of a C++ direct initializer, eg. "int x(1);".
@@ -6044,7 +6114,12 @@ void Parser::ParseDirectDeclarator(Declarator &D) {
       ParsedAttributes attrs(AttrFactory);
       BalancedDelimiterTracker T(*this, tok::l_paren);
       T.consumeOpen();
+      if (IsFunctionDeclaration)
+        Actions.ActOnStartFunctionDeclarationDeclarator(D,
+                                                        TemplateParameterDepth);
       ParseFunctionDeclarator(D, attrs, T, IsAmbiguous);
+      if (IsFunctionDeclaration)
+        Actions.ActOnFinishFunctionDeclarationDeclarator(D);
       PrototypeScope.Exit();
     } else if (Tok.is(tok::l_square)) {
       ParseBracketDeclarator(D);
@@ -6360,7 +6435,7 @@ void Parser::ParseFunctionDeclarator(Declarator &D,
     ProhibitAttributes(FnAttrs);
   } else {
     if (Tok.isNot(tok::r_paren))
-      ParseParameterDeclarationClause(D, FirstArgAttrs, ParamInfo,
+      ParseParameterDeclarationClause(D.getContext(), FirstArgAttrs, ParamInfo,
                                       EllipsisLoc);
     else if (RequiresArg)
       Diag(Tok, diag::err_argument_required_after_attribute);
@@ -6578,9 +6653,9 @@ void Parser::ParseFunctionDeclaratorIdentifierList(
 /// after the opening parenthesis. This function will not parse a K&R-style
 /// identifier list.
 ///
-/// D is the declarator being parsed.  If FirstArgAttrs is non-null, then the
-/// caller parsed those arguments immediately after the open paren - they should
-/// be considered to be part of the first parameter.
+/// DeclContext is the context of the declarator being parsed.  If FirstArgAttrs
+/// is non-null, then the caller parsed those attributes immediately after the
+/// open paren - they should be considered to be part of the first parameter.
 ///
 /// After returning, ParamInfo will hold the parsed parameters. EllipsisLoc will
 /// be the location of the ellipsis, if any was parsed.
@@ -6606,7 +6681,7 @@ void Parser::ParseFunctionDeclaratorIdentifierList(
 /// [C++11] attribute-specifier-seq parameter-declaration
 ///
 void Parser::ParseParameterDeclarationClause(
-       Declarator &D,
+       DeclaratorContext DeclaratorCtx,
        ParsedAttributes &FirstArgAttrs,
        SmallVectorImpl<DeclaratorChunk::ParamInfo> &ParamInfo,
        SourceLocation &EllipsisLoc) {
@@ -6655,9 +6730,11 @@ void Parser::ParseParameterDeclarationClause(
     // "LambdaExprParameterContext", because we must accept either
     // 'declarator' or 'abstract-declarator' here.
     Declarator ParmDeclarator(
-        DS, D.getContext() == DeclaratorContext::LambdaExprContext
-                ? DeclaratorContext::LambdaExprParameterContext
-                : DeclaratorContext::PrototypeContext);
+        DS, DeclaratorCtx == DeclaratorContext::RequiresExprContext
+                ? DeclaratorContext::RequiresExprContext
+                : DeclaratorCtx == DeclaratorContext::LambdaExprContext
+                      ? DeclaratorContext::LambdaExprParameterContext
+                      : DeclaratorContext::PrototypeContext);
     ParseDeclarator(ParmDeclarator);
 
     // Parse GNU attributes, if present.
@@ -6711,7 +6788,7 @@ void Parser::ParseParameterDeclarationClause(
         SourceLocation EqualLoc = Tok.getLocation();
 
         // Parse the default argument
-        if (D.getContext() == DeclaratorContext::MemberContext) {
+        if (DeclaratorCtx == DeclaratorContext::MemberContext) {
           // If we're inside a class definition, cache the tokens
           // corresponding to the default argument. We'll actually parse
           // them when we see the end of the class definition.
diff --git a/clang/lib/Parse/ParseDeclCXX.cpp b/clang/lib/Parse/ParseDeclCXX.cpp
index 081d4d8b1209..f872aa3a950c 100644
--- a/clang/lib/Parse/ParseDeclCXX.cpp
+++ b/clang/lib/Parse/ParseDeclCXX.cpp
@@ -1142,7 +1142,7 @@ TypeResult Parser::ParseBaseTypeSpecifier(SourceLocation &BaseLoc,
     if (TemplateId->Kind == TNK_Type_template ||
         TemplateId->Kind == TNK_Dependent_template_name ||
         TemplateId->Kind == TNK_Undeclared_template) {
-      AnnotateTemplateIdTokenAsType(/*IsClassName*/true);
+      AnnotateTemplateIdTokenAsType(SS, /*IsClassName*/true);
 
       assert(Tok.is(tok::annot_typename) && "template-id -> type failed");
       ParsedType Type = getTypeAnnotation(Tok);
@@ -1193,7 +1193,7 @@ TypeResult Parser::ParseBaseTypeSpecifier(SourceLocation &BaseLoc,
                                 TemplateName))
       return true;
     if (TNK == TNK_Type_template || TNK == TNK_Dependent_template_name)
-      AnnotateTemplateIdTokenAsType(/*IsClassName*/true);
+      AnnotateTemplateIdTokenAsType(SS, /*IsClassName*/true);
 
     // If we didn't end up with a typename token, there's nothing more we
     // can do.
@@ -1826,7 +1826,7 @@ void Parser::ParseClassSpecifier(tok::TokenKind TagTokKind,
                 TemplateInfo.Kind == ParsedTemplateInfo::NonTemplate)) {
       ProhibitAttributes(attrs);
       TypeResult = Actions.ActOnTagTemplateIdType(TUK, TagType, StartLoc,
-                                                  TemplateId->SS,
+                                                  SS,
                                                   TemplateId->TemplateKWLoc,
                                                   TemplateId->Template,
                                                   TemplateId->TemplateNameLoc,
@@ -1876,7 +1876,7 @@ void Parser::ParseClassSpecifier(tok::TokenKind TagTokKind,
       // Build the class template specialization.
       TagOrTempResult = Actions.ActOnClassTemplateSpecialization(
           getCurScope(), TagType, TUK, StartLoc, DS.getModulePrivateSpecLoc(),
-          *TemplateId, attrs,
+          SS, *TemplateId, attrs,
           MultiTemplateParamsArg(TemplateParams ? &(*TemplateParams)[0]
                                                 : nullptr,
                                  TemplateParams ? TemplateParams->size() : 0),
@@ -2642,6 +2642,8 @@ Parser::ParseCXXClassMemberDeclaration(AccessSpecifier AS,
   }
 
   ParsingDeclarator DeclaratorInfo(*this, DS, DeclaratorContext::MemberContext);
+  if (TemplateInfo.TemplateParams)
+    DeclaratorInfo.setTemplateParameterLists(TemplateParams);
   VirtSpecifiers VS;
 
   // Hold late-parsed attributes so we can attach a Decl to them later.
@@ -3520,7 +3522,7 @@ MemInitResult Parser::ParseMemInitializer(Decl *ConstructorDecl) {
     if (TemplateId && (TemplateId->Kind == TNK_Type_template ||
                        TemplateId->Kind == TNK_Dependent_template_name ||
                        TemplateId->Kind == TNK_Undeclared_template)) {
-      AnnotateTemplateIdTokenAsType(/*IsClassName*/true);
+      AnnotateTemplateIdTokenAsType(SS, /*IsClassName*/true);
       assert(Tok.is(tok::annot_typename) && "template-id -> type failed");
       TemplateTypeTy = getTypeAnnotation(Tok);
       ConsumeAnnotationToken();
diff --git a/clang/lib/Parse/ParseExpr.cpp b/clang/lib/Parse/ParseExpr.cpp
index 1442df046bb9..1eb3ad6afd1c 100644
--- a/clang/lib/Parse/ParseExpr.cpp
+++ b/clang/lib/Parse/ParseExpr.cpp
@@ -234,7 +234,7 @@ ExprResult Parser::ParseCaseExpression(SourceLocation CaseLoc) {
 /// \endverbatim
 ExprResult Parser::ParseConstraintExpression() {
   EnterExpressionEvaluationContext ConstantEvaluated(
-      Actions, Sema::ExpressionEvaluationContext::ConstantEvaluated);
+      Actions, Sema::ExpressionEvaluationContext::Unevaluated);
   ExprResult LHS(ParseCastExpression(AnyCastExpr));
   ExprResult Res(ParseRHSOfBinaryExpression(LHS, prec::LogicalOr));
   if (Res.isUsable() && !Actions.CheckConstraintExpression(Res.get())) {
@@ -256,7 +256,7 @@ ExprResult Parser::ParseConstraintExpression() {
 ExprResult
 Parser::ParseConstraintLogicalAndExpression(bool IsTrailingRequiresClause) {
   EnterExpressionEvaluationContext ConstantEvaluated(
-      Actions, Sema::ExpressionEvaluationContext::ConstantEvaluated);
+      Actions, Sema::ExpressionEvaluationContext::Unevaluated);
   bool NotPrimaryExpression = false;
   auto ParsePrimary = [&] () {
     ExprResult E = ParseCastExpression(PrimaryExprOnly,
@@ -756,6 +756,7 @@ class CastExpressionIdValidator final : public CorrectionCandidateCallback {
 /// [C++11] user-defined-literal
 ///         '(' expression ')'
 /// [C11]   generic-selection
+/// [C++2a] requires-expression
 ///         '__func__'        [C99 6.4.2.2]
 /// [GNU]   '__FUNCTION__'
 /// [MS]    '__FUNCDNAME__'
@@ -1530,7 +1531,7 @@ ExprResult Parser::ParseCastExpression(CastParseKind ParseKind,
         CXXScopeSpec SS;
         ParseOptionalCXXScopeSpecifier(SS, nullptr,
                                        /*EnteringContext=*/false);
-        AnnotateTemplateIdTokenAsType();
+        AnnotateTemplateIdTokenAsType(SS);
         return ParseCastExpression(ParseKind, isAddressOfOperand, NotCastExpr,
                                    isTypeCast, isVectorLiteral,
                                    NotPrimaryExpression);
@@ -1548,7 +1549,8 @@ ExprResult Parser::ParseCastExpression(CastParseKind ParseKind,
       // We have a template-id that we know refers to a type,
       // translate it into a type and continue parsing as a cast
       // expression.
-      AnnotateTemplateIdTokenAsType();
+      CXXScopeSpec SS;
+      AnnotateTemplateIdTokenAsType(SS);
       return ParseCastExpression(ParseKind, isAddressOfOperand,
                                  NotCastExpr, isTypeCast, isVectorLiteral,
                                  NotPrimaryExpression);
@@ -1600,6 +1602,9 @@ ExprResult Parser::ParseCastExpression(CastParseKind ParseKind,
       *NotPrimaryExpression = true;
     return ParseCXXDeleteExpression(false, Tok.getLocation());
 
+  case tok::kw_requires: // [C++2a] requires-expression
+    return ParseRequiresExpression();
+
   case tok::kw_noexcept: { // [C++0x] 'noexcept' '(' expression ')'
     if (NotPrimaryExpression)
       *NotPrimaryExpression = true;
diff --git a/clang/lib/Parse/ParseExprCXX.cpp b/clang/lib/Parse/ParseExprCXX.cpp
index e685d5ea8a9c..036eabb94dd7 100644
--- a/clang/lib/Parse/ParseExprCXX.cpp
+++ b/clang/lib/Parse/ParseExprCXX.cpp
@@ -11,7 +11,9 @@
 //===----------------------------------------------------------------------===//
 #include "clang/Parse/Parser.h"
 #include "clang/AST/ASTContext.h"
+#include "clang/AST/Decl.h"
 #include "clang/AST/DeclTemplate.h"
+#include "clang/AST/ExprCXX.h"
 #include "clang/Basic/PrettyStackTrace.h"
 #include "clang/Lex/LiteralSupport.h"
 #include "clang/Parse/ParseDiagnostic.h"
@@ -165,13 +167,6 @@ bool Parser::ParseOptionalCXXScopeSpecifier(CXXScopeSpec &SS,
     return false;
   }
 
-  if (Tok.is(tok::annot_template_id)) {
-    // If the current token is an annotated template id, it may already have
-    // a scope specifier. Restore it.
-    TemplateIdAnnotation *TemplateId = takeTemplateIdAnnotation(Tok);
-    SS = TemplateId->SS;
-  }
-
   // Has to happen before any "return false"s in this function.
   bool CheckForDestructor = false;
   if (MayBePseudoDestructor && *MayBePseudoDestructor) {
@@ -1306,9 +1301,9 @@ ExprResult Parser::ParseLambdaExpressionAfterIntroducer(
       Actions.RecordParsingTemplateParameterDepth(
           CurTemplateDepthTracker.getOriginalDepth());
 
-      ParseParameterDeclarationClause(D, Attr, ParamInfo, EllipsisLoc);
-
-      // For a generic lambda, each 'auto' within the parameter declaration
+      ParseParameterDeclarationClause(D.getContext(), Attr, ParamInfo,
+                                      EllipsisLoc);
+      // For a generic lambda, each 'auto' within the parameter declaration 
       // clause creates a template type parameter, so increment the depth.
       // If we've parsed any explicit template parameters, then the depth will
       // have already been incremented. So we make sure that at most a single
@@ -2405,7 +2400,7 @@ bool Parser::ParseUnqualifiedIdTemplateId(CXXScopeSpec &SS,
             : Id.OperatorFunctionId.Operator;
 
     TemplateIdAnnotation *TemplateId = TemplateIdAnnotation::Create(
-        SS, TemplateKWLoc, Id.StartLocation, TemplateII, OpKind, Template, TNK,
+        TemplateKWLoc, Id.StartLocation, TemplateII, OpKind, Template, TNK,
         LAngleLoc, RAngleLoc, TemplateArgs, TemplateIds);
 
     Id.setTemplateId(TemplateId);
@@ -3262,6 +3257,324 @@ Parser::ParseCXXDeleteExpression(bool UseGlobal, SourceLocation Start) {
   return Actions.ActOnCXXDelete(Start, UseGlobal, ArrayDelete, Operand.get());
 }
 
+/// ParseRequiresExpression - Parse a C++2a requires-expression.
+/// C++2a [expr.prim.req]p1
+///     A requires-expression provides a concise way to express requirements on
+///     template arguments. A requirement is one that can be checked by name
+///     lookup (6.4) or by checking properties of types and expressions.
+///
+///     requires-expression:
+///         'requires' requirement-parameter-list[opt] requirement-body
+///
+///     requirement-parameter-list:
+///         '(' parameter-declaration-clause[opt] ')'
+///
+///     requirement-body:
+///         '{' requirement-seq '}'
+///
+///     requirement-seq:
+///         requirement
+///         requirement-seq requirement
+///
+///     requirement:
+///         simple-requirement
+///         type-requirement
+///         compound-requirement
+///         nested-requirement
+ExprResult Parser::ParseRequiresExpression() {
+  assert(Tok.is(tok::kw_requires) && "Expected 'requires' keyword");
+  SourceLocation RequiresKWLoc = ConsumeToken(); // Consume 'requires'
+
+  llvm::SmallVector<ParmVarDecl *, 2> LocalParameterDecls;
+  if (Tok.is(tok::l_paren)) {
+    // requirement parameter list is present.
+    ParseScope LocalParametersScope(this, Scope::FunctionPrototypeScope |
+                                    Scope::DeclScope);
+    BalancedDelimiterTracker Parens(*this, tok::l_paren);
+    Parens.consumeOpen();
+    if (!Tok.is(tok::r_paren)) {
+      ParsedAttributes FirstArgAttrs(getAttrFactory());
+      SourceLocation EllipsisLoc;
+      llvm::SmallVector<DeclaratorChunk::ParamInfo, 2> LocalParameters;
+      DiagnosticErrorTrap Trap(Diags);
+      ParseParameterDeclarationClause(DeclaratorContext::RequiresExprContext,
+                                      FirstArgAttrs, LocalParameters,
+                                      EllipsisLoc);
+      if (EllipsisLoc.isValid())
+        Diag(EllipsisLoc, diag::err_requires_expr_parameter_list_ellipsis);
+      for (auto &ParamInfo : LocalParameters)
+        LocalParameterDecls.push_back(cast<ParmVarDecl>(ParamInfo.Param));
+      if (Trap.hasErrorOccurred())
+        SkipUntil(tok::r_paren, StopBeforeMatch);
+    }
+    Parens.consumeClose();
+  }
+
+  BalancedDelimiterTracker Braces(*this, tok::l_brace);
+  if (Braces.expectAndConsume())
+    return ExprError();
+
+  // Start of requirement list
+  llvm::SmallVector<concepts::Requirement *, 2> Requirements;
+
+  // C++2a [expr.prim.req]p2
+  //   Expressions appearing within a requirement-body are unevaluated operands.
+  EnterExpressionEvaluationContext Ctx(
+      Actions, Sema::ExpressionEvaluationContext::Unevaluated);
+
+  ParseScope BodyScope(this, Scope::DeclScope);
+  RequiresExprBodyDecl *Body = Actions.ActOnStartRequiresExpr(
+      RequiresKWLoc, LocalParameterDecls, getCurScope());
+
+  if (Tok.is(tok::r_brace)) {
+    // Grammar does not allow an empty body.
+    // requirement-body:
+    //   { requirement-seq }
+    // requirement-seq:
+    //   requirement
+    //   requirement-seq requirement
+    Diag(Tok, diag::err_empty_requires_expr);
+    // Continue anyway and produce a requires expr with no requirements.
+  } else {
+    while (!Tok.is(tok::r_brace)) {
+      switch (Tok.getKind()) {
+      case tok::l_brace: {
+        // Compound requirement
+        // C++ [expr.prim.req.compound]
+        //     compound-requirement:
+        //         '{' expression '}' 'noexcept'[opt]
+        //             return-type-requirement[opt] ';'
+        //     return-type-requirement:
+        //         trailing-return-type
+        //         '->' cv-qualifier-seq[opt] constrained-parameter
+        //             cv-qualifier-seq[opt] abstract-declarator[opt]
+        BalancedDelimiterTracker ExprBraces(*this, tok::l_brace);
+        ExprBraces.consumeOpen();
+        ExprResult Expression =
+            Actions.CorrectDelayedTyposInExpr(ParseExpression());
+        if (!Expression.isUsable()) {
+          ExprBraces.skipToEnd();
+          SkipUntil(tok::semi, tok::r_brace, SkipUntilFlags::StopBeforeMatch);
+          break;
+        }
+        if (ExprBraces.consumeClose())
+          ExprBraces.skipToEnd();
+
+        concepts::Requirement *Req = nullptr;
+        SourceLocation NoexceptLoc;
+        TryConsumeToken(tok::kw_noexcept, NoexceptLoc);
+        if (Tok.is(tok::semi)) {
+          Req = Actions.ActOnCompoundRequirement(Expression.get(), NoexceptLoc);
+          if (Req)
+            Requirements.push_back(Req);
+          break;
+        }
+        if (!TryConsumeToken(tok::arrow))
+          // User probably forgot the arrow, remind them and try to continue.
+          Diag(Tok, diag::err_requires_expr_missing_arrow)
+              << FixItHint::CreateInsertion(Tok.getLocation(), "->");
+        // Try to parse a 'type-constraint'
+        CXXScopeSpec SS;
+        if (ParseOptionalCXXScopeSpecifier(SS, ParsedType(),
+                                           /*EnteringContext=*/false,
+                                           /*MayBePseudoDestructor=*/nullptr,
+                                           // If this is not a type-constraint,
+                                           // then this scope-spec is part of
+                                           // the typename of a non-type
+                                           // template parameter
+                                           /*IsTypename=*/true,
+                                           /*LastII=*/nullptr,
+                                           // We won't find concepts in
+                                           // non-namespaces anyway, so might as
+                                           // well parse this correctly for
+                                           // possible type names.
+                                           /*OnlyNamespace=*/false,
+                                           /*SuppressDiagnostic=*/true)) {
+          SkipUntil(tok::semi, tok::r_brace, SkipUntilFlags::StopBeforeMatch);
+          break;
+        }
+        if (TryAnnotateTypeConstraint()) {
+          SkipUntil(tok::semi, tok::r_brace, SkipUntilFlags::StopBeforeMatch);
+          break;
+        }
+        if (!isTypeConstraintAnnotation()) {
+          Diag(Tok, diag::err_requires_expr_expected_type_constraint);
+          SkipUntil(tok::semi, tok::r_brace, SkipUntilFlags::StopBeforeMatch);
+          break;
+        }
+        if (Tok.is(tok::annot_cxxscope))
+          ConsumeAnnotationToken();
+
+        Req = Actions.ActOnCompoundRequirement(
+            Expression.get(), NoexceptLoc, SS, takeTemplateIdAnnotation(Tok),
+            TemplateParameterDepth);
+        ConsumeAnnotationToken();
+        if (Req)
+          Requirements.push_back(Req);
+        break;
+      }
+      default: {
+        bool PossibleRequiresExprInSimpleRequirement = false;
+        if (Tok.is(tok::kw_requires)) {
+          auto IsNestedRequirement = [&] {
+            RevertingTentativeParsingAction TPA(*this);
+            ConsumeToken(); // 'requires'
+            if (Tok.is(tok::l_brace))
+              // This is a requires expression
+              // requires (T t) {
+              //   requires { t++; };
+              //   ...      ^
+              // }
+              return false;
+            if (Tok.is(tok::l_paren)) {
+              // This might be the parameter list of a requires expression
+              ConsumeParen();
+              auto Res = TryParseParameterDeclarationClause();
+              if (Res != TPResult::False) {
+                // Skip to the closing parenthesis
+                // FIXME: Don't traverse these tokens twice (here and in
+                //  TryParseParameterDeclarationClause).
+                unsigned Depth = 1;
+                while (Depth != 0) {
+                  if (Tok.is(tok::l_paren))
+                    Depth++;
+                  else if (Tok.is(tok::r_paren))
+                    Depth--;
+                  ConsumeAnyToken();
+                }
+                // requires (T t) {
+                //   requires () ?
+                //   ...         ^
+                //   - OR -
+                //   requires (int x) ?
+                //   ...              ^
+                // }
+                if (Tok.is(tok::l_brace))
+                  // requires (...) {
+                  //                ^ - a requires expression as a
+                  //                    simple-requirement.
+                  return false;
+              }
+            }
+            return true;
+          };
+          if (IsNestedRequirement()) {
+            ConsumeToken();
+            // Nested requirement
+            // C++ [expr.prim.req.nested]
+            //     nested-requirement:
+            //         'requires' constraint-expression ';'
+            ExprResult ConstraintExpr =
+                Actions.CorrectDelayedTyposInExpr(ParseConstraintExpression());
+            if (ConstraintExpr.isInvalid() || !ConstraintExpr.isUsable()) {
+              SkipUntil(tok::semi, tok::r_brace,
+                        SkipUntilFlags::StopBeforeMatch);
+              break;
+            }
+            if (auto *Req =
+                    Actions.ActOnNestedRequirement(ConstraintExpr.get()))
+              Requirements.push_back(Req);
+            else {
+              SkipUntil(tok::semi, tok::r_brace,
+                        SkipUntilFlags::StopBeforeMatch);
+              break;
+            }
+            break;
+          } else
+            PossibleRequiresExprInSimpleRequirement = true;
+        } else if (Tok.is(tok::kw_typename)) {
+          // This might be 'typename T::value_type;' (a type requirement) or
+          // 'typename T::value_type{};' (a simple requirement).
+          TentativeParsingAction TPA(*this);
+
+          // We need to consume the typename to allow 'requires { typename a; }'
+          SourceLocation TypenameKWLoc = ConsumeToken();
+          if (TryAnnotateCXXScopeToken()) {
+            SkipUntil(tok::semi, tok::r_brace, SkipUntilFlags::StopBeforeMatch);
+            break;
+          }
+          CXXScopeSpec SS;
+          if (Tok.is(tok::annot_cxxscope)) {
+            Actions.RestoreNestedNameSpecifierAnnotation(
+                Tok.getAnnotationValue(), Tok.getAnnotationRange(), SS);
+            ConsumeAnnotationToken();
+          }
+
+          if (Tok.isOneOf(tok::identifier, tok::annot_template_id) &&
+              !NextToken().isOneOf(tok::l_brace, tok::l_paren)) {
+            TPA.Commit();
+            SourceLocation NameLoc = Tok.getLocation();
+            IdentifierInfo *II = nullptr;
+            TemplateIdAnnotation *TemplateId = nullptr;
+            if (Tok.is(tok::identifier)) {
+              II = Tok.getIdentifierInfo();
+              ConsumeToken();
+            } else {
+              TemplateId = takeTemplateIdAnnotation(Tok);
+              ConsumeAnnotationToken();
+            }
+
+            if (auto *Req = Actions.ActOnTypeRequirement(TypenameKWLoc, SS,
+                                                         NameLoc, II,
+                                                         TemplateId)) {
+              Requirements.push_back(Req);
+            }
+            break;
+          }
+          TPA.Revert();
+        }
+        // Simple requirement
+        // C++ [expr.prim.req.simple]
+        //     simple-requirement:
+        //         expression ';'
+        SourceLocation StartLoc = Tok.getLocation();
+        ExprResult Expression =
+            Actions.CorrectDelayedTyposInExpr(ParseExpression());
+        if (!Expression.isUsable()) {
+          SkipUntil(tok::semi, tok::r_brace, SkipUntilFlags::StopBeforeMatch);
+          break;
+        }
+        if (!Expression.isInvalid() && PossibleRequiresExprInSimpleRequirement)
+          Diag(StartLoc, diag::warn_requires_expr_in_simple_requirement)
+              << FixItHint::CreateInsertion(StartLoc, "requires");
+        if (auto *Req = Actions.ActOnSimpleRequirement(Expression.get()))
+          Requirements.push_back(Req);
+        else {
+          SkipUntil(tok::semi, tok::r_brace, SkipUntilFlags::StopBeforeMatch);
+          break;
+        }
+        // User may have tried to put some compound requirement stuff here
+        if (Tok.is(tok::kw_noexcept)) {
+          Diag(Tok, diag::err_requires_expr_simple_requirement_noexcept)
+              << FixItHint::CreateInsertion(StartLoc, "{")
+              << FixItHint::CreateInsertion(Tok.getLocation(), "}");
+          SkipUntil(tok::semi, tok::r_brace, SkipUntilFlags::StopBeforeMatch);
+          break;
+        }
+        break;
+      }
+      }
+      if (ExpectAndConsumeSemi(diag::err_expected_semi_requirement)) {
+        SkipUntil(tok::semi, tok::r_brace, SkipUntilFlags::StopBeforeMatch);
+        TryConsumeToken(tok::semi);
+        break;
+      }
+    }
+    if (Requirements.empty()) {
+      // Don't emit an empty requires expr here to avoid confusing the user with
+      // other diagnostics quoting an empty requires expression they never
+      // wrote.
+      Braces.consumeClose();
+      Actions.ActOnFinishRequiresExpr();
+      return ExprError();
+    }
+  }
+  Braces.consumeClose();
+  Actions.ActOnFinishRequiresExpr();
+  return Actions.ActOnRequiresExpr(RequiresKWLoc, Body, LocalParameterDecls,
+                                   Requirements, Braces.getCloseLocation());
+}
+
 static TypeTrait TypeTraitFromTokKind(tok::TokenKind kind) {
   switch (kind) {
   default: llvm_unreachable("Not a known type trait");
diff --git a/clang/lib/Parse/ParseTemplate.cpp b/clang/lib/Parse/ParseTemplate.cpp
index 1b9301b6591d..3bc4e3596f12 100644
--- a/clang/lib/Parse/ParseTemplate.cpp
+++ b/clang/lib/Parse/ParseTemplate.cpp
@@ -240,6 +240,8 @@ Decl *Parser::ParseSingleDeclarationAfterTemplate(
 
   // Parse the declarator.
   ParsingDeclarator DeclaratorInfo(*this, DS, (DeclaratorContext)Context);
+  if (TemplateInfo.TemplateParams)
+    DeclaratorInfo.setTemplateParameterLists(*TemplateInfo.TemplateParams);
   ParseDeclarator(DeclaratorInfo);
   // Error parsing the declarator?
   if (!DeclaratorInfo.hasName()) {
@@ -499,10 +501,7 @@ Parser::ParseTemplateParameterList(const unsigned Depth,
 
 /// Determine whether the parser is at the start of a template
 /// type parameter.
-/// \param ScopeError will receive true if there was an error parsing a
-/// scope specifier at the current location.
-bool Parser::isStartOfTemplateTypeParameter(bool &ScopeError) {
-  ScopeError = false;
+Parser::TPResult Parser::isStartOfTemplateTypeParameter() {
   if (Tok.is(tok::kw_class)) {
     // "class" may be the start of an elaborated-type-specifier or a
     // type-parameter. Per C++ [temp.param]p3, we prefer the type-parameter.
@@ -512,7 +511,7 @@ bool Parser::isStartOfTemplateTypeParameter(bool &ScopeError) {
     case tok::greater:
     case tok::greatergreater:
     case tok::ellipsis:
-      return true;
+      return TPResult::True;
 
     case tok::identifier:
       // This may be either a type-parameter or an elaborated-type-specifier.
@@ -520,7 +519,7 @@ bool Parser::isStartOfTemplateTypeParameter(bool &ScopeError) {
       break;
 
     default:
-      return false;
+      return TPResult::False;
     }
 
     switch (GetLookAheadToken(2).getKind()) {
@@ -528,51 +527,28 @@ bool Parser::isStartOfTemplateTypeParameter(bool &ScopeError) {
     case tok::comma:
     case tok::greater:
     case tok::greatergreater:
-      return true;
+      return TPResult::True;
 
     default:
-      return false;
+      return TPResult::False;
     }
   }
 
-  bool WasScopeAnnotation = Tok.is(tok::annot_cxxscope);
-  CXXScopeSpec SS;
-  ScopeError =
-      ParseOptionalCXXScopeSpecifier(SS, ParsedType(),
-                                     /*EnteringContext=*/false,
-                                     /*MayBePseudoDestructor=*/nullptr,
-                                     // If this is not a type-constraint, then
-                                     // this scope-spec is part of the typename
-                                     // of a non-type template parameter
-                                     /*IsTypename=*/true, /*LastII=*/nullptr,
-                                     // We won't find concepts in
-                                     // non-namespaces anyway, so might as well
-                                     // parse this correctly for possible type
-                                     // names.
-                                     /*OnlyNamespace=*/false);
-  if (ScopeError)
-    return false;
-  if (TryAnnotateTypeConstraint(SS))
-    return false;
-  bool IsTypeConstraint = isTypeConstraintAnnotation();
-  if (!IsTypeConstraint && SS.isNotEmpty()) {
-    // This isn't a type-constraint but we've already parsed this scope
-    // specifier - annotate it.
-    AnnotateScopeToken(SS, /*isNewAnnotation=*/!WasScopeAnnotation);
-    return false;
-  }
+  if (TryAnnotateTypeConstraint())
+    return TPResult::Error;
 
-  if (IsTypeConstraint &&
+  if (isTypeConstraintAnnotation() &&
       // Next token might be 'auto' or 'decltype', indicating that this
       // type-constraint is in fact part of a placeholder-type-specifier of a
       // non-type template parameter.
-      !NextToken().isOneOf(tok::kw_auto, tok::kw_decltype))
-    return true;
+      !GetLookAheadToken(Tok.is(tok::annot_cxxscope) ? 2 : 1)
+           .isOneOf(tok::kw_auto, tok::kw_decltype))
+    return TPResult::True;
 
   // 'typedef' is a reasonably-common typo/thinko for 'typename', and is
   // ill-formed otherwise.
   if (Tok.isNot(tok::kw_typename) && Tok.isNot(tok::kw_typedef))
-    return false;
+    return TPResult::False;
 
   // C++ [temp.param]p2:
   //   There is no semantic difference between class and typename in a
@@ -592,17 +568,17 @@ bool Parser::isStartOfTemplateTypeParameter(bool &ScopeError) {
   case tok::greater:
   case tok::greatergreater:
   case tok::ellipsis:
-    return true;
+    return TPResult::True;
 
   case tok::kw_typename:
   case tok::kw_typedef:
   case tok::kw_class:
     // These indicate that a comma was missed after a type parameter, not that
     // we have found a non-type parameter.
-    return true;
+    return TPResult::True;
 
   default:
-    return false;
+    return TPResult::False;
   }
 }
 
@@ -627,13 +603,9 @@ bool Parser::isStartOfTemplateTypeParameter(bool &ScopeError) {
 ///         typename
 ///
 NamedDecl *Parser::ParseTemplateParameter(unsigned Depth, unsigned Position) {
-  // We could be facing a type-constraint, which (could) start a type parameter.
-  // Annotate it now (we might end up not using it if we determine this
-  // type-constraint is in fact part of a placeholder-type-specifier of a
-  // non-type template parameter.
 
-  bool ScopeError;
-  if (isStartOfTemplateTypeParameter(ScopeError)) {
+  switch (isStartOfTemplateTypeParameter()) {
+  case TPResult::True:
     // Is there just a typo in the input code? ('typedef' instead of
     // 'typename')
     if (Tok.is(tok::kw_typedef)) {
@@ -649,8 +621,10 @@ NamedDecl *Parser::ParseTemplateParameter(unsigned Depth, unsigned Position) {
     }
 
     return ParseTypeParameter(Depth, Position);
-  }
-  if (ScopeError) {
+  case TPResult::False:
+    break;
+
+  case TPResult::Error: {
     // We return an invalid parameter as opposed to null to avoid having bogus
     // diagnostics about an empty template parameter list.
     // FIXME: Fix ParseTemplateParameterList to better handle nullptr results
@@ -670,6 +644,11 @@ NamedDecl *Parser::ParseTemplateParameter(unsigned Depth, unsigned Position) {
               StopAtSemi | StopBeforeMatch);
     return ErrorParam;
   }
+
+  case TPResult::Ambiguous:
+    llvm_unreachable("template param classification can't be ambiguous");
+  }
+
   if (Tok.is(tok::kw_template))
     return ParseTemplateTemplateParameter(Depth, Position);
 
@@ -682,15 +661,15 @@ NamedDecl *Parser::ParseTemplateParameter(unsigned Depth, unsigned Position) {
 /// Check whether the current token is a template-id annotation denoting a
 /// type-constraint.
 bool Parser::isTypeConstraintAnnotation() {
-  if (Tok.isNot(tok::annot_template_id))
+  const Token &T = Tok.is(tok::annot_cxxscope) ? NextToken() : Tok;
+  if (T.isNot(tok::annot_template_id))
     return false;
   const auto *ExistingAnnot =
-      static_cast<TemplateIdAnnotation *>(Tok.getAnnotationValue());
+      static_cast<TemplateIdAnnotation *>(T.getAnnotationValue());
   return ExistingAnnot->Kind == TNK_Concept_template;
 }
 
-/// Try parsing a type-constraint construct at the current location, after the
-/// optional scope specifier.
+/// Try parsing a type-constraint at the current location.
 ///
 ///     type-constraint:
 ///       nested-name-specifier[opt] concept-name
@@ -698,35 +677,61 @@ bool Parser::isTypeConstraintAnnotation() {
 ///           '<' template-argument-list[opt] '>'[opt]
 ///
 /// \returns true if an error occurred, and false otherwise.
-bool Parser::TryAnnotateTypeConstraint(CXXScopeSpec &SS) {
-  if (!getLangOpts().ConceptsTS || Tok.isNot(tok::identifier))
+bool Parser::TryAnnotateTypeConstraint() {
+  if (!getLangOpts().CPlusPlus2a)
     return false;
+  CXXScopeSpec SS;
+  bool WasScopeAnnotation = Tok.is(tok::annot_cxxscope);
+  if (ParseOptionalCXXScopeSpecifier(
+          SS, ParsedType(),
+          /*EnteringContext=*/false,
+          /*MayBePseudoDestructor=*/nullptr,
+          // If this is not a type-constraint, then
+          // this scope-spec is part of the typename
+          // of a non-type template parameter
+          /*IsTypename=*/true, /*LastII=*/nullptr,
+          // We won't find concepts in
+          // non-namespaces anyway, so might as well
+          // parse this correctly for possible type
+          // names.
+          /*OnlyNamespace=*/false))
+    return true;
 
-  UnqualifiedId PossibleConceptName;
-  PossibleConceptName.setIdentifier(Tok.getIdentifierInfo(),
-                                    Tok.getLocation());
-
-  TemplateTy PossibleConcept;
-  bool MemberOfUnknownSpecialization = false;
-  auto TNK = Actions.isTemplateName(getCurScope(), SS,
-                                    /*hasTemplateKeyword=*/false,
-                                    PossibleConceptName,
-                                    /*ObjectType=*/ParsedType(),
-                                    /*EnteringContext=*/false,
-                                    PossibleConcept,
-                                    MemberOfUnknownSpecialization);
-  assert(!MemberOfUnknownSpecialization
-         && "Member when we only allowed namespace scope qualifiers??");
-  if (!PossibleConcept || TNK != TNK_Concept_template)
-    return false;
+  if (Tok.is(tok::identifier)) {
+    UnqualifiedId PossibleConceptName;
+    PossibleConceptName.setIdentifier(Tok.getIdentifierInfo(),
+                                      Tok.getLocation());
+
+    TemplateTy PossibleConcept;
+    bool MemberOfUnknownSpecialization = false;
+    auto TNK = Actions.isTemplateName(getCurScope(), SS,
+                                      /*hasTemplateKeyword=*/false,
+                                      PossibleConceptName,
+                                      /*ObjectType=*/ParsedType(),
+                                      /*EnteringContext=*/false,
+                                      PossibleConcept,
+                                      MemberOfUnknownSpecialization);
+    if (MemberOfUnknownSpecialization || !PossibleConcept ||
+        TNK != TNK_Concept_template) {
+      if (SS.isNotEmpty())
+        AnnotateScopeToken(SS, !WasScopeAnnotation);
+      return false;
+    }
 
-  // At this point we're sure we're dealing with a constrained parameter. It
-  // may or may not have a template parameter list following the concept name.
-  return AnnotateTemplateIdToken(PossibleConcept, TNK, SS,
-                                 /*TemplateKWLoc=*/SourceLocation(),
-                                 PossibleConceptName,
-                                 /*AllowTypeAnnotation=*/false,
-                                 /*TypeConstraint=*/true);
+    // At this point we're sure we're dealing with a constrained parameter. It
+    // may or may not have a template parameter list following the concept
+    // name.
+    if (AnnotateTemplateIdToken(PossibleConcept, TNK, SS,
+                                   /*TemplateKWLoc=*/SourceLocation(),
+                                   PossibleConceptName,
+                                   /*AllowTypeAnnotation=*/false,
+                                   /*TypeConstraint=*/true))
+      return true;
+  }
+
+  if (SS.isNotEmpty())
+    AnnotateScopeToken(SS, !WasScopeAnnotation);
+  return false;
 }
 
 /// ParseTypeParameter - Parse a template type parameter (C++ [temp.param]).
@@ -739,13 +744,17 @@ bool Parser::TryAnnotateTypeConstraint(CXXScopeSpec &SS) {
 ///         'typename' ...[opt][C++0x] identifier[opt]
 ///         'typename' identifier[opt] '=' type-id
 NamedDecl *Parser::ParseTypeParameter(unsigned Depth, unsigned Position) {
-  assert(Tok.isOneOf(tok::kw_class, tok::kw_typename, tok::annot_template_id) &&
+  assert((Tok.isOneOf(tok::kw_class, tok::kw_typename) ||
+          isTypeConstraintAnnotation()) &&
          "A type-parameter starts with 'class', 'typename' or a "
          "type-constraint");
 
+  CXXScopeSpec TypeConstraintSS;
   TemplateIdAnnotation *TypeConstraint = nullptr;
   bool TypenameKeyword = false;
   SourceLocation KeyLoc;
+  ParseOptionalCXXScopeSpecifier(TypeConstraintSS, nullptr,
+                                 /*EnteringContext*/ false);
   if (Tok.is(tok::annot_template_id)) {
     // Consume the 'type-constraint'.
     TypeConstraint =
@@ -754,6 +763,9 @@ NamedDecl *Parser::ParseTypeParameter(unsigned Depth, unsigned Position) {
            "stray non-concept template-id annotation");
     KeyLoc = ConsumeAnnotationToken();
   } else {
+    assert(TypeConstraintSS.isEmpty() &&
+           "expected type constraint after scope specifier");
+
     // Consume the 'class' or 'typename' keyword.
     TypenameKeyword = Tok.is(tok::kw_typename);
     KeyLoc = ConsumeToken();
@@ -795,7 +807,8 @@ NamedDecl *Parser::ParseTypeParameter(unsigned Depth, unsigned Position) {
   ParsedType DefaultArg;
   if (TryConsumeToken(tok::equal, EqualLoc))
     DefaultArg = ParseTypeName(/*Range=*/nullptr,
-                               DeclaratorContext::TemplateTypeArgContext).get();
+                               DeclaratorContext::TemplateTypeArgContext)
+                     .get();
 
   NamedDecl *NewDecl = Actions.ActOnTypeParameter(getCurScope(),
                                                   TypenameKeyword, EllipsisLoc,
@@ -804,10 +817,11 @@ NamedDecl *Parser::ParseTypeParameter(unsigned Depth, unsigned Position) {
                                                   DefaultArg,
                                                   TypeConstraint != nullptr);
 
-  if (TypeConstraint)
-    Actions.ActOnTypeConstraint(TypeConstraint,
+  if (TypeConstraint) {
+    Actions.ActOnTypeConstraint(TypeConstraintSS, TypeConstraint,
                                 cast<TemplateTypeParmDecl>(NewDecl),
                                 EllipsisLoc);
+  }
 
   return NewDecl;
 }
@@ -1331,8 +1345,8 @@ bool Parser::AnnotateTemplateIdToken(TemplateTy Template, TemplateNameKind TNK,
             : TemplateName.OperatorFunctionId.Operator;
 
     TemplateIdAnnotation *TemplateId = TemplateIdAnnotation::Create(
-      SS, TemplateKWLoc, TemplateNameLoc, TemplateII, OpKind, Template, TNK,
-      LAngleLoc, RAngleLoc, TemplateArgs, TemplateIds);
+        TemplateKWLoc, TemplateNameLoc, TemplateII, OpKind, Template, TNK,
+        LAngleLoc, RAngleLoc, TemplateArgs, TemplateIds);
 
     Tok.setAnnotationValue(TemplateId);
     if (TemplateKWLoc.isValid())
@@ -1357,11 +1371,14 @@ bool Parser::AnnotateTemplateIdToken(TemplateTy Template, TemplateNameKind TNK,
 /// a type annotation token will still be created, but will have a
 /// NULL type pointer to signify an error.
 ///
+/// \param SS The scope specifier appearing before the template-id, if any.
+///
 /// \param IsClassName Is this template-id appearing in a context where we
 /// know it names a class, such as in an elaborated-type-specifier or
 /// base-specifier? ('typename' and 'template' are unneeded and disallowed
 /// in those contexts.)
-void Parser::AnnotateTemplateIdTokenAsType(bool IsClassName) {
+void Parser::AnnotateTemplateIdTokenAsType(CXXScopeSpec &SS,
+                                           bool IsClassName) {
   assert(Tok.is(tok::annot_template_id) && "Requires template-id tokens");
 
   TemplateIdAnnotation *TemplateId = takeTemplateIdAnnotation(Tok);
@@ -1375,7 +1392,7 @@ void Parser::AnnotateTemplateIdTokenAsType(bool IsClassName) {
 
   TypeResult Type
     = Actions.ActOnTemplateIdType(getCurScope(),
-                                  TemplateId->SS,
+                                  SS,
                                   TemplateId->TemplateKWLoc,
                                   TemplateId->Template,
                                   TemplateId->Name,
@@ -1388,8 +1405,8 @@ void Parser::AnnotateTemplateIdTokenAsType(bool IsClassName) {
   // Create the new "type" annotation token.
   Tok.setKind(tok::annot_typename);
   setTypeAnnotation(Tok, Type.isInvalid() ? nullptr : Type.get());
-  if (TemplateId->SS.isNotEmpty()) // it was a C++ qualified type name.
-    Tok.setLocation(TemplateId->SS.getBeginLoc());
+  if (SS.isNotEmpty()) // it was a C++ qualified type name.
+    Tok.setLocation(SS.getBeginLoc());
   // End location stays the same
 
   // Replace the template-id annotation token, and possible the scope-specifier
diff --git a/clang/lib/Parse/ParseTentative.cpp b/clang/lib/Parse/ParseTentative.cpp
index 4d69fb4693fb..ad0a15b0c8a6 100644
--- a/clang/lib/Parse/ParseTentative.cpp
+++ b/clang/lib/Parse/ParseTentative.cpp
@@ -202,9 +202,7 @@ Parser::TPResult Parser::TryConsumeDeclarationSpecifier() {
       }
     }
 
-    if (Tok.isOneOf(tok::identifier, tok::coloncolon, tok::kw_decltype,
-                    tok::annot_template_id) &&
-        TryAnnotateCXXScopeToken())
+    if (TryAnnotateOptionalCXXScopeToken())
       return TPResult::Error;
     if (Tok.is(tok::annot_cxxscope))
       ConsumeAnnotationToken();
@@ -785,9 +783,8 @@ Parser::isCXX11AttributeSpecifier(bool Disambiguate,
 
 Parser::TPResult Parser::TryParsePtrOperatorSeq() {
   while (true) {
-    if (Tok.isOneOf(tok::coloncolon, tok::identifier))
-      if (TryAnnotateCXXScopeToken(true))
-        return TPResult::Error;
+    if (TryAnnotateOptionalCXXScopeToken(true))
+      return TPResult::Error;
 
     if (Tok.isOneOf(tok::star, tok::amp, tok::caret, tok::ampamp) ||
         (Tok.is(tok::annot_cxxscope) && NextToken().is(tok::star))) {
@@ -1316,6 +1313,18 @@ public:
 Parser::TPResult
 Parser::isCXXDeclarationSpecifier(Parser::TPResult BracedCastResult,
                                   bool *InvalidAsDeclSpec) {
+  auto IsPlaceholderSpecifier = [&] (TemplateIdAnnotation *TemplateId,
+                                     int Lookahead) {
+    // We have a placeholder-constraint (we check for 'auto' or 'decltype' to
+    // distinguish 'C<int>;' from 'C<int> auto c = 1;')
+    return TemplateId->Kind == TNK_Concept_template &&
+        GetLookAheadToken(Lookahead + 1).isOneOf(tok::kw_auto, tok::kw_decltype,
+            // If we have an identifier here, the user probably forgot the
+            // 'auto' in the placeholder constraint, e.g. 'C<int> x = 2;'
+            // This will be diagnosed nicely later, so disambiguate as a
+            // declaration.
+            tok::identifier);
+  };
   switch (Tok.getKind()) {
   case tok::identifier: {
     // Check for need to substitute AltiVec __vector keyword
@@ -1519,10 +1528,12 @@ Parser::isCXXDeclarationSpecifier(Parser::TPResult BracedCastResult,
       *InvalidAsDeclSpec = NextToken().is(tok::l_paren);
       return TPResult::Ambiguous;
     }
+    if (IsPlaceholderSpecifier(TemplateId, /*Lookahead=*/0))
+      return TPResult::True;
     if (TemplateId->Kind != TNK_Type_template)
       return TPResult::False;
     CXXScopeSpec SS;
-    AnnotateTemplateIdTokenAsType();
+    AnnotateTemplateIdTokenAsType(SS);
     assert(Tok.is(tok::annot_typename));
     goto case_typename;
   }
@@ -1532,6 +1543,13 @@ Parser::isCXXDeclarationSpecifier(Parser::TPResult BracedCastResult,
     if (TryAnnotateTypeOrScopeToken())
       return TPResult::Error;
     if (!Tok.is(tok::annot_typename)) {
+      if (Tok.is(tok::annot_cxxscope) &&
+          NextToken().is(tok::annot_template_id)) {
+        TemplateIdAnnotation *TemplateId =
+            takeTemplateIdAnnotation(NextToken());
+        if (IsPlaceholderSpecifier(TemplateId, /*Lookahead=*/1))
+          return TPResult::True;
+      }
       // If the next token is an identifier or a type qualifier, then this
       // can't possibly be a valid expression either.
       if (Tok.is(tok::annot_cxxscope) && NextToken().is(tok::identifier)) {
@@ -2137,3 +2155,58 @@ Parser::TPResult Parser::isTemplateArgumentList(unsigned TokensToSkip) {
     return TPResult::Ambiguous;
   return TPResult::False;
 }
+
+/// Determine whether we might be looking at the '(' of a C++20 explicit(bool)
+/// in an earlier language mode.
+Parser::TPResult Parser::isExplicitBool() {
+  assert(Tok.is(tok::l_paren) && "expected to be looking at a '(' token");
+
+  RevertingTentativeParsingAction PA(*this);
+  ConsumeParen();
+
+  // We can only have 'explicit' on a constructor, conversion function, or
+  // deduction guide. The declarator of a deduction guide cannot be
+  // parenthesized, so we know this isn't a deduction guide. So the only
+  // thing we need to check for is some number of parens followed by either
+  // the current class name or 'operator'.
+  while (Tok.is(tok::l_paren))
+    ConsumeParen();
+
+  if (TryAnnotateOptionalCXXScopeToken())
+    return TPResult::Error;
+
+  // Class-scope constructor and conversion function names can't really be
+  // qualified, but we get better diagnostics if we assume they can be.
+  CXXScopeSpec SS;
+  if (Tok.is(tok::annot_cxxscope)) {
+    Actions.RestoreNestedNameSpecifierAnnotation(Tok.getAnnotationValue(),
+                                                 Tok.getAnnotationRange(),
+                                                 SS);
+    ConsumeAnnotationToken();
+  }
+
+  // 'explicit(operator' might be explicit(bool) or the declaration of a
+  // conversion function, but it's probably a conversion function.
+  if (Tok.is(tok::kw_operator))
+    return TPResult::Ambiguous;
+
+  // If this can't be a constructor name, it can only be explicit(bool).
+  if (Tok.isNot(tok::identifier) && Tok.isNot(tok::annot_template_id))
+    return TPResult::True;
+  if (!Actions.isCurrentClassName(Tok.is(tok::identifier)
+                                      ? *Tok.getIdentifierInfo()
+                                      : *takeTemplateIdAnnotation(Tok)->Name,
+                                  getCurScope(), &SS))
+    return TPResult::True;
+  // Formally, we must have a right-paren after the constructor name to match
+  // the grammar for a constructor. But clang permits a parenthesized
+  // constructor declarator, so also allow a constructor declarator to follow
+  // with no ')' token after the constructor name.
+  if (!NextToken().is(tok::r_paren) &&
+      !isConstructorDeclarator(/*Unqualified=*/SS.isEmpty(),
+                               /*DeductionGuide=*/false))
+    return TPResult::True;
+
+  // Might be explicit(bool) or a parenthesized constructor name.
+  return TPResult::Ambiguous;
+}
diff --git a/clang/lib/Parse/Parser.cpp b/clang/lib/Parse/Parser.cpp
index 4249de361b89..0b778bd24277 100644
--- a/clang/lib/Parse/Parser.cpp
+++ b/clang/lib/Parse/Parser.cpp
@@ -1136,6 +1136,7 @@ Decl *Parser::ParseFunctionDefinition(ParsingDeclarator &D,
   // Poison SEH identifiers so they are flagged as illegal in function bodies.
   PoisonSEHIdentifiersRAIIObject PoisonSEHIdentifiers(*this, true);
   const DeclaratorChunk::FunctionTypeInfo &FTI = D.getFunctionTypeInfo();
+  TemplateParameterDepthRAII CurTemplateDepthTracker(TemplateParameterDepth);
 
   // If this is C90 and the declspecs were completely missing, fudge in an
   // implicit int.  We do this here because this is the only place where
@@ -1262,6 +1263,15 @@ Decl *Parser::ParseFunctionDefinition(ParsingDeclarator &D,
   // safe because we're always the sole owner.
   D.getMutableDeclSpec().abort();
 
+  // With abbreviated function templates - we need to explicitly add depth to
+  // account for the implicit template parameter list induced by the template.
+  if (auto *Template = dyn_cast_or_null<FunctionTemplateDecl>(Res))
+    if (Template->isAbbreviated() &&
+        Template->getTemplateParameters()->getParam(0)->isImplicit())
+      // First template parameter is implicit - meaning no explicit template
+      // parameter list was specified.
+      CurTemplateDepthTracker.addDepth(1);
+
   if (TryConsumeToken(tok::equal)) {
     assert(getLangOpts().CPlusPlus && "Only C++ function definitions have '='");
 
@@ -1732,6 +1742,20 @@ Parser::TryAnnotateName(CorrectionCandidateCallback *CCC) {
       return ANK_Error;
     return ANK_Success;
   }
+  case Sema::NC_Concept: {
+    UnqualifiedId Id;
+    Id.setIdentifier(Name, NameLoc);
+    if (Next.is(tok::less))
+      // We have a concept name followed by '<'. Consume the identifier token so
+      // we reach the '<' and annotate it.
+      ConsumeToken();
+    if (AnnotateTemplateIdToken(
+            TemplateTy::make(Classification.getTemplateName()),
+            Classification.getTemplateNameKind(), SS, SourceLocation(), Id,
+            /*AllowTypeAnnotation=*/false, /*TypeConstraint=*/true))
+      return ANK_Error;
+    return ANK_Success;
+  }
   }
 
   // Unable to classify the name, but maybe we can annotate a scope specifier.
@@ -1810,7 +1834,7 @@ bool Parser::TryAnnotateTypeOrScopeToken() {
                                        /*EnteringContext=*/false, nullptr,
                                        /*IsTypename*/ true))
       return true;
-    if (!SS.isSet()) {
+    if (SS.isEmpty()) {
       if (Tok.is(tok::identifier) || Tok.is(tok::annot_template_id) ||
           Tok.is(tok::annot_decltype)) {
         // Attempt to recover by skipping the invalid 'typename'
@@ -1983,7 +2007,7 @@ bool Parser::TryAnnotateTypeOrScopeTokenAfterScopeSpec(CXXScopeSpec &SS,
       // template-id annotation in a context where we weren't allowed
       // to produce a type annotation token. Update the template-id
       // annotation token to a type annotation token now.
-      AnnotateTemplateIdTokenAsType();
+      AnnotateTemplateIdTokenAsType(SS);
       return false;
     }
   }
@@ -2005,10 +2029,7 @@ bool Parser::TryAnnotateTypeOrScopeTokenAfterScopeSpec(CXXScopeSpec &SS,
 bool Parser::TryAnnotateCXXScopeToken(bool EnteringContext) {
   assert(getLangOpts().CPlusPlus &&
          "Call sites of this function should be guarded by checking for C++");
-  assert((Tok.is(tok::identifier) || Tok.is(tok::coloncolon) ||
-          (Tok.is(tok::annot_template_id) && NextToken().is(tok::coloncolon)) ||
-          Tok.is(tok::kw_decltype) || Tok.is(tok::kw___super)) &&
-         "Cannot be a type or scope token!");
+  assert(MightBeCXXScopeToken() && "Cannot be a type or scope token!");
 
   CXXScopeSpec SS;
   if (ParseOptionalCXXScopeSpecifier(SS, nullptr, EnteringContext))
diff --git a/clang/lib/Sema/DeclSpec.cpp b/clang/lib/Sema/DeclSpec.cpp
index 639231c87232..94d87974624e 100644
--- a/clang/lib/Sema/DeclSpec.cpp
+++ b/clang/lib/Sema/DeclSpec.cpp
@@ -784,6 +784,15 @@ bool DeclSpec::SetTypeSpecType(TST T, SourceLocation TagKwLoc,
   return false;
 }
 
+bool DeclSpec::SetTypeSpecType(TST T, SourceLocation Loc, const char *&PrevSpec,
+                               unsigned &DiagID, TemplateIdAnnotation *Rep,
+                               const PrintingPolicy &Policy) {
+  assert(T == TST_auto || T == TST_decltype_auto);
+  ConstrainedAuto = true;
+  TemplateIdRep = Rep;
+  return SetTypeSpecType(T, Loc, PrevSpec, DiagID, Policy);
+}
+
 bool DeclSpec::SetTypeSpecType(TST T, SourceLocation Loc,
                                const char *&PrevSpec,
                                unsigned &DiagID,
diff --git a/clang/lib/Sema/Sema.cpp b/clang/lib/Sema/Sema.cpp
index 2cd158a8b43c..9cfce5a63b1d 100644
--- a/clang/lib/Sema/Sema.cpp
+++ b/clang/lib/Sema/Sema.cpp
@@ -52,6 +52,21 @@ SourceLocation Sema::getLocForEndOfToken(SourceLocation Loc, unsigned Offset) {
 
 ModuleLoader &Sema::getModuleLoader() const { return PP.getModuleLoader(); }
 
+IdentifierInfo *
+Sema::InventAbbreviatedTemplateParameterTypeName(IdentifierInfo *ParamName,
+                                                 unsigned int Index) {
+  std::string InventedName;
+  llvm::raw_string_ostream OS(InventedName);
+
+  if (!ParamName)
+    OS << "auto:" << Index + 1;
+  else
+    OS << ParamName->getName() << ":auto";
+
+  OS.flush();
+  return &Context.Idents.get(OS.str());
+}
+
 PrintingPolicy Sema::getPrintingPolicy(const ASTContext &Context,
                                        const Preprocessor &PP) {
   PrintingPolicy Policy = Context.getPrintingPolicy();
@@ -153,10 +168,10 @@ Sema::Sema(Preprocessor &pp, ASTContext &ctxt, ASTConsumer &consumer,
       TUKind(TUKind), NumSFINAEErrors(0),
       FullyCheckedComparisonCategories(
           static_cast<unsigned>(ComparisonCategoryType::Last) + 1),
-      AccessCheckingSFINAE(false), InNonInstantiationSFINAEContext(false),
-      NonInstantiationEntries(0), ArgumentPackSubstitutionIndex(-1),
-      CurrentInstantiationScope(nullptr), DisableTypoCorrection(false),
-      TyposCorrected(0), AnalysisWarnings(*this),
+      SatisfactionCache(Context), AccessCheckingSFINAE(false),
+      InNonInstantiationSFINAEContext(false), NonInstantiationEntries(0),
+      ArgumentPackSubstitutionIndex(-1), CurrentInstantiationScope(nullptr),
+      DisableTypoCorrection(false), TyposCorrected(0), AnalysisWarnings(*this),
       ThreadSafetyDeclCache(nullptr), VarDataSharingAttributesStack(nullptr),
       CurScope(nullptr), Ident_super(nullptr), Ident___float128(nullptr) {
   TUScope = nullptr;
@@ -379,6 +394,14 @@ Sema::~Sema() {
   if (isMultiplexExternalSource)
     delete ExternalSource;
 
+  // Delete cached satisfactions.
+  std::vector<ConstraintSatisfaction *> Satisfactions;
+  Satisfactions.reserve(Satisfactions.size());
+  for (auto &Node : SatisfactionCache)
+    Satisfactions.push_back(&Node);
+  for (auto *Node : Satisfactions)
+    delete Node;
+
   threadSafety::threadSafetyCleanup(ThreadSafetyDeclCache);
 
   // Destroys data sharing attributes stack for OpenMP
@@ -1261,7 +1284,8 @@ DeclContext *Sema::getFunctionLevelDeclContext() {
   DeclContext *DC = CurContext;
 
   while (true) {
-    if (isa<BlockDecl>(DC) || isa<EnumDecl>(DC) || isa<CapturedDecl>(DC)) {
+    if (isa<BlockDecl>(DC) || isa<EnumDecl>(DC) || isa<CapturedDecl>(DC) ||
+        isa<RequiresExprBodyDecl>(DC)) {
       DC = DC->getParent();
     } else if (isa<CXXMethodDecl>(DC) &&
                cast<CXXMethodDecl>(DC)->getOverloadedOperator() == OO_Call &&
diff --git a/clang/lib/Sema/SemaConcept.cpp b/clang/lib/Sema/SemaConcept.cpp
index 018ac2d7dc9d..81601b09ce0d 100644
--- a/clang/lib/Sema/SemaConcept.cpp
+++ b/clang/lib/Sema/SemaConcept.cpp
@@ -17,7 +17,10 @@
 #include "clang/Sema/SemaDiagnostic.h"
 #include "clang/Sema/TemplateDeduction.h"
 #include "clang/Sema/Template.h"
-#include "clang/AST/ExprCXX.h"
+#include "clang/Sema/Overload.h"
+#include "clang/Sema/Initialization.h"
+#include "clang/Sema/SemaInternal.h"
+#include "clang/AST/ExprConcepts.h"
 #include "clang/AST/RecursiveASTVisitor.h"
 #include "clang/Basic/OperatorPrecedence.h"
 #include "llvm/ADT/DenseMap.h"
@@ -269,36 +272,56 @@ static bool CheckConstraintSatisfaction(Sema &S, TemplateDeclT *Template,
   return false;
 }
 
-bool Sema::CheckConstraintSatisfaction(TemplateDecl *Template,
-                                       ArrayRef<const Expr *> ConstraintExprs,
-                                       ArrayRef<TemplateArgument> TemplateArgs,
-                                       SourceRange TemplateIDRange,
-                                       ConstraintSatisfaction &Satisfaction) {
-  return ::CheckConstraintSatisfaction(*this, Template, ConstraintExprs,
-                                       TemplateArgs, TemplateIDRange,
-                                       Satisfaction);
-}
+bool Sema::CheckConstraintSatisfaction(
+    NamedDecl *Template, ArrayRef<const Expr *> ConstraintExprs,
+    ArrayRef<TemplateArgument> TemplateArgs, SourceRange TemplateIDRange,
+    ConstraintSatisfaction &OutSatisfaction) {
+  if (ConstraintExprs.empty()) {
+    OutSatisfaction.IsSatisfied = true;
+    return false;
+  }
 
-bool
-Sema::CheckConstraintSatisfaction(ClassTemplatePartialSpecializationDecl* Part,
-                                  ArrayRef<const Expr *> ConstraintExprs,
-                                  ArrayRef<TemplateArgument> TemplateArgs,
-                                  SourceRange TemplateIDRange,
-                                  ConstraintSatisfaction &Satisfaction) {
-  return ::CheckConstraintSatisfaction(*this, Part, ConstraintExprs,
-                                       TemplateArgs, TemplateIDRange,
-                                       Satisfaction);
-}
+  llvm::FoldingSetNodeID ID;
+  void *InsertPos;
+  ConstraintSatisfaction *Satisfaction = nullptr;
+  if (LangOpts.ConceptSatisfactionCaching) {
+    ConstraintSatisfaction::Profile(ID, Context, Template, TemplateArgs);
+    Satisfaction = SatisfactionCache.FindNodeOrInsertPos(ID, InsertPos);
+    if (Satisfaction) {
+      OutSatisfaction = *Satisfaction;
+      return false;
+    }
+    Satisfaction = new ConstraintSatisfaction(Template, TemplateArgs);
+  } else {
+    Satisfaction = &OutSatisfaction;
+  }
+  bool Failed;
+  if (auto *T = dyn_cast<TemplateDecl>(Template))
+    Failed = ::CheckConstraintSatisfaction(*this, T, ConstraintExprs,
+                                           TemplateArgs, TemplateIDRange,
+                                           *Satisfaction);
+  else if (auto *P =
+               dyn_cast<ClassTemplatePartialSpecializationDecl>(Template))
+    Failed = ::CheckConstraintSatisfaction(*this, P, ConstraintExprs,
+                                           TemplateArgs, TemplateIDRange,
+                                           *Satisfaction);
+  else
+    Failed = ::CheckConstraintSatisfaction(
+        *this, cast<VarTemplatePartialSpecializationDecl>(Template),
+        ConstraintExprs, TemplateArgs, TemplateIDRange, *Satisfaction);
+  if (Failed) {
+    if (LangOpts.ConceptSatisfactionCaching)
+      delete Satisfaction;
+    return true;
+  }
 
-bool
-Sema::CheckConstraintSatisfaction(VarTemplatePartialSpecializationDecl* Partial,
-                                  ArrayRef<const Expr *> ConstraintExprs,
-                                  ArrayRef<TemplateArgument> TemplateArgs,
-                                  SourceRange TemplateIDRange,
-                                  ConstraintSatisfaction &Satisfaction) {
-  return ::CheckConstraintSatisfaction(*this, Partial, ConstraintExprs,
-                                       TemplateArgs, TemplateIDRange,
-                                       Satisfaction);
+  if (LangOpts.ConceptSatisfactionCaching) {
+    // We cannot use InsertNode here because CheckConstraintSatisfaction might
+    // have invalidated it.
+    SatisfactionCache.InsertNode(Satisfaction);
+    OutSatisfaction = *Satisfaction;
+  }
+  return false;
 }
 
 bool Sema::CheckConstraintSatisfaction(const Expr *ConstraintExpr,
@@ -336,6 +359,118 @@ bool Sema::EnsureTemplateArgumentListConstraints(
   return false;
 }
 
+static void diagnoseUnsatisfiedRequirement(Sema &S,
+                                           concepts::ExprRequirement *Req,
+                                           bool First) {
+  assert(!Req->isSatisfied()
+         && "Diagnose() can only be used on an unsatisfied requirement");
+  switch (Req->getSatisfactionStatus()) {
+    case concepts::ExprRequirement::SS_Dependent:
+      llvm_unreachable("Diagnosing a dependent requirement");
+      break;
+    case concepts::ExprRequirement::SS_ExprSubstitutionFailure: {
+      auto *SubstDiag = Req->getExprSubstitutionDiagnostic();
+      if (!SubstDiag->DiagMessage.empty())
+        S.Diag(SubstDiag->DiagLoc,
+               diag::note_expr_requirement_expr_substitution_error)
+               << (int)First << SubstDiag->SubstitutedEntity
+               << SubstDiag->DiagMessage;
+      else
+        S.Diag(SubstDiag->DiagLoc,
+               diag::note_expr_requirement_expr_unknown_substitution_error)
+            << (int)First << SubstDiag->SubstitutedEntity;
+      break;
+    }
+    case concepts::ExprRequirement::SS_NoexceptNotMet:
+      S.Diag(Req->getNoexceptLoc(),
+             diag::note_expr_requirement_noexcept_not_met)
+          << (int)First << Req->getExpr();
+      break;
+    case concepts::ExprRequirement::SS_TypeRequirementSubstitutionFailure: {
+      auto *SubstDiag =
+          Req->getReturnTypeRequirement().getSubstitutionDiagnostic();
+      if (!SubstDiag->DiagMessage.empty())
+        S.Diag(SubstDiag->DiagLoc,
+               diag::note_expr_requirement_type_requirement_substitution_error)
+            << (int)First << SubstDiag->SubstitutedEntity
+            << SubstDiag->DiagMessage;
+      else
+        S.Diag(SubstDiag->DiagLoc,
+               diag::note_expr_requirement_type_requirement_unknown_substitution_error)
+            << (int)First << SubstDiag->SubstitutedEntity;
+      break;
+    }
+    case concepts::ExprRequirement::SS_ConstraintsNotSatisfied: {
+      ConceptSpecializationExpr *ConstraintExpr =
+          Req->getReturnTypeRequirementSubstitutedConstraintExpr();
+      if (ConstraintExpr->getTemplateArgsAsWritten()->NumTemplateArgs == 1)
+        // A simple case - expr type is the type being constrained and the concept
+        // was not provided arguments.
+        S.Diag(ConstraintExpr->getBeginLoc(),
+               diag::note_expr_requirement_constraints_not_satisfied_simple)
+            << (int)First << S.BuildDecltypeType(Req->getExpr(),
+                                                 Req->getExpr()->getBeginLoc())
+            << ConstraintExpr->getNamedConcept();
+      else
+        S.Diag(ConstraintExpr->getBeginLoc(),
+               diag::note_expr_requirement_constraints_not_satisfied)
+            << (int)First << ConstraintExpr;
+      S.DiagnoseUnsatisfiedConstraint(ConstraintExpr->getSatisfaction());
+      break;
+    }
+    case concepts::ExprRequirement::SS_Satisfied:
+      llvm_unreachable("We checked this above");
+  }
+}
+
+static void diagnoseUnsatisfiedRequirement(Sema &S,
+                                           concepts::TypeRequirement *Req,
+                                           bool First) {
+  assert(!Req->isSatisfied()
+         && "Diagnose() can only be used on an unsatisfied requirement");
+  switch (Req->getSatisfactionStatus()) {
+  case concepts::TypeRequirement::SS_Dependent:
+    llvm_unreachable("Diagnosing a dependent requirement");
+    return;
+  case concepts::TypeRequirement::SS_SubstitutionFailure: {
+    auto *SubstDiag = Req->getSubstitutionDiagnostic();
+    if (!SubstDiag->DiagMessage.empty())
+      S.Diag(SubstDiag->DiagLoc,
+             diag::note_type_requirement_substitution_error) << (int)First
+          << SubstDiag->SubstitutedEntity << SubstDiag->DiagMessage;
+    else
+      S.Diag(SubstDiag->DiagLoc,
+             diag::note_type_requirement_unknown_substitution_error)
+          << (int)First << SubstDiag->SubstitutedEntity;
+    return;
+  }
+  default:
+    llvm_unreachable("Unknown satisfaction status");
+    return;
+  }
+}
+
+static void diagnoseUnsatisfiedRequirement(Sema &S,
+                                           concepts::NestedRequirement *Req,
+                                           bool First) {
+  if (Req->isSubstitutionFailure()) {
+    concepts::Requirement::SubstitutionDiagnostic *SubstDiag =
+        Req->getSubstitutionDiagnostic();
+    if (!SubstDiag->DiagMessage.empty())
+      S.Diag(SubstDiag->DiagLoc,
+             diag::note_nested_requirement_substitution_error)
+             << (int)First << SubstDiag->SubstitutedEntity
+             << SubstDiag->DiagMessage;
+    else
+      S.Diag(SubstDiag->DiagLoc,
+             diag::note_nested_requirement_unknown_substitution_error)
+          << (int)First << SubstDiag->SubstitutedEntity;
+    return;
+  }
+  S.DiagnoseUnsatisfiedConstraint(Req->getConstraintSatisfaction(), First);
+}
+
+
 static void diagnoseWellFormedUnsatisfiedConstraintExpr(Sema &S,
                                                         Expr *SubstExpr,
                                                         bool First = true) {
@@ -412,6 +547,19 @@ static void diagnoseWellFormedUnsatisfiedConstraintExpr(Sema &S,
     }
     S.DiagnoseUnsatisfiedConstraint(CSE->getSatisfaction());
     return;
+  } else if (auto *RE = dyn_cast<RequiresExpr>(SubstExpr)) {
+    for (concepts::Requirement *Req : RE->getRequirements())
+      if (!Req->isDependent() && !Req->isSatisfied()) {
+        if (auto *E = dyn_cast<concepts::ExprRequirement>(Req))
+          diagnoseUnsatisfiedRequirement(S, E, First);
+        else if (auto *T = dyn_cast<concepts::TypeRequirement>(Req))
+          diagnoseUnsatisfiedRequirement(S, T, First);
+        else
+          diagnoseUnsatisfiedRequirement(
+              S, cast<concepts::NestedRequirement>(Req), First);
+        break;
+      }
+    return;
   }
 
   S.Diag(SubstExpr->getSourceRange().getBegin(),
@@ -434,11 +582,11 @@ static void diagnoseUnsatisfiedConstraintExpr(
       Record.template get<Expr *>(), First);
 }
 
-void Sema::DiagnoseUnsatisfiedConstraint(
-    const ConstraintSatisfaction& Satisfaction) {
+void
+Sema::DiagnoseUnsatisfiedConstraint(const ConstraintSatisfaction& Satisfaction,
+                                    bool First) {
   assert(!Satisfaction.IsSatisfied &&
          "Attempted to diagnose a satisfied constraint");
-  bool First = true;
   for (auto &Pair : Satisfaction.Details) {
     diagnoseUnsatisfiedConstraintExpr(*this, Pair.first, Pair.second, First);
     First = false;
@@ -446,10 +594,10 @@ void Sema::DiagnoseUnsatisfiedConstraint(
 }
 
 void Sema::DiagnoseUnsatisfiedConstraint(
-    const ASTConstraintSatisfaction &Satisfaction) {
+    const ASTConstraintSatisfaction &Satisfaction,
+    bool First) {
   assert(!Satisfaction.IsSatisfied &&
          "Attempted to diagnose a satisfied constraint");
-  bool First = true;
   for (auto &Pair : Satisfaction) {
     diagnoseUnsatisfiedConstraintExpr(*this, Pair.first, Pair.second, First);
     First = false;
@@ -826,3 +974,67 @@ bool Sema::MaybeEmitAmbiguousAtomicConstraintsDiagnostic(NamedDecl *D1,
       << AmbiguousAtomic2->getSourceRange();
   return true;
 }
+
+concepts::ExprRequirement::ExprRequirement(
+    Expr *E, bool IsSimple, SourceLocation NoexceptLoc,
+    ReturnTypeRequirement Req, SatisfactionStatus Status,
+    ConceptSpecializationExpr *SubstitutedConstraintExpr) :
+    Requirement(IsSimple ? RK_Simple : RK_Compound, Status == SS_Dependent,
+                Status == SS_Dependent &&
+                (E->containsUnexpandedParameterPack() ||
+                 Req.containsUnexpandedParameterPack()),
+                Status == SS_Satisfied), Value(E), NoexceptLoc(NoexceptLoc),
+    TypeReq(Req), SubstitutedConstraintExpr(SubstitutedConstraintExpr),
+    Status(Status) {
+  assert((!IsSimple || (Req.isEmpty() && NoexceptLoc.isInvalid())) &&
+         "Simple requirement must not have a return type requirement or a "
+         "noexcept specification");
+  assert((Status > SS_TypeRequirementSubstitutionFailure && Req.isTypeConstraint()) ==
+         (SubstitutedConstraintExpr != nullptr));
+}
+
+concepts::ExprRequirement::ExprRequirement(
+    SubstitutionDiagnostic *ExprSubstDiag, bool IsSimple,
+    SourceLocation NoexceptLoc, ReturnTypeRequirement Req) :
+    Requirement(IsSimple ? RK_Simple : RK_Compound, Req.isDependent(),
+                Req.containsUnexpandedParameterPack(), /*IsSatisfied=*/false),
+    Value(ExprSubstDiag), NoexceptLoc(NoexceptLoc), TypeReq(Req),
+    Status(SS_ExprSubstitutionFailure) {
+  assert((!IsSimple || (Req.isEmpty() && NoexceptLoc.isInvalid())) &&
+         "Simple requirement must not have a return type requirement or a "
+         "noexcept specification");
+}
+
+concepts::ExprRequirement::ReturnTypeRequirement::
+ReturnTypeRequirement(TemplateParameterList *TPL) :
+    TypeConstraintInfo(TPL, 0) {
+  assert(TPL->size() == 1);
+  const TypeConstraint *TC =
+      cast<TemplateTypeParmDecl>(TPL->getParam(0))->getTypeConstraint();
+  assert(TC &&
+         "TPL must have a template type parameter with a type constraint");
+  auto *Constraint =
+      cast_or_null<ConceptSpecializationExpr>(
+          TC->getImmediatelyDeclaredConstraint());
+  bool Dependent = false;
+  if (Constraint->getTemplateArgsAsWritten()) {
+    for (auto &ArgLoc :
+         Constraint->getTemplateArgsAsWritten()->arguments().drop_front(1)) {
+      if (ArgLoc.getArgument().isDependent()) {
+        Dependent = true;
+        break;
+      }
+    }
+  }
+  TypeConstraintInfo.setInt(Dependent ? 1 : 0);
+}
+
+concepts::TypeRequirement::TypeRequirement(TypeSourceInfo *T) :
+    Requirement(RK_Type, T->getType()->isDependentType(),
+                T->getType()->containsUnexpandedParameterPack(),
+                // We reach this ctor with either dependent types (in which
+                // IsSatisfied doesn't matter) or with non-dependent type in
+                // which the existence of the type indicates satisfaction.
+                /*IsSatisfied=*/true
+                ), Value(T),
+    Status(T->getType()->isDependentType() ? SS_Dependent : SS_Satisfied) {}
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 507e4a6cd436..0bf490336537 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -10,6 +10,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "TreeTransform.h"
 #include "TypeLocBuilder.h"
 #include "clang/AST/ASTConsumer.h"
 #include "clang/AST/ASTContext.h"
@@ -1153,6 +1154,10 @@ Corrected:
     return ParsedType::make(T);
   }
 
+  if (isa<ConceptDecl>(FirstDecl))
+    return NameClassification::Concept(
+        TemplateName(cast<TemplateDecl>(FirstDecl)));
+
   // We can have a type template here if we're classifying a template argument.
   if (isa<TemplateDecl>(FirstDecl) && !isa<FunctionTemplateDecl>(FirstDecl) &&
       !isa<VarTemplateDecl>(FirstDecl))
@@ -6468,6 +6473,8 @@ static bool shouldConsiderLinkage(const VarDecl *VD) {
     return true;
   if (DC->isRecord())
     return false;
+  if (isa<RequiresExprBodyDecl>(DC))
+    return false;
   llvm_unreachable("Unexpected context");
 }
 
@@ -8654,11 +8661,21 @@ static Scope *getTagInjectionScope(Scope *S, const LangOptions &LangOpts) {
 NamedDecl*
 Sema::ActOnFunctionDeclarator(Scope *S, Declarator &D, DeclContext *DC,
                               TypeSourceInfo *TInfo, LookupResult &Previous,
-                              MultiTemplateParamsArg TemplateParamLists,
+                              MultiTemplateParamsArg TemplateParamListsRef,
                               bool &AddToScope) {
   QualType R = TInfo->getType();
 
   assert(R->isFunctionType());
+  SmallVector<TemplateParameterList *, 4> TemplateParamLists;
+  for (TemplateParameterList *TPL : TemplateParamListsRef)
+    TemplateParamLists.push_back(TPL);
+  if (TemplateParameterList *Invented = D.getInventedTemplateParameterList()) {
+    if (!TemplateParamLists.empty() &&
+        Invented->getDepth() == TemplateParamLists.back()->getDepth())
+      TemplateParamLists.back() = Invented;
+    else
+      TemplateParamLists.push_back(Invented);
+  }
 
   // TODO: consider using NameInfo for diagnostic.
   DeclarationNameInfo NameInfo = GetNameForDeclarator(D);
@@ -8738,15 +8755,16 @@ Sema::ActOnFunctionDeclarator(Scope *S, Declarator &D, DeclContext *DC,
     // Match up the template parameter lists with the scope specifier, then
     // determine whether we have a template or a template specialization.
     bool Invalid = false;
-    if (TemplateParameterList *TemplateParams =
-            MatchTemplateParametersToScopeSpecifier(
-                D.getDeclSpec().getBeginLoc(), D.getIdentifierLoc(),
-                D.getCXXScopeSpec(),
-                D.getName().getKind() == UnqualifiedIdKind::IK_TemplateId
-                    ? D.getName().TemplateId
-                    : nullptr,
-                TemplateParamLists, isFriend, isMemberSpecialization,
-                Invalid)) {
+    TemplateParameterList *TemplateParams =
+        MatchTemplateParametersToScopeSpecifier(
+            D.getDeclSpec().getBeginLoc(), D.getIdentifierLoc(),
+            D.getCXXScopeSpec(),
+            D.getName().getKind() == UnqualifiedIdKind::IK_TemplateId
+                ? D.getName().TemplateId
+                : nullptr,
+            TemplateParamLists, isFriend, isMemberSpecialization,
+            Invalid);
+    if (TemplateParams) {
       if (TemplateParams->size() > 0) {
         // This is a function template
 
@@ -8779,7 +8797,8 @@ Sema::ActOnFunctionDeclarator(Scope *S, Declarator &D, DeclContext *DC,
         // For source fidelity, store the other template param lists.
         if (TemplateParamLists.size() > 1) {
           NewFD->setTemplateParameterListsInfo(Context,
-                                               TemplateParamLists.drop_back(1));
+              ArrayRef<TemplateParameterList *>(TemplateParamLists)
+                  .drop_back(1));
         }
       } else {
         // This is a function template specialization.
diff --git a/clang/lib/Sema/SemaDeclAttr.cpp b/clang/lib/Sema/SemaDeclAttr.cpp
index 5c51b0f9b8cb..849bc09063b3 100644
--- a/clang/lib/Sema/SemaDeclAttr.cpp
+++ b/clang/lib/Sema/SemaDeclAttr.cpp
@@ -4924,9 +4924,9 @@ static void handlePatchableFunctionEntryAttr(Sema &S, Decl *D,
     Expr *Arg = AL.getArgAsExpr(1);
     if (!checkUInt32Argument(S, AL, Arg, Offset, 1, true))
       return;
-    if (Offset) {
+    if (Count < Offset) {
       S.Diag(getAttrLoc(AL), diag::err_attribute_argument_out_of_range)
-          << &AL << 0 << 0 << Arg->getBeginLoc();
+          << &AL << 0 << Count << Arg->getBeginLoc();
       return;
     }
   }
diff --git a/clang/lib/Sema/SemaDeclCXX.cpp b/clang/lib/Sema/SemaDeclCXX.cpp
index 9916d3be77e1..9fa5691983a1 100644
--- a/clang/lib/Sema/SemaDeclCXX.cpp
+++ b/clang/lib/Sema/SemaDeclCXX.cpp
@@ -17386,3 +17386,50 @@ MSPropertyDecl *Sema::HandleMSProperty(Scope *S, RecordDecl *Record,
 
   return NewPD;
 }
+
+void Sema::ActOnStartFunctionDeclarationDeclarator(
+    Declarator &Declarator, unsigned TemplateParameterDepth) {
+  auto &Info = InventedParameterInfos.emplace_back();
+  TemplateParameterList *ExplicitParams = nullptr;
+  ArrayRef<TemplateParameterList *> ExplicitLists =
+      Declarator.getTemplateParameterLists();
+  if (!ExplicitLists.empty()) {
+    bool IsMemberSpecialization, IsInvalid;
+    ExplicitParams = MatchTemplateParametersToScopeSpecifier(
+        Declarator.getBeginLoc(), Declarator.getIdentifierLoc(),
+        Declarator.getCXXScopeSpec(), /*TemplateId=*/nullptr,
+        ExplicitLists, /*IsFriend=*/false, IsMemberSpecialization, IsInvalid,
+        /*SuppressDiagnostic=*/true);
+  }
+  if (ExplicitParams) {
+    Info.AutoTemplateParameterDepth = ExplicitParams->getDepth();
+    for (NamedDecl *Param : *ExplicitParams)
+      Info.TemplateParams.push_back(Param);
+    Info.NumExplicitTemplateParams = ExplicitParams->size();
+  } else {
+    Info.AutoTemplateParameterDepth = TemplateParameterDepth;
+    Info.NumExplicitTemplateParams = 0;
+  }
+}
+
+void Sema::ActOnFinishFunctionDeclarationDeclarator(Declarator &Declarator) {
+  auto &FSI = InventedParameterInfos.back();
+  if (FSI.TemplateParams.size() > FSI.NumExplicitTemplateParams) {
+    if (FSI.NumExplicitTemplateParams != 0) {
+      TemplateParameterList *ExplicitParams =
+          Declarator.getTemplateParameterLists().back();
+      Declarator.setInventedTemplateParameterList(
+          TemplateParameterList::Create(
+              Context, ExplicitParams->getTemplateLoc(),
+              ExplicitParams->getLAngleLoc(), FSI.TemplateParams,
+              ExplicitParams->getRAngleLoc(),
+              ExplicitParams->getRequiresClause()));
+    } else {
+      Declarator.setInventedTemplateParameterList(
+          TemplateParameterList::Create(
+              Context, SourceLocation(), SourceLocation(), FSI.TemplateParams,
+              SourceLocation(), /*RequiresClause=*/nullptr));
+    }
+  }
+  InventedParameterInfos.pop_back();
+}
diff --git a/clang/lib/Sema/SemaExceptionSpec.cpp b/clang/lib/Sema/SemaExceptionSpec.cpp
index 5aedbe7644e4..193eaa3e01f9 100644
--- a/clang/lib/Sema/SemaExceptionSpec.cpp
+++ b/clang/lib/Sema/SemaExceptionSpec.cpp
@@ -1386,6 +1386,7 @@ CanThrowResult Sema::canThrow(const Stmt *S) {
   case Expr::StringLiteralClass:
   case Expr::SourceLocExprClass:
   case Expr::ConceptSpecializationExprClass:
+  case Expr::RequiresExprClass:
     // These expressions can never throw.
     return CT_Cannot;
 
diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
index 5f4071924d3f..ea4b93ee6a5a 100644
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/clang/lib/Sema/SemaExpr.cpp
@@ -350,6 +350,17 @@ bool Sema::DiagnoseUseOfDecl(NamedDecl *D, ArrayRef<SourceLocation> Locs,
     }
   }
 
+  if (isa<ParmVarDecl>(D) && isa<RequiresExprBodyDecl>(D->getDeclContext()) &&
+      !isUnevaluatedContext()) {
+    // C++ [expr.prim.req.nested] p3
+    //   A local parameter shall only appear as an unevaluated operand
+    //   (Clause 8) within the constraint-expression.
+    Diag(Loc, diag::err_requires_expr_parameter_referenced_in_evaluated_context)
+        << D;
+    Diag(D->getLocation(), diag::note_entity_declared_at) << D;
+    return true;
+  }
+
   return false;
 }
 
@@ -1904,7 +1915,7 @@ Sema::BuildDeclRefExpr(ValueDecl *D, QualType Ty, ExprValueKind VK,
   bool RefersToCapturedVariable =
       isa<VarDecl>(D) &&
       NeedToCaptureVariable(cast<VarDecl>(D), NameInfo.getLoc());
-
+  
   DeclRefExpr *E = DeclRefExpr::Create(
       Context, NNS, TemplateKWLoc, D, RefersToCapturedVariable, NameInfo, Ty,
       VK, FoundD, TemplateArgs, getNonOdrUseReasonInCurrentContext(D));
diff --git a/clang/lib/Sema/SemaExprCXX.cpp b/clang/lib/Sema/SemaExprCXX.cpp
index a73e6906fceb..192c237b6c1c 100644
--- a/clang/lib/Sema/SemaExprCXX.cpp
+++ b/clang/lib/Sema/SemaExprCXX.cpp
@@ -11,6 +11,7 @@
 ///
 //===----------------------------------------------------------------------===//
 
+#include "clang/Sema/Template.h"
 #include "clang/Sema/SemaInternal.h"
 #include "TreeTransform.h"
 #include "TypeLocBuilder.h"
@@ -7317,7 +7318,7 @@ ExprResult Sema::ActOnPseudoDestructorExpr(Scope *S, Expr *Base,
     ASTTemplateArgsPtr TemplateArgsPtr(TemplateId->getTemplateArgs(),
                                        TemplateId->NumArgs);
     TypeResult T = ActOnTemplateIdType(S,
-                                       TemplateId->SS,
+                                       SS,
                                        TemplateId->TemplateKWLoc,
                                        TemplateId->Template,
                                        TemplateId->Name,
@@ -7370,7 +7371,7 @@ ExprResult Sema::ActOnPseudoDestructorExpr(Scope *S, Expr *Base,
       ASTTemplateArgsPtr TemplateArgsPtr(TemplateId->getTemplateArgs(),
                                          TemplateId->NumArgs);
       TypeResult T = ActOnTemplateIdType(S,
-                                         TemplateId->SS,
+                                         SS,
                                          TemplateId->TemplateKWLoc,
                                          TemplateId->Template,
                                          TemplateId->Name,
@@ -8331,3 +8332,215 @@ Sema::CheckMicrosoftIfExistsSymbol(Scope *S, SourceLocation KeywordLoc,
 
   return CheckMicrosoftIfExistsSymbol(S, SS, TargetNameInfo);
 }
+
+concepts::Requirement *Sema::ActOnSimpleRequirement(Expr *E) {
+  return BuildExprRequirement(E, /*IsSimple=*/true,
+                              /*NoexceptLoc=*/SourceLocation(),
+                              /*ReturnTypeRequirement=*/{});
+}
+
+concepts::Requirement *
+Sema::ActOnTypeRequirement(SourceLocation TypenameKWLoc, CXXScopeSpec &SS,
+                           SourceLocation NameLoc, IdentifierInfo *TypeName,
+                           TemplateIdAnnotation *TemplateId) {
+  assert(((!TypeName && TemplateId) || (TypeName && !TemplateId)) &&
+         "Exactly one of TypeName and TemplateId must be specified.");
+  TypeSourceInfo *TSI = nullptr;
+  if (TypeName) {
+    QualType T = CheckTypenameType(ETK_Typename, TypenameKWLoc,
+                                   SS.getWithLocInContext(Context), *TypeName,
+                                   NameLoc, &TSI, /*DeducedTypeContext=*/false);
+    if (T.isNull())
+      return nullptr;
+  } else {
+    ASTTemplateArgsPtr ArgsPtr(TemplateId->getTemplateArgs(),
+                               TemplateId->NumArgs);
+    TypeResult T = ActOnTypenameType(CurScope, TypenameKWLoc, SS,
+                                     TemplateId->TemplateKWLoc,
+                                     TemplateId->Template, TemplateId->Name,
+                                     TemplateId->TemplateNameLoc,
+                                     TemplateId->LAngleLoc, ArgsPtr,
+                                     TemplateId->RAngleLoc);
+    if (T.isInvalid())
+      return nullptr;
+    if (GetTypeFromParser(T.get(), &TSI).isNull())
+      return nullptr;
+  }
+  return BuildTypeRequirement(TSI);
+}
+
+concepts::Requirement *
+Sema::ActOnCompoundRequirement(Expr *E, SourceLocation NoexceptLoc) {
+  return BuildExprRequirement(E, /*IsSimple=*/false, NoexceptLoc,
+                              /*ReturnTypeRequirement=*/{});
+}
+
+concepts::Requirement *
+Sema::ActOnCompoundRequirement(
+    Expr *E, SourceLocation NoexceptLoc, CXXScopeSpec &SS,
+    TemplateIdAnnotation *TypeConstraint, unsigned Depth) {
+  // C++2a [expr.prim.req.compound] p1.3.3
+  //   [..] the expression is deduced against an invented function template
+  //   F [...] F is a void function template with a single type template
+  //   parameter T declared with the constrained-parameter. Form a new
+  //   cv-qualifier-seq cv by taking the union of const and volatile specifiers
+  //   around the constrained-parameter. F has a single parameter whose
+  //   type-specifier is cv T followed by the abstract-declarator. [...]
+  //
+  // The cv part is done in the calling function - we get the concept with
+  // arguments and the abstract declarator with the correct CV qualification and
+  // have to synthesize T and the single parameter of F.
+  auto &II = Context.Idents.get("expr-type");
+  auto *TParam = TemplateTypeParmDecl::Create(Context, CurContext,
+                                              SourceLocation(),
+                                              SourceLocation(), Depth,
+                                              /*Index=*/0, &II,
+                                              /*Typename=*/true,
+                                              /*ParameterPack=*/false,
+                                              /*HasTypeConstraint=*/true);
+
+  if (ActOnTypeConstraint(SS, TypeConstraint, TParam,
+                          /*EllpsisLoc=*/SourceLocation()))
+    // Just produce a requirement with no type requirements.
+    return BuildExprRequirement(E, /*IsSimple=*/false, NoexceptLoc, {});
+
+  auto *TPL = TemplateParameterList::Create(Context, SourceLocation(),
+                                            SourceLocation(),
+                                            ArrayRef<NamedDecl *>(TParam),
+                                            SourceLocation(),
+                                            /*RequiresClause=*/nullptr);
+  return BuildExprRequirement(
+      E, /*IsSimple=*/false, NoexceptLoc,
+      concepts::ExprRequirement::ReturnTypeRequirement(TPL));
+}
+
+concepts::ExprRequirement *
+Sema::BuildExprRequirement(
+    Expr *E, bool IsSimple, SourceLocation NoexceptLoc,
+    concepts::ExprRequirement::ReturnTypeRequirement ReturnTypeRequirement) {
+  auto Status = concepts::ExprRequirement::SS_Satisfied;
+  ConceptSpecializationExpr *SubstitutedConstraintExpr = nullptr;
+  if (E->isInstantiationDependent() || ReturnTypeRequirement.isDependent())
+    Status = concepts::ExprRequirement::SS_Dependent;
+  else if (NoexceptLoc.isValid() && canThrow(E) == CanThrowResult::CT_Can)
+    Status = concepts::ExprRequirement::SS_NoexceptNotMet;
+  else if (ReturnTypeRequirement.isSubstitutionFailure())
+    Status = concepts::ExprRequirement::SS_TypeRequirementSubstitutionFailure;
+  else if (ReturnTypeRequirement.isTypeConstraint()) {
+    // C++2a [expr.prim.req]p1.3.3
+    //     The immediately-declared constraint ([temp]) of decltype((E)) shall
+    //     be satisfied.
+    TemplateParameterList *TPL =
+        ReturnTypeRequirement.getTypeConstraintTemplateParameterList();
+    QualType MatchedType =
+        BuildDecltypeType(E, E->getBeginLoc()).getCanonicalType();
+    llvm::SmallVector<TemplateArgument, 1> Args;
+    Args.push_back(TemplateArgument(MatchedType));
+    TemplateArgumentList TAL(TemplateArgumentList::OnStack, Args);
+    MultiLevelTemplateArgumentList MLTAL(TAL);
+    for (unsigned I = 0; I < TPL->getDepth(); ++I)
+      MLTAL.addOuterRetainedLevel();
+    Expr *IDC =
+        cast<TemplateTypeParmDecl>(TPL->getParam(0))->getTypeConstraint()
+            ->getImmediatelyDeclaredConstraint();
+    ExprResult Constraint = SubstExpr(IDC, MLTAL);
+    assert(!Constraint.isInvalid() &&
+           "Substitution cannot fail as it is simply putting a type template "
+           "argument into a concept specialization expression's parameter.");
+
+    SubstitutedConstraintExpr =
+        cast<ConceptSpecializationExpr>(Constraint.get());
+    if (!SubstitutedConstraintExpr->isSatisfied())
+      Status = concepts::ExprRequirement::SS_ConstraintsNotSatisfied;
+  }
+  return new (Context) concepts::ExprRequirement(E, IsSimple, NoexceptLoc,
+                                                 ReturnTypeRequirement, Status,
+                                                 SubstitutedConstraintExpr);
+}
+
+concepts::ExprRequirement *
+Sema::BuildExprRequirement(
+    concepts::Requirement::SubstitutionDiagnostic *ExprSubstitutionDiagnostic,
+    bool IsSimple, SourceLocation NoexceptLoc,
+    concepts::ExprRequirement::ReturnTypeRequirement ReturnTypeRequirement) {
+  return new (Context) concepts::ExprRequirement(ExprSubstitutionDiagnostic,
+                                                 IsSimple, NoexceptLoc,
+                                                 ReturnTypeRequirement);
+}
+
+concepts::TypeRequirement *
+Sema::BuildTypeRequirement(TypeSourceInfo *Type) {
+  return new (Context) concepts::TypeRequirement(Type);
+}
+
+concepts::TypeRequirement *
+Sema::BuildTypeRequirement(
+    concepts::Requirement::SubstitutionDiagnostic *SubstDiag) {
+  return new (Context) concepts::TypeRequirement(SubstDiag);
+}
+
+concepts::Requirement *Sema::ActOnNestedRequirement(Expr *Constraint) {
+  return BuildNestedRequirement(Constraint);
+}
+
+concepts::NestedRequirement *
+Sema::BuildNestedRequirement(Expr *Constraint) {
+  ConstraintSatisfaction Satisfaction;
+  if (!Constraint->isInstantiationDependent() &&
+      CheckConstraintSatisfaction(Constraint, Satisfaction))
+    return nullptr;
+  return new (Context) concepts::NestedRequirement(Context, Constraint,
+                                                   Satisfaction);
+}
+
+concepts::NestedRequirement *
+Sema::BuildNestedRequirement(
+    concepts::Requirement::SubstitutionDiagnostic *SubstDiag) {
+  return new (Context) concepts::NestedRequirement(SubstDiag);
+}
+
+RequiresExprBodyDecl *
+Sema::ActOnStartRequiresExpr(SourceLocation RequiresKWLoc,
+                             ArrayRef<ParmVarDecl *> LocalParameters,
+                             Scope *BodyScope) {
+  assert(BodyScope);
+
+  RequiresExprBodyDecl *Body = RequiresExprBodyDecl::Create(Context, CurContext,
+                                                            RequiresKWLoc);
+
+  PushDeclContext(BodyScope, Body);
+
+  for (ParmVarDecl *Param : LocalParameters) {
+    if (Param->hasDefaultArg())
+      // C++2a [expr.prim.req] p4
+      //     [...] A local parameter of a requires-expression shall not have a
+      //     default argument. [...]
+      Diag(Param->getDefaultArgRange().getBegin(),
+           diag::err_requires_expr_local_parameter_default_argument);
+    // Ignore default argument and move on
+
+    Param->setDeclContext(Body);
+    // If this has an identifier, add it to the scope stack.
+    if (Param->getIdentifier()) {
+      CheckShadow(BodyScope, Param);
+      PushOnScopeChains(Param, BodyScope);
+    }
+  }
+  return Body;
+}
+
+void Sema::ActOnFinishRequiresExpr() {
+  assert(CurContext && "DeclContext imbalance!");
+  CurContext = CurContext->getLexicalParent();
+  assert(CurContext && "Popped translation unit!");
+}
+
+ExprResult
+Sema::ActOnRequiresExpr(SourceLocation RequiresKWLoc,
+                        RequiresExprBodyDecl *Body,
+                        ArrayRef<ParmVarDecl *> LocalParameters,
+                        ArrayRef<concepts::Requirement *> Requirements,
+                        SourceLocation ClosingBraceLoc) {
+  return RequiresExpr::Create(Context, RequiresKWLoc, Body, LocalParameters,
+                              Requirements, ClosingBraceLoc);
+}
diff --git a/clang/lib/Sema/SemaLambda.cpp b/clang/lib/Sema/SemaLambda.cpp
index c2d14a44f53d..ae89b146c409 100644
--- a/clang/lib/Sema/SemaLambda.cpp
+++ b/clang/lib/Sema/SemaLambda.cpp
@@ -791,7 +791,8 @@ QualType Sema::buildLambdaInitCaptureInitialization(
   // deduce against.
   QualType DeductType = Context.getAutoDeductType();
   TypeLocBuilder TLB;
-  TLB.pushTypeSpec(DeductType).setNameLoc(Loc);
+  AutoTypeLoc TL = TLB.push<AutoTypeLoc>(DeductType);
+  TL.setNameLoc(Loc);
   if (ByRef) {
     DeductType = BuildReferenceType(DeductType, true, Loc, Id);
     assert(!DeductType.isNull() && "can't build reference to auto");
diff --git a/clang/lib/Sema/SemaLookup.cpp b/clang/lib/Sema/SemaLookup.cpp
index 0ed51de0cc13..8d96404a5c27 100644
--- a/clang/lib/Sema/SemaLookup.cpp
+++ b/clang/lib/Sema/SemaLookup.cpp
@@ -1575,7 +1575,9 @@ llvm::DenseSet<Module*> &Sema::getLookupModules() {
   unsigned N = CodeSynthesisContexts.size();
   for (unsigned I = CodeSynthesisContextLookupModules.size();
        I != N; ++I) {
-    Module *M = getDefiningModule(*this, CodeSynthesisContexts[I].Entity);
+    Module *M = CodeSynthesisContexts[I].Entity ?
+                getDefiningModule(*this, CodeSynthesisContexts[I].Entity) :
+                nullptr;
     if (M && !LookupModulesCache.insert(M).second)
       M = nullptr;
     CodeSynthesisContextLookupModules.push_back(M);
diff --git a/clang/lib/Sema/SemaStmt.cpp b/clang/lib/Sema/SemaStmt.cpp
index d6c3af9e84c8..ff6481006280 100644
--- a/clang/lib/Sema/SemaStmt.cpp
+++ b/clang/lib/Sema/SemaStmt.cpp
@@ -2838,6 +2838,9 @@ static void DiagnoseForRangeConstVariableCopies(Sema &SemaRef,
 ///    Suggest "const foo &x" to prevent the copy.
 static void DiagnoseForRangeVariableCopies(Sema &SemaRef,
                                            const CXXForRangeStmt *ForStmt) {
+  if (SemaRef.inTemplateInstantiation())
+    return;
+
   if (SemaRef.Diags.isIgnored(diag::warn_for_range_const_reference_copy,
                               ForStmt->getBeginLoc()) &&
       SemaRef.Diags.isIgnored(diag::warn_for_range_variable_always_copy,
@@ -2860,6 +2863,9 @@ static void DiagnoseForRangeVariableCopies(Sema &SemaRef,
   if (!InitExpr)
     return;
 
+  if (InitExpr->getExprLoc().isMacroID())
+    return;
+
   if (VariableType->isReferenceType()) {
     DiagnoseForRangeReferenceVariableCopies(SemaRef, VD,
                                             ForStmt->getRangeInit()->getType());
diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
index 1184446796eb..f961244da072 100644
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/clang/lib/Sema/SemaTemplate.cpp
@@ -1050,7 +1050,8 @@ makeTemplateArgumentListInfo(Sema &S, TemplateIdAnnotation &TemplateId) {
   return TemplateArgs;
 }
 
-bool Sema::ActOnTypeConstraint(TemplateIdAnnotation *TypeConstr,
+bool Sema::ActOnTypeConstraint(const CXXScopeSpec &SS,
+                               TemplateIdAnnotation *TypeConstr,
                                TemplateTypeParmDecl *ConstrainedParameter,
                                SourceLocation EllipsisLoc) {
   ConceptDecl *CD =
@@ -1080,14 +1081,57 @@ bool Sema::ActOnTypeConstraint(TemplateIdAnnotation *TypeConstr,
         makeTemplateArgumentListInfo(*this, *TypeConstr);
   }
   return AttachTypeConstraint(
-      TypeConstr->SS.isSet() ? TypeConstr->SS.getWithLocInContext(Context) :
-      NestedNameSpecifierLoc(),
+      SS.isSet() ? SS.getWithLocInContext(Context) : NestedNameSpecifierLoc(),
       DeclarationNameInfo(DeclarationName(TypeConstr->Name),
                           TypeConstr->TemplateNameLoc), CD,
       TypeConstr->LAngleLoc.isValid() ? &TemplateArgs : nullptr,
       ConstrainedParameter, EllipsisLoc);
 }
 
+template<typename ArgumentLocAppender>
+static ExprResult formImmediatelyDeclaredConstraint(
+    Sema &S, NestedNameSpecifierLoc NS, DeclarationNameInfo NameInfo,
+    ConceptDecl *NamedConcept, SourceLocation LAngleLoc,
+    SourceLocation RAngleLoc, QualType ConstrainedType,
+    SourceLocation ParamNameLoc, ArgumentLocAppender Appender,
+    SourceLocation EllipsisLoc) {
+
+  TemplateArgumentListInfo ConstraintArgs;
+  ConstraintArgs.addArgument(
+    S.getTrivialTemplateArgumentLoc(TemplateArgument(ConstrainedType),
+                                    /*NTTPType=*/QualType(), ParamNameLoc));
+
+  ConstraintArgs.setRAngleLoc(RAngleLoc);
+  ConstraintArgs.setLAngleLoc(LAngleLoc);
+  Appender(ConstraintArgs);
+
+  // C++2a [temp.param]p4:
+  //     [...] This constraint-expression E is called the immediately-declared
+  //     constraint of T. [...]
+  CXXScopeSpec SS;
+  SS.Adopt(NS);
+  ExprResult ImmediatelyDeclaredConstraint = S.CheckConceptTemplateId(
+      SS, /*TemplateKWLoc=*/SourceLocation(), NameInfo,
+      /*FoundDecl=*/NamedConcept, NamedConcept, &ConstraintArgs);
+  if (ImmediatelyDeclaredConstraint.isInvalid() || !EllipsisLoc.isValid())
+    return ImmediatelyDeclaredConstraint;
+
+  // C++2a [temp.param]p4:
+  //     [...] If T is not a pack, then E is E', otherwise E is (E' && ...).
+  //
+  // We have the following case:
+  //
+  // template<typename T> concept C1 = true;
+  // template<C1... T> struct s1;
+  //
+  // The constraint: (C1<T> && ...)
+  return S.BuildCXXFoldExpr(/*LParenLoc=*/SourceLocation(),
+                            ImmediatelyDeclaredConstraint.get(), BO_LAnd,
+                            EllipsisLoc, /*RHS=*/nullptr,
+                            /*RParenLoc=*/SourceLocation(),
+                            /*NumExpansions=*/None);
+}
+
 /// Attach a type-constraint to a template parameter.
 /// \returns true if an error occured. This can happen if the
 /// immediately-declared constraint could not be formed (e.g. incorrect number
@@ -1106,51 +1150,21 @@ bool Sema::AttachTypeConstraint(NestedNameSpecifierLoc NS,
                                                        *TemplateArgs) : nullptr;
 
   QualType ParamAsArgument(ConstrainedParameter->getTypeForDecl(), 0);
-  TemplateArgumentListInfo ConstraintArgs;
-  ConstraintArgs.addArgument(
-    TemplateArgumentLoc(
-        TemplateArgument(ParamAsArgument),
-        TemplateArgumentLocInfo(
-            Context.getTrivialTypeSourceInfo(ParamAsArgument,
-                ConstrainedParameter->getLocation()))));
-  if (TemplateArgs) {
-    ConstraintArgs.setRAngleLoc(TemplateArgs->getRAngleLoc());
-    ConstraintArgs.setLAngleLoc(TemplateArgs->getLAngleLoc());
-    for (const TemplateArgumentLoc &ArgLoc : TemplateArgs->arguments())
-      ConstraintArgs.addArgument(ArgLoc);
-  }
 
-  // C++2a [temp.param]p4:
-  //     [...] This constraint-expression E is called the immediately-declared
-  //     constraint of T. [...]
-  CXXScopeSpec SS;
-  SS.Adopt(NS);
-  ExprResult ImmediatelyDeclaredConstraint = CheckConceptTemplateId(SS,
-      /*TemplateKWLoc=*/SourceLocation(), NameInfo, /*FoundDecl=*/NamedConcept,
-      NamedConcept, &ConstraintArgs);
+  ExprResult ImmediatelyDeclaredConstraint =
+      formImmediatelyDeclaredConstraint(
+          *this, NS, NameInfo, NamedConcept,
+          TemplateArgs ? TemplateArgs->getLAngleLoc() : SourceLocation(),
+          TemplateArgs ? TemplateArgs->getRAngleLoc() : SourceLocation(),
+          ParamAsArgument, ConstrainedParameter->getLocation(),
+          [&] (TemplateArgumentListInfo &ConstraintArgs) {
+            if (TemplateArgs)
+              for (const auto &ArgLoc : TemplateArgs->arguments())
+                ConstraintArgs.addArgument(ArgLoc);
+          }, EllipsisLoc);
   if (ImmediatelyDeclaredConstraint.isInvalid())
     return true;
 
-  if (ConstrainedParameter->isParameterPack()) {
-    // C++2a [temp.param]p4:
-    //     [...] If T is not a pack, then E is E', otherwise E is (E' && ...).
-    //
-    // We have the following case:
-    //
-    // template<typename T> concept C1 = true;
-    // template<C1... T> struct s1;
-    //
-    // The constraint: (C1<T> && ...)
-    ImmediatelyDeclaredConstraint =
-        BuildCXXFoldExpr(/*LParenLoc=*/SourceLocation(),
-                         ImmediatelyDeclaredConstraint.get(), BO_LAnd,
-                         EllipsisLoc, /*RHS=*/nullptr,
-                         /*RParenLoc=*/SourceLocation(),
-                         /*NumExpansions=*/None).get();
-    if (ImmediatelyDeclaredConstraint.isInvalid())
-      return true;
-  }
-
   ConstrainedParameter->setTypeConstraint(NS, NameInfo,
                                           /*FoundDecl=*/NamedConcept,
                                           NamedConcept, ArgsAsWritten,
@@ -1158,6 +1172,38 @@ bool Sema::AttachTypeConstraint(NestedNameSpecifierLoc NS,
   return false;
 }
 
+bool Sema::AttachTypeConstraint(AutoTypeLoc TL, NonTypeTemplateParmDecl *NTTP,
+                                SourceLocation EllipsisLoc) {
+  if (NTTP->getType() != TL.getType() ||
+      TL.getAutoKeyword() != AutoTypeKeyword::Auto) {
+    Diag(NTTP->getTypeSourceInfo()->getTypeLoc().getBeginLoc(),
+         diag::err_unsupported_placeholder_constraint)
+       << NTTP->getTypeSourceInfo()->getTypeLoc().getSourceRange();
+    return true;
+  }
+  // FIXME: Concepts: This should be the type of the placeholder, but this is
+  // unclear in the wording right now.
+  DeclRefExpr *Ref = BuildDeclRefExpr(NTTP, NTTP->getType(), VK_RValue,
+                                      NTTP->getLocation());
+  if (!Ref)
+    return true;
+  ExprResult ImmediatelyDeclaredConstraint =
+      formImmediatelyDeclaredConstraint(
+          *this, TL.getNestedNameSpecifierLoc(), TL.getConceptNameInfo(),
+          TL.getNamedConcept(), TL.getLAngleLoc(), TL.getRAngleLoc(),
+          BuildDecltypeType(Ref, NTTP->getLocation()), NTTP->getLocation(),
+          [&] (TemplateArgumentListInfo &ConstraintArgs) {
+            for (unsigned I = 0, C = TL.getNumArgs(); I != C; ++I)
+              ConstraintArgs.addArgument(TL.getArgLoc(I));
+          }, EllipsisLoc);
+  if (ImmediatelyDeclaredConstraint.isInvalid() ||
+     !ImmediatelyDeclaredConstraint.isUsable())
+    return true;
+
+  NTTP->setPlaceholderTypeConstraint(ImmediatelyDeclaredConstraint.get());
+  return false;
+}
+
 /// Check that the type of a non-type template parameter is
 /// well-formed.
 ///
@@ -1319,6 +1365,11 @@ NamedDecl *Sema::ActOnNonTypeTemplateParameter(Scope *S, Declarator &D,
       TInfo);
   Param->setAccess(AS_public);
 
+  if (AutoTypeLoc TL = TInfo->getTypeLoc().getContainedAutoTypeLoc())
+    if (TL.isConstrained())
+      if (AttachTypeConstraint(TL, Param, D.getEllipsisLoc()))
+        Invalid = true;
+
   if (Invalid)
     Param->setInvalidDecl();
 
@@ -2762,7 +2813,7 @@ TemplateParameterList *Sema::MatchTemplateParametersToScopeSpecifier(
     SourceLocation DeclStartLoc, SourceLocation DeclLoc, const CXXScopeSpec &SS,
     TemplateIdAnnotation *TemplateId,
     ArrayRef<TemplateParameterList *> ParamLists, bool IsFriend,
-    bool &IsMemberSpecialization, bool &Invalid) {
+    bool &IsMemberSpecialization, bool &Invalid, bool SuppressDiagnostic) {
   IsMemberSpecialization = false;
   Invalid = false;
 
@@ -2870,8 +2921,9 @@ TemplateParameterList *Sema::MatchTemplateParametersToScopeSpecifier(
 
   auto CheckExplicitSpecialization = [&](SourceRange Range, bool Recovery) {
     if (SawNonEmptyTemplateParameterList) {
-      Diag(DeclLoc, diag::err_specialize_member_of_template)
-        << !Recovery << Range;
+      if (!SuppressDiagnostic)
+        Diag(DeclLoc, diag::err_specialize_member_of_template)
+          << !Recovery << Range;
       Invalid = true;
       IsMemberSpecialization = false;
       return true;
@@ -2892,9 +2944,10 @@ TemplateParameterList *Sema::MatchTemplateParametersToScopeSpecifier(
     else
       ExpectedTemplateLoc = DeclStartLoc;
 
-    Diag(DeclLoc, diag::err_template_spec_needs_header)
-      << Range
-      << FixItHint::CreateInsertion(ExpectedTemplateLoc, "template<> ");
+    if (!SuppressDiagnostic)
+      Diag(DeclLoc, diag::err_template_spec_needs_header)
+        << Range
+        << FixItHint::CreateInsertion(ExpectedTemplateLoc, "template<> ");
     return false;
   };
 
@@ -2984,12 +3037,13 @@ TemplateParameterList *Sema::MatchTemplateParametersToScopeSpecifier(
       if (ParamIdx < ParamLists.size()) {
         if (ParamLists[ParamIdx]->size() > 0) {
           // The header has template parameters when it shouldn't. Complain.
-          Diag(ParamLists[ParamIdx]->getTemplateLoc(),
-               diag::err_template_param_list_matches_nontemplate)
-            << T
-            << SourceRange(ParamLists[ParamIdx]->getLAngleLoc(),
-                           ParamLists[ParamIdx]->getRAngleLoc())
-            << getRangeOfTypeInNestedNameSpecifier(Context, T, SS);
+          if (!SuppressDiagnostic)
+            Diag(ParamLists[ParamIdx]->getTemplateLoc(),
+                 diag::err_template_param_list_matches_nontemplate)
+              << T
+              << SourceRange(ParamLists[ParamIdx]->getLAngleLoc(),
+                             ParamLists[ParamIdx]->getRAngleLoc())
+              << getRangeOfTypeInNestedNameSpecifier(Context, T, SS);
           Invalid = true;
           return nullptr;
         }
@@ -3025,7 +3079,7 @@ TemplateParameterList *Sema::MatchTemplateParametersToScopeSpecifier(
         if (ExpectedTemplateParams &&
             !TemplateParameterListsAreEqual(ParamLists[ParamIdx],
                                             ExpectedTemplateParams,
-                                            true, TPL_TemplateMatch))
+                                            !SuppressDiagnostic, TPL_TemplateMatch))
           Invalid = true;
 
         if (!Invalid &&
@@ -3037,9 +3091,10 @@ TemplateParameterList *Sema::MatchTemplateParametersToScopeSpecifier(
         continue;
       }
 
-      Diag(DeclLoc, diag::err_template_spec_needs_template_parameters)
-        << T
-        << getRangeOfTypeInNestedNameSpecifier(Context, T, SS);
+      if (!SuppressDiagnostic)
+        Diag(DeclLoc, diag::err_template_spec_needs_template_parameters)
+          << T
+          << getRangeOfTypeInNestedNameSpecifier(Context, T, SS);
       Invalid = true;
       continue;
     }
@@ -3075,16 +3130,18 @@ TemplateParameterList *Sema::MatchTemplateParametersToScopeSpecifier(
         AllExplicitSpecHeaders = false;
     }
 
-    Diag(ParamLists[ParamIdx]->getTemplateLoc(),
-         AllExplicitSpecHeaders ? diag::warn_template_spec_extra_headers
-                                : diag::err_template_spec_extra_headers)
-        << SourceRange(ParamLists[ParamIdx]->getTemplateLoc(),
-                       ParamLists[ParamLists.size() - 2]->getRAngleLoc());
+    if (!SuppressDiagnostic)
+      Diag(ParamLists[ParamIdx]->getTemplateLoc(),
+           AllExplicitSpecHeaders ? diag::warn_template_spec_extra_headers
+                                  : diag::err_template_spec_extra_headers)
+          << SourceRange(ParamLists[ParamIdx]->getTemplateLoc(),
+                         ParamLists[ParamLists.size() - 2]->getRAngleLoc());
 
     // If there was a specialization somewhere, such that 'template<>' is
     // not required, and there were any 'template<>' headers, note where the
     // specialization occurred.
-    if (ExplicitSpecLoc.isValid() && HasAnyExplicitSpecHeader)
+    if (ExplicitSpecLoc.isValid() && HasAnyExplicitSpecHeader &&
+        !SuppressDiagnostic)
       Diag(ExplicitSpecLoc,
            diag::note_explicit_template_spec_does_not_need_header)
         << NestedTypes.back();
@@ -4044,7 +4101,7 @@ DeclResult Sema::ActOnVarTemplateSpecialization(
 
     if (isSameAsPrimaryTemplate(VarTemplate->getTemplateParameters(),
                                 Converted) &&
-        (!Context.getLangOpts().ConceptsTS ||
+        (!Context.getLangOpts().CPlusPlus2a ||
          !TemplateParams->hasAssociatedConstraints())) {
       // C++ [temp.class.spec]p9b3:
       //
@@ -6530,7 +6587,12 @@ ExprResult Sema::CheckTemplateArgument(NonTypeTemplateParmDecl *Param,
       DeductionArg = PE->getPattern();
     if (DeduceAutoType(
             Context.getTrivialTypeSourceInfo(ParamType, Param->getLocation()),
-            DeductionArg, ParamType, Depth) == DAR_Failed) {
+            DeductionArg, ParamType, Depth,
+            // We do not check constraints right now because the
+            // immediately-declared constraint of the auto type is also an
+            // associated constraint, and will be checked along with the other
+            // associated constraints after checking the template argument list.
+            /*IgnoreConstraints=*/true) == DAR_Failed) {
       Diag(Arg->getExprLoc(),
            diag::err_non_type_template_parm_type_deduction_failure)
         << Param->getDeclName() << Param->getType() << Arg->getType()
@@ -7102,6 +7164,11 @@ bool Sema::CheckTemplateTemplateArgument(TemplateTemplateParmDecl *Param,
       //   [temp.constr.order].
       SmallVector<const Expr *, 3> ParamsAC, TemplateAC;
       Params->getAssociatedConstraints(ParamsAC);
+      // C++2a[temp.arg.template]p3
+      //   [...] In this comparison, if P is unconstrained, the constraints on A
+      //   are not considered.
+      if (ParamsAC.empty())
+        return false;
       Template->getAssociatedConstraints(TemplateAC);
       bool IsParamAtLeastAsConstrained;
       if (IsAtLeastAsConstrained(Param, ParamsAC, Template, TemplateAC,
@@ -7872,13 +7939,11 @@ bool Sema::CheckTemplatePartialSpecializationArgs(
 
 DeclResult Sema::ActOnClassTemplateSpecialization(
     Scope *S, unsigned TagSpec, TagUseKind TUK, SourceLocation KWLoc,
-    SourceLocation ModulePrivateLoc, TemplateIdAnnotation &TemplateId,
-    const ParsedAttributesView &Attr,
+    SourceLocation ModulePrivateLoc, CXXScopeSpec &SS,
+    TemplateIdAnnotation &TemplateId, const ParsedAttributesView &Attr,
     MultiTemplateParamsArg TemplateParameterLists, SkipBodyInfo *SkipBody) {
   assert(TUK != TUK_Reference && "References are not specializations");
 
-  CXXScopeSpec &SS = TemplateId.SS;
-
   // NOTE: KWLoc is the location of the tag keyword. This will instead
   // store the location of the outermost template keyword in the declaration.
   SourceLocation TemplateKWLoc = TemplateParameterLists.size() > 0
@@ -8048,7 +8113,7 @@ DeclResult Sema::ActOnClassTemplateSpecialization(
 
     if (Context.hasSameType(CanonType,
                         ClassTemplate->getInjectedClassNameSpecialization()) &&
-        (!Context.getLangOpts().ConceptsTS ||
+        (!Context.getLangOpts().CPlusPlus2a ||
          !TemplateParams->hasAssociatedConstraints())) {
       // C++ [temp.class.spec]p9b3:
       //
@@ -10012,24 +10077,12 @@ Sema::ActOnTypenameType(Scope *S, SourceLocation TypenameLoc,
       << FixItHint::CreateRemoval(TypenameLoc);
 
   NestedNameSpecifierLoc QualifierLoc = SS.getWithLocInContext(Context);
+  TypeSourceInfo *TSI = nullptr;
   QualType T = CheckTypenameType(TypenameLoc.isValid()? ETK_Typename : ETK_None,
-                                 TypenameLoc, QualifierLoc, II, IdLoc);
+                                 TypenameLoc, QualifierLoc, II, IdLoc, &TSI,
+                                 /*DeducedTSTContext=*/true);
   if (T.isNull())
     return true;
-
-  TypeSourceInfo *TSI = Context.CreateTypeSourceInfo(T);
-  if (isa<DependentNameType>(T)) {
-    DependentNameTypeLoc TL = TSI->getTypeLoc().castAs<DependentNameTypeLoc>();
-    TL.setElaboratedKeywordLoc(TypenameLoc);
-    TL.setQualifierLoc(QualifierLoc);
-    TL.setNameLoc(IdLoc);
-  } else {
-    ElaboratedTypeLoc TL = TSI->getTypeLoc().castAs<ElaboratedTypeLoc>();
-    TL.setElaboratedKeywordLoc(TypenameLoc);
-    TL.setQualifierLoc(QualifierLoc);
-    TL.getNamedTypeLoc().castAs<TypeSpecTypeLoc>().setNameLoc(IdLoc);
-  }
-
   return CreateParsedType(T, TSI);
 }
 
@@ -10166,6 +10219,35 @@ static bool isEnableIf(NestedNameSpecifierLoc NNS, const IdentifierInfo &II,
   return true;
 }
 
+QualType
+Sema::CheckTypenameType(ElaboratedTypeKeyword Keyword,
+                        SourceLocation KeywordLoc,
+                        NestedNameSpecifierLoc QualifierLoc,
+                        const IdentifierInfo &II,
+                        SourceLocation IILoc,
+                        TypeSourceInfo **TSI,
+                        bool DeducedTSTContext) {
+  QualType T = CheckTypenameType(Keyword, KeywordLoc, QualifierLoc, II, IILoc,
+                                 DeducedTSTContext);
+  if (T.isNull())
+    return QualType();
+
+  *TSI = Context.CreateTypeSourceInfo(T);
+  if (isa<DependentNameType>(T)) {
+    DependentNameTypeLoc TL =
+        (*TSI)->getTypeLoc().castAs<DependentNameTypeLoc>();
+    TL.setElaboratedKeywordLoc(KeywordLoc);
+    TL.setQualifierLoc(QualifierLoc);
+    TL.setNameLoc(IILoc);
+  } else {
+    ElaboratedTypeLoc TL = (*TSI)->getTypeLoc().castAs<ElaboratedTypeLoc>();
+    TL.setElaboratedKeywordLoc(KeywordLoc);
+    TL.setQualifierLoc(QualifierLoc);
+    TL.getNamedTypeLoc().castAs<TypeSpecTypeLoc>().setNameLoc(IILoc);
+  }
+  return T;
+}
+
 /// Build the type that describes a C++ typename specifier,
 /// e.g., "typename T::type".
 QualType
@@ -10173,32 +10255,38 @@ Sema::CheckTypenameType(ElaboratedTypeKeyword Keyword,
                         SourceLocation KeywordLoc,
                         NestedNameSpecifierLoc QualifierLoc,
                         const IdentifierInfo &II,
-                        SourceLocation IILoc) {
+                        SourceLocation IILoc, bool DeducedTSTContext) {
   CXXScopeSpec SS;
   SS.Adopt(QualifierLoc);
 
-  DeclContext *Ctx = computeDeclContext(SS);
-  if (!Ctx) {
-    // If the nested-name-specifier is dependent and couldn't be
-    // resolved to a type, build a typename type.
-    assert(QualifierLoc.getNestedNameSpecifier()->isDependent());
-    return Context.getDependentNameType(Keyword,
-                                        QualifierLoc.getNestedNameSpecifier(),
-                                        &II);
+  DeclContext *Ctx = nullptr;
+  if (QualifierLoc) {
+    Ctx = computeDeclContext(SS);
+    if (!Ctx) {
+      // If the nested-name-specifier is dependent and couldn't be
+      // resolved to a type, build a typename type.
+      assert(QualifierLoc.getNestedNameSpecifier()->isDependent());
+      return Context.getDependentNameType(Keyword,
+                                          QualifierLoc.getNestedNameSpecifier(),
+                                          &II);
+    }
+
+    // If the nested-name-specifier refers to the current instantiation,
+    // the "typename" keyword itself is superfluous. In C++03, the
+    // program is actually ill-formed. However, DR 382 (in C++0x CD1)
+    // allows such extraneous "typename" keywords, and we retroactively
+    // apply this DR to C++03 code with only a warning. In any case we continue.
+
+    if (RequireCompleteDeclContext(SS, Ctx))
+      return QualType();
   }
 
-  // If the nested-name-specifier refers to the current instantiation,
-  // the "typename" keyword itself is superfluous. In C++03, the
-  // program is actually ill-formed. However, DR 382 (in C++0x CD1)
-  // allows such extraneous "typename" keywords, and we retroactively
-  // apply this DR to C++03 code with only a warning. In any case we continue.
-
-  if (RequireCompleteDeclContext(SS, Ctx))
-    return QualType();
-
   DeclarationName Name(&II);
   LookupResult Result(*this, Name, IILoc, LookupOrdinaryName);
-  LookupQualifiedName(Result, Ctx, SS);
+  if (Ctx)
+    LookupQualifiedName(Result, Ctx, SS);
+  else
+    LookupName(Result, CurScope);
   unsigned DiagID = 0;
   Decl *Referenced = nullptr;
   switch (Result.getResultKind()) {
@@ -10207,7 +10295,7 @@ Sema::CheckTypenameType(ElaboratedTypeKeyword Keyword,
     // a more specific diagnostic.
     SourceRange CondRange;
     Expr *Cond = nullptr;
-    if (isEnableIf(QualifierLoc, II, CondRange, Cond)) {
+    if (Ctx && isEnableIf(QualifierLoc, II, CondRange, Cond)) {
       // If we have a condition, narrow it down to the specific failed
       // condition.
       if (Cond) {
@@ -10223,12 +10311,14 @@ Sema::CheckTypenameType(ElaboratedTypeKeyword Keyword,
         return QualType();
       }
 
-      Diag(CondRange.getBegin(), diag::err_typename_nested_not_found_enable_if)
+      Diag(CondRange.getBegin(),
+           diag::err_typename_nested_not_found_enable_if)
           << Ctx << CondRange;
       return QualType();
     }
 
-    DiagID = diag::err_typename_nested_not_found;
+    DiagID = Ctx ? diag::err_typename_nested_not_found
+                 : diag::err_unknown_typename;
     break;
   }
 
@@ -10294,6 +10384,19 @@ Sema::CheckTypenameType(ElaboratedTypeKeyword Keyword,
     //   is a placeholder for a deduced class type [...].
     if (getLangOpts().CPlusPlus17) {
       if (auto *TD = getAsTypeTemplateDecl(Result.getFoundDecl())) {
+        if (!DeducedTSTContext) {
+          QualType T(QualifierLoc
+                         ? QualifierLoc.getNestedNameSpecifier()->getAsType()
+                         : nullptr, 0);
+          if (!T.isNull())
+            Diag(IILoc, diag::err_dependent_deduced_tst)
+              << (int)getTemplateNameKindForDiagnostics(TemplateName(TD)) << T;
+          else
+            Diag(IILoc, diag::err_deduced_tst)
+              << (int)getTemplateNameKindForDiagnostics(TemplateName(TD));
+          Diag(TD->getLocation(), diag::note_template_decl_here);
+          return QualType();
+        }
         return Context.getElaboratedType(
             Keyword, QualifierLoc.getNestedNameSpecifier(),
             Context.getDeducedTemplateSpecializationType(TemplateName(TD),
@@ -10301,12 +10404,14 @@ Sema::CheckTypenameType(ElaboratedTypeKeyword Keyword,
       }
     }
 
-    DiagID = diag::err_typename_nested_not_type;
+    DiagID = Ctx ? diag::err_typename_nested_not_type
+                 : diag::err_typename_not_type;
     Referenced = Result.getFoundDecl();
     break;
 
   case LookupResult::FoundOverloaded:
-    DiagID = diag::err_typename_nested_not_type;
+    DiagID = Ctx ? diag::err_typename_nested_not_type
+                 : diag::err_typename_not_type;
     Referenced = *Result.begin();
     break;
 
@@ -10318,9 +10423,14 @@ Sema::CheckTypenameType(ElaboratedTypeKeyword Keyword,
   // type. Emit an appropriate diagnostic and return an error.
   SourceRange FullRange(KeywordLoc.isValid() ? KeywordLoc : SS.getBeginLoc(),
                         IILoc);
-  Diag(IILoc, DiagID) << FullRange << Name << Ctx;
+  if (Ctx)
+    Diag(IILoc, DiagID) << FullRange << Name << Ctx;
+  else
+    Diag(IILoc, DiagID) << FullRange << Name;
   if (Referenced)
-    Diag(Referenced->getLocation(), diag::note_typename_refers_here)
+    Diag(Referenced->getLocation(),
+         Ctx ? diag::note_typename_member_refers_here
+             : diag::note_typename_refers_here)
       << Name;
   return QualType();
 }
diff --git a/clang/lib/Sema/SemaTemplateDeduction.cpp b/clang/lib/Sema/SemaTemplateDeduction.cpp
index 1b9f1b2144d1..394c81c82794 100644
--- a/clang/lib/Sema/SemaTemplateDeduction.cpp
+++ b/clang/lib/Sema/SemaTemplateDeduction.cpp
@@ -724,38 +724,48 @@ private:
     // Compute the set of template parameter indices that correspond to
     // parameter packs expanded by the pack expansion.
     llvm::SmallBitVector SawIndices(TemplateParams->size());
+    llvm::SmallVector<TemplateArgument, 4> ExtraDeductions;
 
     auto AddPack = [&](unsigned Index) {
       if (SawIndices[Index])
         return;
       SawIndices[Index] = true;
       addPack(Index);
+
+      // Deducing a parameter pack that is a pack expansion also constrains the
+      // packs appearing in that parameter to have the same deduced arity. Also,
+      // in C++17 onwards, deducing a non-type template parameter deduces its
+      // type, so we need to collect the pending deduced values for those packs.
+      if (auto *NTTP = dyn_cast<NonTypeTemplateParmDecl>(
+              TemplateParams->getParam(Index))) {
+        if (auto *Expansion = dyn_cast<PackExpansionType>(NTTP->getType()))
+          ExtraDeductions.push_back(Expansion->getPattern());
+      }
+      // FIXME: Also collect the unexpanded packs in any type and template
+      // parameter packs that are pack expansions.
     };
 
-    // First look for unexpanded packs in the pattern.
-    SmallVector<UnexpandedParameterPack, 2> Unexpanded;
-    S.collectUnexpandedParameterPacks(Pattern, Unexpanded);
-    for (unsigned I = 0, N = Unexpanded.size(); I != N; ++I) {
-      unsigned Depth, Index;
-      std::tie(Depth, Index) = getDepthAndIndex(Unexpanded[I]);
-      if (Depth == Info.getDeducedDepth())
-        AddPack(Index);
-    }
+    auto Collect = [&](TemplateArgument Pattern) {
+      SmallVector<UnexpandedParameterPack, 2> Unexpanded;
+      S.collectUnexpandedParameterPacks(Pattern, Unexpanded);
+      for (unsigned I = 0, N = Unexpanded.size(); I != N; ++I) {
+        unsigned Depth, Index;
+        std::tie(Depth, Index) = getDepthAndIndex(Unexpanded[I]);
+        if (Depth == Info.getDeducedDepth())
+          AddPack(Index);
+      }
+    };
+
+    // Look for unexpanded packs in the pattern.
+    Collect(Pattern);
     assert(!Packs.empty() && "Pack expansion without unexpanded packs?");
 
     unsigned NumNamedPacks = Packs.size();
 
-    // We can also have deduced template parameters that do not actually
-    // appear in the pattern, but can be deduced by it (the type of a non-type
-    // template parameter pack, in particular). These won't have prevented us
-    // from partially expanding the pack.
-    llvm::SmallBitVector Used(TemplateParams->size());
-    MarkUsedTemplateParameters(S.Context, Pattern, /*OnlyDeduced*/true,
-                               Info.getDeducedDepth(), Used);
-    for (int Index = Used.find_first(); Index != -1;
-         Index = Used.find_next(Index))
-      if (TemplateParams->getParam(Index)->isParameterPack())
-        AddPack(Index);
+    // Also look for unexpanded packs that are indirectly deduced by deducing
+    // the sizes of the packs in this pattern.
+    while (!ExtraDeductions.empty())
+      Collect(ExtraDeductions.pop_back_val());
 
     return NumNamedPacks;
   }
@@ -4404,9 +4414,10 @@ namespace {
 
       QualType Result = SemaRef.Context.getAutoType(
           Replacement, TL.getTypePtr()->getKeyword(), Replacement.isNull(),
-          ReplacementIsPack);
+          ReplacementIsPack, TL.getTypePtr()->getTypeConstraintConcept(),
+          TL.getTypePtr()->getTypeConstraintArguments());
       auto NewTL = TLB.push<AutoTypeLoc>(Result);
-      NewTL.setNameLoc(TL.getNameLoc());
+      NewTL.copy(TL);
       return Result;
     }
 
@@ -4441,9 +4452,10 @@ namespace {
 
 Sema::DeduceAutoResult
 Sema::DeduceAutoType(TypeSourceInfo *Type, Expr *&Init, QualType &Result,
-                     Optional<unsigned> DependentDeductionDepth) {
+                     Optional<unsigned> DependentDeductionDepth,
+                     bool IgnoreConstraints) {
   return DeduceAutoType(Type->getTypeLoc(), Init, Result,
-                        DependentDeductionDepth);
+                        DependentDeductionDepth, IgnoreConstraints);
 }
 
 /// Attempt to produce an informative diagostic explaining why auto deduction
@@ -4471,6 +4483,49 @@ static bool diagnoseAutoDeductionFailure(Sema &S,
   }
 }
 
+static Sema::DeduceAutoResult
+CheckDeducedPlaceholderConstraints(Sema &S, const AutoType &Type,
+                                   AutoTypeLoc TypeLoc, QualType Deduced) {
+  ConstraintSatisfaction Satisfaction;
+  ConceptDecl *Concept = Type.getTypeConstraintConcept();
+  TemplateArgumentListInfo TemplateArgs(TypeLoc.getLAngleLoc(),
+                                        TypeLoc.getRAngleLoc());
+  TemplateArgs.addArgument(
+      TemplateArgumentLoc(TemplateArgument(Deduced),
+                          S.Context.getTrivialTypeSourceInfo(
+                              Deduced, TypeLoc.getNameLoc())));
+  for (unsigned I = 0, C = TypeLoc.getNumArgs(); I != C; ++I)
+    TemplateArgs.addArgument(TypeLoc.getArgLoc(I));
+
+  llvm::SmallVector<TemplateArgument, 4> Converted;
+  if (S.CheckTemplateArgumentList(Concept, SourceLocation(), TemplateArgs,
+                                  /*PartialTemplateArgs=*/false, Converted))
+    return Sema::DAR_FailedAlreadyDiagnosed;
+  if (S.CheckConstraintSatisfaction(Concept, {Concept->getConstraintExpr()},
+                                    Converted, TypeLoc.getLocalSourceRange(),
+                                    Satisfaction))
+    return Sema::DAR_FailedAlreadyDiagnosed;
+  if (!Satisfaction.IsSatisfied) {
+    std::string Buf;
+    llvm::raw_string_ostream OS(Buf);
+    OS << "'" << Concept->getName();
+    if (TypeLoc.hasExplicitTemplateArgs()) {
+      OS << "<";
+      for (const auto &Arg : Type.getTypeConstraintArguments())
+        Arg.print(S.getPrintingPolicy(), OS);
+      OS << ">";
+    }
+    OS << "'";
+    OS.flush();
+    S.Diag(TypeLoc.getConceptNameLoc(),
+           diag::err_placeholder_constraints_not_satisfied)
+         << Deduced << Buf << TypeLoc.getLocalSourceRange();
+    S.DiagnoseUnsatisfiedConstraint(Satisfaction);
+    return Sema::DAR_FailedAlreadyDiagnosed;
+  }
+  return Sema::DAR_Succeeded;
+}
+
 /// Deduce the type for an auto type-specifier (C++11 [dcl.spec.auto]p6)
 ///
 /// Note that this is done even if the initializer is dependent. (This is
@@ -4485,9 +4540,12 @@ static bool diagnoseAutoDeductionFailure(Sema &S,
 ///        dependent cases. This is necessary for template partial ordering with
 ///        'auto' template parameters. The value specified is the template
 ///        parameter depth at which we should perform 'auto' deduction.
+/// \param IgnoreConstraints Set if we should not fail if the deduced type does
+///                          not satisfy the type-constraint in the auto type.
 Sema::DeduceAutoResult
 Sema::DeduceAutoType(TypeLoc Type, Expr *&Init, QualType &Result,
-                     Optional<unsigned> DependentDeductionDepth) {
+                     Optional<unsigned> DependentDeductionDepth,
+                     bool IgnoreConstraints) {
   if (Init->getType()->isNonOverloadPlaceholderType()) {
     ExprResult NonPlaceholder = CheckPlaceholderExpr(Init);
     if (NonPlaceholder.isInvalid())
@@ -4528,6 +4586,14 @@ Sema::DeduceAutoType(TypeLoc Type, Expr *&Init, QualType &Result,
         return DAR_FailedAlreadyDiagnosed;
       // FIXME: Support a non-canonical deduced type for 'auto'.
       Deduced = Context.getCanonicalType(Deduced);
+      if (AT->isConstrained() && !IgnoreConstraints) {
+        auto ConstraintsResult =
+            CheckDeducedPlaceholderConstraints(*this, *AT,
+                                               Type.getContainedAutoTypeLoc(),
+                                               Deduced);
+        if (ConstraintsResult != DAR_Succeeded)
+          return ConstraintsResult;
+      }
       Result = SubstituteDeducedTypeTransform(*this, Deduced).Apply(Type);
       if (Result.isNull())
         return DAR_FailedAlreadyDiagnosed;
@@ -4635,6 +4701,17 @@ Sema::DeduceAutoType(TypeLoc Type, Expr *&Init, QualType &Result,
       return DAR_FailedAlreadyDiagnosed;
   }
 
+  if (const auto *AT = Type.getType()->getAs<AutoType>()) {
+    if (AT->isConstrained() && !IgnoreConstraints) {
+      auto ConstraintsResult =
+          CheckDeducedPlaceholderConstraints(*this, *AT,
+                                             Type.getContainedAutoTypeLoc(),
+                                             DeducedType);
+      if (ConstraintsResult != DAR_Succeeded)
+        return ConstraintsResult;
+    }
+  }
+
   Result = SubstituteDeducedTypeTransform(*this, DeducedType).Apply(Type);
   if (Result.isNull())
     return DAR_FailedAlreadyDiagnosed;
diff --git a/clang/lib/Sema/SemaTemplateInstantiate.cpp b/clang/lib/Sema/SemaTemplateInstantiate.cpp
index af41e231134d..39bc28d62305 100644
--- a/clang/lib/Sema/SemaTemplateInstantiate.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiate.cpp
@@ -26,6 +26,7 @@
 #include "clang/Sema/Template.h"
 #include "clang/Sema/TemplateDeduction.h"
 #include "clang/Sema/TemplateInstCallback.h"
+#include "clang/Sema/SemaConcept.h"
 #include "llvm/Support/TimeProfiler.h"
 
 using namespace clang;
@@ -199,8 +200,10 @@ bool Sema::CodeSynthesisContext::isInstantiationRecord() const {
   case DeducedTemplateArgumentSubstitution:
   case PriorTemplateArgumentSubstitution:
   case ConstraintsCheck:
+  case NestedRequirementConstraintsCheck:
     return true;
 
+  case RequirementInstantiation:
   case DefaultTemplateArgumentChecking:
   case DeclaringSpecialMember:
   case DeclaringImplicitEqualityComparison:
@@ -247,7 +250,7 @@ Sema::InstantiatingTemplate::InstantiatingTemplate(
     Inst.InstantiationRange = InstantiationRange;
     SemaRef.pushCodeSynthesisContext(Inst);
 
-    AlreadyInstantiating =
+    AlreadyInstantiating = !Inst.Entity ? false :
         !SemaRef.InstantiatingSpecializations
              .insert(std::make_pair(Inst.Entity->getCanonicalDecl(), Inst.Kind))
              .second;
@@ -366,6 +369,26 @@ Sema::InstantiatingTemplate::InstantiatingTemplate(
 
 Sema::InstantiatingTemplate::InstantiatingTemplate(
     Sema &SemaRef, SourceLocation PointOfInstantiation,
+    concepts::Requirement *Req, sema::TemplateDeductionInfo &DeductionInfo,
+    SourceRange InstantiationRange)
+    : InstantiatingTemplate(
+          SemaRef, CodeSynthesisContext::RequirementInstantiation,
+          PointOfInstantiation, InstantiationRange, /*Entity=*/nullptr,
+          /*Template=*/nullptr, /*TemplateArgs=*/None, &DeductionInfo) {}
+
+
+Sema::InstantiatingTemplate::InstantiatingTemplate(
+    Sema &SemaRef, SourceLocation PointOfInstantiation,
+    concepts::NestedRequirement *Req, ConstraintsCheck,
+    SourceRange InstantiationRange)
+    : InstantiatingTemplate(
+          SemaRef, CodeSynthesisContext::NestedRequirementConstraintsCheck,
+          PointOfInstantiation, InstantiationRange, /*Entity=*/nullptr,
+          /*Template=*/nullptr, /*TemplateArgs=*/None) {}
+
+
+Sema::InstantiatingTemplate::InstantiatingTemplate(
+    Sema &SemaRef, SourceLocation PointOfInstantiation,
     ConstraintsCheck, NamedDecl *Template,
     ArrayRef<TemplateArgument> TemplateArgs, SourceRange InstantiationRange)
     : InstantiatingTemplate(
@@ -446,8 +469,9 @@ void Sema::InstantiatingTemplate::Clear() {
   if (!Invalid) {
     if (!AlreadyInstantiating) {
       auto &Active = SemaRef.CodeSynthesisContexts.back();
-      SemaRef.InstantiatingSpecializations.erase(
-          std::make_pair(Active.Entity, Active.Kind));
+      if (Active.Entity)
+        SemaRef.InstantiatingSpecializations.erase(
+            std::make_pair(Active.Entity, Active.Kind));
     }
 
     atTemplateEnd(SemaRef.TemplateInstCallbacks, SemaRef,
@@ -684,6 +708,18 @@ void Sema::PrintInstantiationStack() {
         << Active->InstantiationRange;
       break;
 
+    case CodeSynthesisContext::RequirementInstantiation:
+      Diags.Report(Active->PointOfInstantiation,
+                   diag::note_template_requirement_instantiation_here)
+        << Active->InstantiationRange;
+      break;
+
+    case CodeSynthesisContext::NestedRequirementConstraintsCheck:
+      Diags.Report(Active->PointOfInstantiation,
+                   diag::note_nested_requirement_here)
+        << Active->InstantiationRange;
+      break;
+
     case CodeSynthesisContext::DeclaringSpecialMember:
       Diags.Report(Active->PointOfInstantiation,
                    diag::note_in_declaration_of_implicit_special_member)
@@ -788,6 +824,7 @@ Optional<TemplateDeductionInfo *> Sema::isSFINAEContext() const {
     case CodeSynthesisContext::ConstraintsCheck:
     case CodeSynthesisContext::ParameterMappingSubstitution:
     case CodeSynthesisContext::ConstraintNormalization:
+    case CodeSynthesisContext::NestedRequirementConstraintsCheck:
       // This is a template instantiation, so there is no SFINAE.
       return None;
 
@@ -802,9 +839,10 @@ Optional<TemplateDeductionInfo *> Sema::isSFINAEContext() const {
     case CodeSynthesisContext::ExplicitTemplateArgumentSubstitution:
     case CodeSynthesisContext::DeducedTemplateArgumentSubstitution:
     case CodeSynthesisContext::ConstraintSubstitution:
-      // We're either substituting explicitly-specified template arguments
-      // or deduced template arguments or a constraint expression, so SFINAE
-      // applies.
+    case CodeSynthesisContext::RequirementInstantiation:
+      // We're either substituting explicitly-specified template arguments,
+      // deduced template arguments, a constraint expression or a requirement
+      // in a requires expression, so SFINAE applies.
       assert(Active->DeductionInfo && "Missing deduction info pointer");
       return Active->DeductionInfo;
 
@@ -1056,6 +1094,41 @@ namespace {
       return TreeTransform<TemplateInstantiator>::TransformLambdaExpr(E);
     }
 
+    ExprResult TransformRequiresExpr(RequiresExpr *E) {
+      LocalInstantiationScope Scope(SemaRef, /*CombineWithOuterScope=*/true);
+      return TreeTransform<TemplateInstantiator>::TransformRequiresExpr(E);
+    }
+
+    bool TransformRequiresExprRequirements(
+        ArrayRef<concepts::Requirement *> Reqs,
+        SmallVectorImpl<concepts::Requirement *> &Transformed) {
+      bool SatisfactionDetermined = false;
+      for (concepts::Requirement *Req : Reqs) {
+        concepts::Requirement *TransReq = nullptr;
+        if (!SatisfactionDetermined) {
+          if (auto *TypeReq = dyn_cast<concepts::TypeRequirement>(Req))
+            TransReq = TransformTypeRequirement(TypeReq);
+          else if (auto *ExprReq = dyn_cast<concepts::ExprRequirement>(Req))
+            TransReq = TransformExprRequirement(ExprReq);
+          else
+            TransReq = TransformNestedRequirement(
+                cast<concepts::NestedRequirement>(Req));
+          if (!TransReq)
+            return true;
+          if (!TransReq->isDependent() && !TransReq->isSatisfied())
+            // [expr.prim.req]p6
+            //   [...]  The substitution and semantic constraint checking
+            //   proceeds in lexical order and stops when a condition that
+            //   determines the result of the requires-expression is
+            //   encountered. [..]
+            SatisfactionDetermined = true;
+        } else
+          TransReq = Req;
+        Transformed.push_back(TransReq);
+      }
+      return false;
+    }
+
     TemplateParameterList *TransformTemplateParameterList(
                               TemplateParameterList *OrigTPL)  {
       if (!OrigTPL || !OrigTPL->size()) return OrigTPL;
@@ -1065,6 +1138,14 @@ namespace {
                         /* DeclContext *Owner */ Owner, TemplateArgs);
       return DeclInstantiator.SubstTemplateParams(OrigTPL);
     }
+
+    concepts::TypeRequirement *
+    TransformTypeRequirement(concepts::TypeRequirement *Req);
+    concepts::ExprRequirement *
+    TransformExprRequirement(concepts::ExprRequirement *Req);
+    concepts::NestedRequirement *
+    TransformNestedRequirement(concepts::NestedRequirement *Req);
+
   private:
     ExprResult transformNonTypeTemplateParmRef(NonTypeTemplateParmDecl *parm,
                                                SourceLocation loc,
@@ -1669,6 +1750,163 @@ TemplateInstantiator::TransformSubstTemplateTypeParmPackType(
   return Result;
 }
 
+template<typename EntityPrinter>
+static concepts::Requirement::SubstitutionDiagnostic *
+createSubstDiag(Sema &S, TemplateDeductionInfo &Info, EntityPrinter Printer) {
+  SmallString<128> Message;
+  SourceLocation ErrorLoc;
+  if (Info.hasSFINAEDiagnostic()) {
+    PartialDiagnosticAt PDA(SourceLocation(),
+                            PartialDiagnostic::NullDiagnostic{});
+    Info.takeSFINAEDiagnostic(PDA);
+    PDA.second.EmitToString(S.getDiagnostics(), Message);
+    ErrorLoc = PDA.first;
+  } else {
+    ErrorLoc = Info.getLocation();
+  }
+  char *MessageBuf = new (S.Context) char[Message.size()];
+  std::copy(Message.begin(), Message.end(), MessageBuf);
+  SmallString<128> Entity;
+  llvm::raw_svector_ostream OS(Entity);
+  Printer(OS);
+  char *EntityBuf = new (S.Context) char[Entity.size()];
+  std::copy(Entity.begin(), Entity.end(), EntityBuf);
+  return new (S.Context) concepts::Requirement::SubstitutionDiagnostic{
+      StringRef(EntityBuf, Entity.size()), ErrorLoc,
+      StringRef(MessageBuf, Message.size())};
+}
+
+concepts::TypeRequirement *
+TemplateInstantiator::TransformTypeRequirement(concepts::TypeRequirement *Req) {
+  if (!Req->isDependent() && !AlwaysRebuild())
+    return Req;
+  if (Req->isSubstitutionFailure()) {
+    if (AlwaysRebuild())
+      return RebuildTypeRequirement(
+              Req->getSubstitutionDiagnostic());
+    return Req;
+  }
+
+  Sema::SFINAETrap Trap(SemaRef);
+  TemplateDeductionInfo Info(Req->getType()->getTypeLoc().getBeginLoc());
+  Sema::InstantiatingTemplate TypeInst(SemaRef,
+      Req->getType()->getTypeLoc().getBeginLoc(), Req, Info,
+      Req->getType()->getTypeLoc().getSourceRange());
+  if (TypeInst.isInvalid())
+    return nullptr;
+  TypeSourceInfo *TransType = TransformType(Req->getType());
+  if (!TransType || Trap.hasErrorOccurred())
+    return RebuildTypeRequirement(createSubstDiag(SemaRef, Info,
+        [&] (llvm::raw_ostream& OS) {
+            Req->getType()->getType().print(OS, SemaRef.getPrintingPolicy());
+        }));
+  return RebuildTypeRequirement(TransType);
+}
+
+concepts::ExprRequirement *
+TemplateInstantiator::TransformExprRequirement(concepts::ExprRequirement *Req) {
+  if (!Req->isDependent() && !AlwaysRebuild())
+    return Req;
+
+  Sema::SFINAETrap Trap(SemaRef);
+  TemplateDeductionInfo Info(Req->getExpr()->getBeginLoc());
+
+  llvm::PointerUnion<Expr *, concepts::Requirement::SubstitutionDiagnostic *>
+      TransExpr;
+  if (Req->isExprSubstitutionFailure())
+    TransExpr = Req->getExprSubstitutionDiagnostic();
+  else {
+    Sema::InstantiatingTemplate ExprInst(SemaRef, Req->getExpr()->getBeginLoc(),
+                                         Req, Info,
+                                         Req->getExpr()->getSourceRange());
+    if (ExprInst.isInvalid())
+      return nullptr;
+    ExprResult TransExprRes = TransformExpr(Req->getExpr());
+    if (TransExprRes.isInvalid() || Trap.hasErrorOccurred())
+      TransExpr = createSubstDiag(SemaRef, Info,
+          [&] (llvm::raw_ostream& OS) {
+              Req->getExpr()->printPretty(OS, nullptr,
+                                          SemaRef.getPrintingPolicy());
+          });
+    else
+      TransExpr = TransExprRes.get();
+  }
+
+  llvm::Optional<concepts::ExprRequirement::ReturnTypeRequirement> TransRetReq;
+  const auto &RetReq = Req->getReturnTypeRequirement();
+  if (RetReq.isEmpty())
+    TransRetReq.emplace();
+  else if (RetReq.isSubstitutionFailure())
+    TransRetReq.emplace(RetReq.getSubstitutionDiagnostic());
+  else if (RetReq.isTypeConstraint()) {
+    TemplateParameterList *OrigTPL =
+        RetReq.getTypeConstraintTemplateParameterList();
+    Sema::InstantiatingTemplate TPLInst(SemaRef, OrigTPL->getTemplateLoc(),
+                                        Req, Info, OrigTPL->getSourceRange());
+    if (TPLInst.isInvalid())
+      return nullptr;
+    TemplateParameterList *TPL =
+        TransformTemplateParameterList(OrigTPL);
+    if (!TPL)
+      TransRetReq.emplace(createSubstDiag(SemaRef, Info,
+          [&] (llvm::raw_ostream& OS) {
+              RetReq.getTypeConstraint()->getImmediatelyDeclaredConstraint()
+                  ->printPretty(OS, nullptr, SemaRef.getPrintingPolicy());
+          }));
+    else {
+      TPLInst.Clear();
+      TransRetReq.emplace(TPL);
+    }
+  }
+  assert(TransRetReq.hasValue() &&
+         "All code paths leading here must set TransRetReq");
+  if (Expr *E = TransExpr.dyn_cast<Expr *>())
+    return RebuildExprRequirement(E, Req->isSimple(), Req->getNoexceptLoc(),
+                                  std::move(*TransRetReq));
+  return RebuildExprRequirement(
+      TransExpr.get<concepts::Requirement::SubstitutionDiagnostic *>(),
+      Req->isSimple(), Req->getNoexceptLoc(), std::move(*TransRetReq));
+}
+
+concepts::NestedRequirement *
+TemplateInstantiator::TransformNestedRequirement(
+    concepts::NestedRequirement *Req) {
+  if (!Req->isDependent() && !AlwaysRebuild())
+    return Req;
+  if (Req->isSubstitutionFailure()) {
+    if (AlwaysRebuild())
+      return RebuildNestedRequirement(
+          Req->getSubstitutionDiagnostic());
+    return Req;
+  }
+  Sema::InstantiatingTemplate ReqInst(SemaRef,
+      Req->getConstraintExpr()->getBeginLoc(), Req,
+      Sema::InstantiatingTemplate::ConstraintsCheck{},
+      Req->getConstraintExpr()->getSourceRange());
+
+  ExprResult TransConstraint;
+  TemplateDeductionInfo Info(Req->getConstraintExpr()->getBeginLoc());
+  {
+    EnterExpressionEvaluationContext ContextRAII(
+        SemaRef, Sema::ExpressionEvaluationContext::ConstantEvaluated);
+    Sema::SFINAETrap Trap(SemaRef);
+    Sema::InstantiatingTemplate ConstrInst(SemaRef,
+        Req->getConstraintExpr()->getBeginLoc(), Req, Info,
+        Req->getConstraintExpr()->getSourceRange());
+    if (ConstrInst.isInvalid())
+      return nullptr;
+    TransConstraint = TransformExpr(Req->getConstraintExpr());
+    if (TransConstraint.isInvalid() || Trap.hasErrorOccurred())
+      return RebuildNestedRequirement(createSubstDiag(SemaRef, Info,
+          [&] (llvm::raw_ostream& OS) {
+              Req->getConstraintExpr()->printPretty(OS, nullptr,
+                                                    SemaRef.getPrintingPolicy());
+          }));
+  }
+  return RebuildNestedRequirement(TransConstraint.get());
+}
+
+
 /// Perform substitution on the type T with a given set of template
 /// arguments.
 ///
diff --git a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
index 64500d0a26d5..fbbab8f00703 100644
--- a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -1848,6 +1848,8 @@ Decl *TemplateDeclInstantiator::VisitFunctionDecl(
   // FIXME: Concepts: Do not substitute into constraint expressions
   Expr *TrailingRequiresClause = D->getTrailingRequiresClause();
   if (TrailingRequiresClause) {
+    EnterExpressionEvaluationContext ConstantEvaluated(
+        SemaRef, Sema::ExpressionEvaluationContext::Unevaluated);
     ExprResult SubstRC = SemaRef.SubstExpr(TrailingRequiresClause,
                                            TemplateArgs);
     if (SubstRC.isInvalid())
@@ -2186,6 +2188,8 @@ Decl *TemplateDeclInstantiator::VisitCXXMethodDecl(
   // FIXME: Concepts: Do not substitute into constraint expressions
   Expr *TrailingRequiresClause = D->getTrailingRequiresClause();
   if (TrailingRequiresClause) {
+    EnterExpressionEvaluationContext ConstantEvaluated(
+        SemaRef, Sema::ExpressionEvaluationContext::Unevaluated);
     ExprResult SubstRC = SemaRef.SubstExpr(TrailingRequiresClause,
                                            TemplateArgs);
     if (SubstRC.isInvalid())
@@ -2685,6 +2689,16 @@ Decl *TemplateDeclInstantiator::VisitNonTypeTemplateParmDecl(
         D->getDepth() - TemplateArgs.getNumSubstitutedLevels(),
         D->getPosition(), D->getIdentifier(), T, D->isParameterPack(), DI);
 
+  if (AutoTypeLoc AutoLoc = DI->getTypeLoc().getContainedAutoTypeLoc())
+    if (AutoLoc.isConstrained())
+      if (SemaRef.AttachTypeConstraint(
+              AutoLoc, Param,
+              IsExpandedParameterPack
+                ? DI->getTypeLoc().getAs<PackExpansionTypeLoc>()
+                    .getEllipsisLoc()
+                : SourceLocation()))
+        Invalid = true;
+
   Param->setAccess(AS_public);
   Param->setImplicit(D->isImplicit());
   if (Invalid)
@@ -3600,6 +3614,12 @@ Decl *TemplateDeclInstantiator::VisitConceptDecl(ConceptDecl *D) {
   llvm_unreachable("Concept definitions cannot reside inside a template");
 }
 
+Decl *
+TemplateDeclInstantiator::VisitRequiresExprBodyDecl(RequiresExprBodyDecl *D) {
+  return RequiresExprBodyDecl::Create(SemaRef.Context, D->getDeclContext(),
+                                      D->getBeginLoc());
+}
+
 Decl *TemplateDeclInstantiator::VisitDecl(Decl *D) {
   llvm_unreachable("Unexpected decl");
 }
@@ -3713,6 +3733,8 @@ TemplateDeclInstantiator::SubstTemplateParams(TemplateParameterList *L) {
   // checking satisfaction.
   Expr *InstRequiresClause = nullptr;
   if (Expr *E = L->getRequiresClause()) {
+    EnterExpressionEvaluationContext ConstantEvaluated(
+        SemaRef, Sema::ExpressionEvaluationContext::Unevaluated);
     ExprResult Res = SemaRef.SubstExpr(E, TemplateArgs);
     if (Res.isInvalid() || !Res.isUsable()) {
       return nullptr;
@@ -4236,9 +4258,9 @@ bool Sema::CheckInstantiatedFunctionTemplateConstraints(
         MLTAL.getInnermost(), SourceRange());
     if (Inst.isInvalid())
       return true;
-    if (addInstantiatedParametersToScope(*this, Decl,
-                                        Decl->getTemplateInstantiationPattern(),
-                                         Scope, MLTAL))
+    if (addInstantiatedParametersToScope(
+            *this, Decl, Decl->getPrimaryTemplate()->getTemplatedDecl(),
+            Scope, MLTAL))
       return true;
   }
 
diff --git a/clang/lib/Sema/SemaType.cpp b/clang/lib/Sema/SemaType.cpp
index 3884fdae8fe7..93ddd047e09b 100644
--- a/clang/lib/Sema/SemaType.cpp
+++ b/clang/lib/Sema/SemaType.cpp
@@ -11,6 +11,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "TypeLocBuilder.h"
+#include "TreeTransform.h"
 #include "clang/AST/ASTConsumer.h"
 #include "clang/AST/ASTContext.h"
 #include "clang/AST/ASTMutationListener.h"
@@ -27,6 +28,7 @@
 #include "clang/Sema/DeclSpec.h"
 #include "clang/Sema/DelayedDiagnostic.h"
 #include "clang/Sema/Lookup.h"
+#include "clang/Sema/ParsedTemplate.h"
 #include "clang/Sema/ScopeInfo.h"
 #include "clang/Sema/SemaInternal.h"
 #include "clang/Sema/Template.h"
@@ -1251,6 +1253,26 @@ getImageAccess(const ParsedAttributesView &Attrs) {
   return OpenCLAccessAttr::Keyword_read_only;
 }
 
+static QualType ConvertConstrainedAutoDeclSpecToType(Sema &S, DeclSpec &DS,
+                                                     AutoTypeKeyword AutoKW) {
+  assert(DS.isConstrainedAuto());
+  TemplateIdAnnotation *TemplateId = DS.getRepAsTemplateId();
+  TemplateArgumentListInfo TemplateArgsInfo;
+  TemplateArgsInfo.setLAngleLoc(TemplateId->LAngleLoc);
+  TemplateArgsInfo.setRAngleLoc(TemplateId->RAngleLoc);
+  ASTTemplateArgsPtr TemplateArgsPtr(TemplateId->getTemplateArgs(),
+                                     TemplateId->NumArgs);
+  S.translateTemplateArguments(TemplateArgsPtr, TemplateArgsInfo);
+  llvm::SmallVector<TemplateArgument, 8> TemplateArgs;
+  for (auto &ArgLoc : TemplateArgsInfo.arguments())
+    TemplateArgs.push_back(ArgLoc.getArgument());
+  return S.Context.getAutoType(QualType(), AutoTypeKeyword::Auto, false,
+                               /*IsPack=*/false,
+                               cast<ConceptDecl>(TemplateId->Template.get()
+                                                 .getAsTemplateDecl()),
+                               TemplateArgs);
+}
+
 /// Convert the specified declspec to the appropriate type
 /// object.
 /// \param state Specifies the declarator containing the declaration specifier
@@ -1595,6 +1617,11 @@ static QualType ConvertDeclSpecToType(TypeProcessingState &state) {
     break;
 
   case DeclSpec::TST_auto:
+    if (DS.isConstrainedAuto()) {
+      Result = ConvertConstrainedAutoDeclSpecToType(S, DS,
+                                                    AutoTypeKeyword::Auto);
+      break;
+    }
     Result = Context.getAutoType(QualType(), AutoTypeKeyword::Auto, false);
     break;
 
@@ -1603,6 +1630,12 @@ static QualType ConvertDeclSpecToType(TypeProcessingState &state) {
     break;
 
   case DeclSpec::TST_decltype_auto:
+    if (DS.isConstrainedAuto()) {
+      Result =
+          ConvertConstrainedAutoDeclSpecToType(S, DS,
+                                               AutoTypeKeyword::DecltypeAuto);
+      break;
+    }
     Result = Context.getAutoType(QualType(), AutoTypeKeyword::DecltypeAuto,
                                  /*IsDependent*/ false);
     break;
@@ -2921,6 +2954,87 @@ static void diagnoseRedundantReturnTypeQualifiers(Sema &S, QualType RetTy,
                               D.getDeclSpec().getUnalignedSpecLoc());
 }
 
+static void CopyTypeConstraintFromAutoType(Sema &SemaRef, const AutoType *Auto,
+                                           AutoTypeLoc AutoLoc,
+                                           TemplateTypeParmDecl *TP,
+                                           SourceLocation EllipsisLoc) {
+
+  TemplateArgumentListInfo TAL(AutoLoc.getLAngleLoc(), AutoLoc.getRAngleLoc());
+  for (unsigned Idx = 0; Idx < AutoLoc.getNumArgs(); ++Idx)
+    TAL.addArgument(AutoLoc.getArgLoc(Idx));
+
+  SemaRef.AttachTypeConstraint(
+      AutoLoc.getNestedNameSpecifierLoc(), AutoLoc.getConceptNameInfo(),
+      AutoLoc.getNamedConcept(),
+      AutoLoc.hasExplicitTemplateArgs() ? &TAL : nullptr, TP, EllipsisLoc);
+}
+
+static QualType InventTemplateParameter(
+    TypeProcessingState &state, QualType T, TypeSourceInfo *TSI, AutoType *Auto,
+    InventedTemplateParameterInfo &Info) {
+  Sema &S = state.getSema();
+  Declarator &D = state.getDeclarator();
+
+  const unsigned TemplateParameterDepth = Info.AutoTemplateParameterDepth;
+  const unsigned AutoParameterPosition = Info.TemplateParams.size();
+  const bool IsParameterPack = D.hasEllipsis();
+
+  // If auto is mentioned in a lambda parameter or abbreviated function
+  // template context, convert it to a template parameter type.
+
+  // Create the TemplateTypeParmDecl here to retrieve the corresponding
+  // template parameter type. Template parameters are temporarily added
+  // to the TU until the associated TemplateDecl is created.
+  TemplateTypeParmDecl *InventedTemplateParam =
+      TemplateTypeParmDecl::Create(
+          S.Context, S.Context.getTranslationUnitDecl(),
+          /*KeyLoc=*/D.getDeclSpec().getTypeSpecTypeLoc(),
+          /*NameLoc=*/D.getIdentifierLoc(),
+          TemplateParameterDepth, AutoParameterPosition,
+          S.InventAbbreviatedTemplateParameterTypeName(
+              D.getIdentifier(), AutoParameterPosition), false,
+          IsParameterPack, /*HasTypeConstraint=*/Auto->isConstrained());
+  InventedTemplateParam->setImplicit();
+  Info.TemplateParams.push_back(InventedTemplateParam);
+  // Attach type constraints
+  if (Auto->isConstrained()) {
+    if (TSI) {
+      CopyTypeConstraintFromAutoType(
+          S, Auto, TSI->getTypeLoc().getContainedAutoTypeLoc(),
+          InventedTemplateParam, D.getEllipsisLoc());
+    } else {
+      TemplateIdAnnotation *TemplateId = D.getDeclSpec().getRepAsTemplateId();
+      TemplateArgumentListInfo TemplateArgsInfo;
+      if (TemplateId->LAngleLoc.isValid()) {
+        ASTTemplateArgsPtr TemplateArgsPtr(TemplateId->getTemplateArgs(),
+                                           TemplateId->NumArgs);
+        S.translateTemplateArguments(TemplateArgsPtr, TemplateArgsInfo);
+      }
+      S.AttachTypeConstraint(
+          D.getDeclSpec().getTypeSpecScope().getWithLocInContext(S.Context),
+          DeclarationNameInfo(DeclarationName(TemplateId->Name),
+                              TemplateId->TemplateNameLoc),
+          cast<ConceptDecl>(TemplateId->Template.get().getAsTemplateDecl()),
+          TemplateId->LAngleLoc.isValid() ? &TemplateArgsInfo : nullptr,
+          InventedTemplateParam, D.getEllipsisLoc());
+    }
+  }
+
+  // If TSI is nullptr, this is a constrained declspec auto and the type
+  // constraint will be attached later in TypeSpecLocFiller
+
+  // Replace the 'auto' in the function parameter with this invented
+  // template type parameter.
+  // FIXME: Retain some type sugar to indicate that this was written
+  //  as 'auto'?
+  return state.ReplaceAutoType(
+      T, QualType(InventedTemplateParam->getTypeForDecl(), 0));
+}
+
+static TypeSourceInfo *
+GetTypeSourceInfoForDeclarator(TypeProcessingState &State,
+                               QualType T, TypeSourceInfo *ReturnTypeInfo);
+
 static QualType GetDeclSpecTypeForDeclarator(TypeProcessingState &state,
                                              TypeSourceInfo *&ReturnTypeInfo) {
   Sema &SemaRef = state.getSema();
@@ -2991,43 +3105,43 @@ static QualType GetDeclSpecTypeForDeclarator(TypeProcessingState &state,
       break;
     case DeclaratorContext::ObjCParameterContext:
     case DeclaratorContext::ObjCResultContext:
-    case DeclaratorContext::PrototypeContext:
       Error = 0;
       break;
-    case DeclaratorContext::LambdaExprParameterContext:
-      // In C++14, generic lambdas allow 'auto' in their parameters.
-      if (!SemaRef.getLangOpts().CPlusPlus14 ||
-          !Auto || Auto->getKeyword() != AutoTypeKeyword::Auto)
-        Error = 16;
-      else {
-        // If auto is mentioned in a lambda parameter context, convert it to a
-        // template parameter type.
-        sema::LambdaScopeInfo *LSI = SemaRef.getCurLambda();
-        assert(LSI && "No LambdaScopeInfo on the stack!");
-        const unsigned TemplateParameterDepth = LSI->AutoTemplateParameterDepth;
-        const unsigned AutoParameterPosition = LSI->TemplateParams.size();
-        const bool IsParameterPack = D.hasEllipsis();
-
-        // Create the TemplateTypeParmDecl here to retrieve the corresponding
-        // template parameter type. Template parameters are temporarily added
-        // to the TU until the associated TemplateDecl is created.
-        TemplateTypeParmDecl *CorrespondingTemplateParam =
-            TemplateTypeParmDecl::Create(
-                SemaRef.Context, SemaRef.Context.getTranslationUnitDecl(),
-                /*KeyLoc*/ SourceLocation(), /*NameLoc*/ D.getBeginLoc(),
-                TemplateParameterDepth, AutoParameterPosition,
-                /*Identifier*/ nullptr, false, IsParameterPack,
-                /*HasTypeConstraint=*/false);
-        CorrespondingTemplateParam->setImplicit();
-        LSI->TemplateParams.push_back(CorrespondingTemplateParam);
-        // Replace the 'auto' in the function parameter with this invented
-        // template type parameter.
-        // FIXME: Retain some type sugar to indicate that this was written
-        // as 'auto'.
-        T = state.ReplaceAutoType(
-            T, QualType(CorrespondingTemplateParam->getTypeForDecl(), 0));
+    case DeclaratorContext::RequiresExprContext:
+      Error = 22;
+      break;
+    case DeclaratorContext::PrototypeContext:
+    case DeclaratorContext::LambdaExprParameterContext: {
+      InventedTemplateParameterInfo *Info = nullptr;
+      if (D.getContext() == DeclaratorContext::PrototypeContext) {
+        // With concepts we allow 'auto' in function parameters.
+        if (!SemaRef.getLangOpts().CPlusPlus2a || !Auto ||
+            Auto->getKeyword() != AutoTypeKeyword::Auto) {
+          Error = 0;
+          break;
+        } else if (!SemaRef.getCurScope()->isFunctionDeclarationScope()) {
+          Error = 21;
+          break;
+        } else if (D.hasTrailingReturnType()) {
+          // This might be OK, but we'll need to convert the trailing return
+          // type later.
+          break;
+        }
+
+        Info = &SemaRef.InventedParameterInfos.back();
+      } else {
+        // In C++14, generic lambdas allow 'auto' in their parameters.
+        if (!SemaRef.getLangOpts().CPlusPlus14 || !Auto ||
+            Auto->getKeyword() != AutoTypeKeyword::Auto) {
+          Error = 16;
+          break;
+        }
+        Info = SemaRef.getCurLambda();
+        assert(Info && "No LambdaScopeInfo on the stack!");
       }
+      T = InventTemplateParameter(state, T, nullptr, Auto, *Info);
       break;
+    }
     case DeclaratorContext::MemberContext: {
       if (D.getDeclSpec().getStorageClassSpec() == DeclSpec::SCS_static ||
           D.isFunctionDeclarator())
@@ -3221,6 +3335,7 @@ static QualType GetDeclSpecTypeForDeclarator(TypeProcessingState &state,
     case DeclaratorContext::ObjCParameterContext:
     case DeclaratorContext::ObjCResultContext:
     case DeclaratorContext::KNRTypeListContext:
+    case DeclaratorContext::RequiresExprContext:
       // C++ [dcl.fct]p6:
       //   Types shall not be defined in return or parameter types.
       DiagID = diag::err_type_defined_in_param_type;
@@ -4028,10 +4143,6 @@ static bool DiagnoseMultipleAddrSpaceAttributes(Sema &S, LangAS ASOld,
   return false;
 }
 
-static TypeSourceInfo *
-GetTypeSourceInfoForDeclarator(TypeProcessingState &State,
-                               QualType T, TypeSourceInfo *ReturnTypeInfo);
-
 static TypeSourceInfo *GetFullTypeForDeclarator(TypeProcessingState &state,
                                                 QualType declSpecType,
                                                 TypeSourceInfo *TInfo) {
@@ -4279,6 +4390,7 @@ static TypeSourceInfo *GetFullTypeForDeclarator(TypeProcessingState &state,
     case DeclaratorContext::TemplateTypeArgContext:
     case DeclaratorContext::TypeNameContext:
     case DeclaratorContext::FunctionalCastContext:
+    case DeclaratorContext::RequiresExprContext:
       // Don't infer in these contexts.
       break;
     }
@@ -4606,7 +4718,8 @@ static TypeSourceInfo *GetFullTypeForDeclarator(TypeProcessingState &state,
           } else if (D.getContext() != DeclaratorContext::LambdaExprContext &&
                      (T.hasQualifiers() || !isa<AutoType>(T) ||
                       cast<AutoType>(T)->getKeyword() !=
-                          AutoTypeKeyword::Auto)) {
+                          AutoTypeKeyword::Auto ||
+                      cast<AutoType>(T)->isConstrained())) {
             S.Diag(D.getDeclSpec().getTypeSpecTypeLoc(),
                    diag::err_trailing_return_without_auto)
                 << T << D.getDeclSpec().getSourceRange();
@@ -4617,7 +4730,12 @@ static TypeSourceInfo *GetFullTypeForDeclarator(TypeProcessingState &state,
             // An error occurred parsing the trailing return type.
             T = Context.IntTy;
             D.setInvalidType(true);
-          }
+          } else if (S.getLangOpts().CPlusPlus2a)
+            // Handle cases like: `auto f() -> auto` or `auto f() -> C auto`.
+            if (AutoType *Auto = T->getContainedAutoType())
+              if (S.getCurScope()->isFunctionDeclarationScope())
+                T = InventTemplateParameter(state, T, TInfo, Auto,
+                                            S.InventedParameterInfos.back());
         } else {
           // This function type is not the type of the entity being declared,
           // so checking the 'auto' is not the responsibility of this chunk.
@@ -5227,6 +5345,7 @@ static TypeSourceInfo *GetFullTypeForDeclarator(TypeProcessingState &state,
     switch (D.getContext()) {
     case DeclaratorContext::PrototypeContext:
     case DeclaratorContext::LambdaExprParameterContext:
+    case DeclaratorContext::RequiresExprContext:
       // C++0x [dcl.fct]p13:
       //   [...] When it is part of a parameter-declaration-clause, the
       //   parameter pack is a function parameter pack (14.5.3). The type T
@@ -5236,7 +5355,8 @@ static TypeSourceInfo *GetFullTypeForDeclarator(TypeProcessingState &state,
       //
       // We represent function parameter packs as function parameters whose
       // type is a pack expansion.
-      if (!T->containsUnexpandedParameterPack()) {
+      if (!T->containsUnexpandedParameterPack() &&
+          (!LangOpts.CPlusPlus2a || !T->getContainedAutoType())) {
         S.Diag(D.getEllipsisLoc(),
              diag::err_function_parameter_pack_without_parameter_packs)
           << T <<  D.getSourceRange();
@@ -5444,14 +5564,15 @@ static void fillAttributedTypeLoc(AttributedTypeLoc TL,
 
 namespace {
   class TypeSpecLocFiller : public TypeLocVisitor<TypeSpecLocFiller> {
+    Sema &SemaRef;
     ASTContext &Context;
     TypeProcessingState &State;
     const DeclSpec &DS;
 
   public:
-    TypeSpecLocFiller(ASTContext &Context, TypeProcessingState &State,
+    TypeSpecLocFiller(Sema &S, ASTContext &Context, TypeProcessingState &State,
                       const DeclSpec &DS)
-        : Context(Context), State(State), DS(DS) {}
+        : SemaRef(S), Context(Context), State(State), DS(DS) {}
 
     void VisitAttributedTypeLoc(AttributedTypeLoc TL) {
       Visit(TL.getModifiedLoc());
@@ -5579,6 +5700,34 @@ namespace {
       TL.copy(
           TInfo->getTypeLoc().castAs<DependentTemplateSpecializationTypeLoc>());
     }
+    void VisitAutoTypeLoc(AutoTypeLoc TL) {
+      assert(DS.getTypeSpecType() == TST_auto ||
+             DS.getTypeSpecType() == TST_decltype_auto ||
+             DS.getTypeSpecType() == TST_auto_type ||
+             DS.getTypeSpecType() == TST_unspecified);
+      TL.setNameLoc(DS.getTypeSpecTypeLoc());
+      if (!DS.isConstrainedAuto())
+        return;
+      TemplateIdAnnotation *TemplateId = DS.getRepAsTemplateId();
+      if (DS.getTypeSpecScope().isNotEmpty())
+        TL.setNestedNameSpecifierLoc(
+            DS.getTypeSpecScope().getWithLocInContext(Context));
+      else
+        TL.setNestedNameSpecifierLoc(NestedNameSpecifierLoc());
+      TL.setTemplateKWLoc(TemplateId->TemplateKWLoc);
+      TL.setConceptNameLoc(TemplateId->TemplateNameLoc);
+      TL.setFoundDecl(nullptr);
+      TL.setLAngleLoc(TemplateId->LAngleLoc);
+      TL.setRAngleLoc(TemplateId->RAngleLoc);
+      if (TemplateId->NumArgs == 0)
+        return;
+      TemplateArgumentListInfo TemplateArgsInfo;
+      ASTTemplateArgsPtr TemplateArgsPtr(TemplateId->getTemplateArgs(),
+                                         TemplateId->NumArgs);
+      SemaRef.translateTemplateArguments(TemplateArgsPtr, TemplateArgsInfo);
+      for (unsigned I = 0; I < TemplateId->NumArgs; ++I)
+        TL.setArgLocInfo(I, TemplateArgsInfo.arguments()[I].getLocInfo());
+    }
     void VisitTagTypeLoc(TagTypeLoc TL) {
       TL.setNameLoc(DS.getTypeSpecTypeNameLoc());
     }
@@ -5848,7 +5997,7 @@ GetTypeSourceInfoForDeclarator(TypeProcessingState &State,
     assert(TL.getFullDataSize() == CurrTL.getFullDataSize());
     memcpy(CurrTL.getOpaqueData(), TL.getOpaqueData(), TL.getFullDataSize());
   } else {
-    TypeSpecLocFiller(S.Context, State, D.getDeclSpec()).Visit(CurrTL);
+    TypeSpecLocFiller(S, S.Context, State, D.getDeclSpec()).Visit(CurrTL);
   }
 
   return TInfo;
diff --git a/clang/lib/Sema/TreeTransform.h b/clang/lib/Sema/TreeTransform.h
index 3b827fbc950b..d6105353bbdf 100644
--- a/clang/lib/Sema/TreeTransform.h
+++ b/clang/lib/Sema/TreeTransform.h
@@ -19,6 +19,7 @@
 #include "clang/AST/DeclObjC.h"
 #include "clang/AST/DeclTemplate.h"
 #include "clang/AST/Expr.h"
+#include "clang/AST/ExprConcepts.h"
 #include "clang/AST/ExprCXX.h"
 #include "clang/AST/ExprObjC.h"
 #include "clang/AST/ExprOpenMP.h"
@@ -509,6 +510,15 @@ public:
   DeclarationNameInfo
   TransformDeclarationNameInfo(const DeclarationNameInfo &NameInfo);
 
+  bool TransformRequiresExprRequirements(ArrayRef<concepts::Requirement *> Reqs,
+      llvm::SmallVectorImpl<concepts::Requirement *> &Transformed);
+  concepts::TypeRequirement *
+  TransformTypeRequirement(concepts::TypeRequirement *Req);
+  concepts::ExprRequirement *
+  TransformExprRequirement(concepts::ExprRequirement *Req);
+  concepts::NestedRequirement *
+  TransformNestedRequirement(concepts::NestedRequirement *Req);
+
   /// Transform the given template name.
   ///
   /// \param SS The nested-name-specifier that qualifies the template
@@ -941,12 +951,16 @@ public:
   /// Build a new C++11 auto type.
   ///
   /// By default, builds a new AutoType with the given deduced type.
-  QualType RebuildAutoType(QualType Deduced, AutoTypeKeyword Keyword) {
+  QualType RebuildAutoType(QualType Deduced, AutoTypeKeyword Keyword,
+                           ConceptDecl *TypeConstraintConcept,
+                           ArrayRef<TemplateArgument> TypeConstraintArgs) {
     // Note, IsDependent is always false here: we implicitly convert an 'auto'
     // which has been deduced to a dependent type into an undeduced 'auto', so
     // that we'll retry deduction after the transformation.
     return SemaRef.Context.getAutoType(Deduced, Keyword,
-                                       /*IsDependent*/ false);
+                                       /*IsDependent*/ false, /*IsPack=*/false,
+                                       TypeConstraintConcept,
+                                       TypeConstraintArgs);
   }
 
   /// By default, builds a new DeducedTemplateSpecializationType with the given
@@ -1056,23 +1070,8 @@ public:
     }
 
     if (Keyword == ETK_None || Keyword == ETK_Typename) {
-      QualType T = SemaRef.CheckTypenameType(Keyword, KeywordLoc, QualifierLoc,
-                                             *Id, IdLoc);
-      // If a dependent name resolves to a deduced template specialization type,
-      // check that we're in one of the syntactic contexts permitting it.
-      if (!DeducedTSTContext) {
-        if (auto *Deduced = dyn_cast_or_null<DeducedTemplateSpecializationType>(
-                T.isNull() ? nullptr : T->getContainedDeducedType())) {
-          SemaRef.Diag(IdLoc, diag::err_dependent_deduced_tst)
-            << (int)SemaRef.getTemplateNameKindForDiagnostics(
-                   Deduced->getTemplateName())
-            << QualType(QualifierLoc.getNestedNameSpecifier()->getAsType(), 0);
-          if (auto *TD = Deduced->getTemplateName().getAsTemplateDecl())
-            SemaRef.Diag(TD->getLocation(), diag::note_template_decl_here);
-          return QualType();
-        }
-      }
-      return T;
+      return SemaRef.CheckTypenameType(Keyword, KeywordLoc, QualifierLoc,
+                                       *Id, IdLoc, DeducedTSTContext);
     }
 
     TagTypeKind Kind = TypeWithKeyword::getTagTypeKindForKeyword(Keyword);
@@ -3078,7 +3077,56 @@ public:
     return Result;
   }
 
-    /// \brief Build a new Objective-C boxed expression.
+  /// \brief Build a new requires expression.
+  ///
+  /// By default, performs semantic analysis to build the new expression.
+  /// Subclasses may override this routine to provide different behavior.
+  ExprResult RebuildRequiresExpr(SourceLocation RequiresKWLoc,
+                                 RequiresExprBodyDecl *Body,
+                                 ArrayRef<ParmVarDecl *> LocalParameters,
+                                 ArrayRef<concepts::Requirement *> Requirements,
+                                 SourceLocation ClosingBraceLoc) {
+    return RequiresExpr::Create(SemaRef.Context, RequiresKWLoc, Body,
+                                LocalParameters, Requirements, ClosingBraceLoc);
+  }
+
+  concepts::TypeRequirement *
+  RebuildTypeRequirement(
+      concepts::Requirement::SubstitutionDiagnostic *SubstDiag) {
+    return SemaRef.BuildTypeRequirement(SubstDiag);
+  }
+
+  concepts::TypeRequirement *RebuildTypeRequirement(TypeSourceInfo *T) {
+    return SemaRef.BuildTypeRequirement(T);
+  }
+
+  concepts::ExprRequirement *
+  RebuildExprRequirement(
+      concepts::Requirement::SubstitutionDiagnostic *SubstDiag, bool IsSimple,
+      SourceLocation NoexceptLoc,
+      concepts::ExprRequirement::ReturnTypeRequirement Ret) {
+    return SemaRef.BuildExprRequirement(SubstDiag, IsSimple, NoexceptLoc,
+                                        std::move(Ret));
+  }
+
+  concepts::ExprRequirement *
+  RebuildExprRequirement(Expr *E, bool IsSimple, SourceLocation NoexceptLoc,
+                         concepts::ExprRequirement::ReturnTypeRequirement Ret) {
+    return SemaRef.BuildExprRequirement(E, IsSimple, NoexceptLoc,
+                                        std::move(Ret));
+  }
+
+  concepts::NestedRequirement *
+  RebuildNestedRequirement(
+      concepts::Requirement::SubstitutionDiagnostic *SubstDiag) {
+    return SemaRef.BuildNestedRequirement(SubstDiag);
+  }
+
+  concepts::NestedRequirement *RebuildNestedRequirement(Expr *Constraint) {
+    return SemaRef.BuildNestedRequirement(Constraint);
+  }
+
+  /// \brief Build a new Objective-C boxed expression.
   ///
   /// By default, performs semantic analysis to build the new expression.
   /// Subclasses may override this routine to provide different behavior.
@@ -4456,7 +4504,10 @@ QualType TreeTransform<Derived>::RebuildQualifiedType(QualType T,
         Deduced =
             SemaRef.Context.getQualifiedType(Deduced.getUnqualifiedType(), Qs);
         T = SemaRef.Context.getAutoType(Deduced, AutoTy->getKeyword(),
-                                        AutoTy->isDependentType());
+                                        AutoTy->isDependentType(),
+                                        /*isPack=*/false,
+                                        AutoTy->getTypeConstraintConcept(),
+                                        AutoTy->getTypeConstraintArguments());
       } else {
         // Otherwise, complain about the addition of a qualifier to an
         // already-qualified type.
@@ -5189,21 +5240,29 @@ bool TreeTransform<Derived>::TransformFunctionTypeParams(
         PackExpansionTypeLoc ExpansionTL = TL.castAs<PackExpansionTypeLoc>();
         TypeLoc Pattern = ExpansionTL.getPatternLoc();
         SemaRef.collectUnexpandedParameterPacks(Pattern, Unexpanded);
-        assert(Unexpanded.size() > 0 && "Could not find parameter packs!");
 
         // Determine whether we should expand the parameter packs.
         bool ShouldExpand = false;
         bool RetainExpansion = false;
-        Optional<unsigned> OrigNumExpansions =
-            ExpansionTL.getTypePtr()->getNumExpansions();
-        NumExpansions = OrigNumExpansions;
-        if (getDerived().TryExpandParameterPacks(ExpansionTL.getEllipsisLoc(),
-                                                 Pattern.getSourceRange(),
-                                                 Unexpanded,
-                                                 ShouldExpand,
-                                                 RetainExpansion,
-                                                 NumExpansions)) {
-          return true;
+        Optional<unsigned> OrigNumExpansions;
+        if (Unexpanded.size() > 0) {
+          OrigNumExpansions = ExpansionTL.getTypePtr()->getNumExpansions();
+          NumExpansions = OrigNumExpansions;
+          if (getDerived().TryExpandParameterPacks(ExpansionTL.getEllipsisLoc(),
+                                                   Pattern.getSourceRange(),
+                                                   Unexpanded,
+                                                   ShouldExpand,
+                                                   RetainExpansion,
+                                                   NumExpansions)) {
+            return true;
+          }
+        } else {
+#ifndef NDEBUG
+          const AutoType *AT =
+              Pattern.getType().getTypePtr()->getContainedAutoType();
+          assert((AT && (!AT->isDeduced() || AT->getDeducedType().isNull())) &&
+                 "Could not find parameter packs or undeduced auto type!");
+#endif
         }
 
         if (ShouldExpand) {
@@ -5263,6 +5322,9 @@ bool TreeTransform<Derived>::TransformFunctionTypeParams(
                                                           indexAdjustment,
                                                           NumExpansions,
                                                   /*ExpectParameterPack=*/true);
+        assert(NewParm->isParameterPack() &&
+               "Parameter pack no longer a parameter pack after "
+               "transformation.");
       } else {
         NewParm = getDerived().TransformFunctionTypeParam(
             OldParm, indexAdjustment, None, /*ExpectParameterPack=*/ false);
@@ -5768,32 +5830,6 @@ QualType TreeTransform<Derived>::TransformUnaryTransformType(
 }
 
 template<typename Derived>
-QualType TreeTransform<Derived>::TransformAutoType(TypeLocBuilder &TLB,
-                                                   AutoTypeLoc TL) {
-  const AutoType *T = TL.getTypePtr();
-  QualType OldDeduced = T->getDeducedType();
-  QualType NewDeduced;
-  if (!OldDeduced.isNull()) {
-    NewDeduced = getDerived().TransformType(OldDeduced);
-    if (NewDeduced.isNull())
-      return QualType();
-  }
-
-  QualType Result = TL.getType();
-  if (getDerived().AlwaysRebuild() || NewDeduced != OldDeduced ||
-      T->isDependentType()) {
-    Result = getDerived().RebuildAutoType(NewDeduced, T->getKeyword());
-    if (Result.isNull())
-      return QualType();
-  }
-
-  AutoTypeLoc NewTL = TLB.push<AutoTypeLoc>(Result);
-  NewTL.setNameLoc(TL.getNameLoc());
-
-  return Result;
-}
-
-template<typename Derived>
 QualType TreeTransform<Derived>::TransformDeducedTemplateSpecializationType(
     TypeLocBuilder &TLB, DeducedTemplateSpecializationTypeLoc TL) {
   const DeducedTemplateSpecializationType *T = TL.getTypePtr();
@@ -6054,6 +6090,71 @@ QualType TreeTransform<Derived>::TransformPipeType(TypeLocBuilder &TLB,
     }
   };
 
+template<typename Derived>
+QualType TreeTransform<Derived>::TransformAutoType(TypeLocBuilder &TLB,
+                                                   AutoTypeLoc TL) {
+  const AutoType *T = TL.getTypePtr();
+  QualType OldDeduced = T->getDeducedType();
+  QualType NewDeduced;
+  if (!OldDeduced.isNull()) {
+    NewDeduced = getDerived().TransformType(OldDeduced);
+    if (NewDeduced.isNull())
+      return QualType();
+  }
+
+  ConceptDecl *NewCD = nullptr;
+  TemplateArgumentListInfo NewTemplateArgs;
+  NestedNameSpecifierLoc NewNestedNameSpec;
+  if (TL.getTypePtr()->isConstrained()) {
+    NewCD = cast_or_null<ConceptDecl>(
+        getDerived().TransformDecl(
+            TL.getConceptNameLoc(),
+            TL.getTypePtr()->getTypeConstraintConcept()));
+
+    NewTemplateArgs.setLAngleLoc(TL.getLAngleLoc());
+    NewTemplateArgs.setRAngleLoc(TL.getRAngleLoc());
+    typedef TemplateArgumentLocContainerIterator<AutoTypeLoc> ArgIterator;
+    if (getDerived().TransformTemplateArguments(ArgIterator(TL, 0),
+                                                ArgIterator(TL,
+                                                            TL.getNumArgs()),
+                                                NewTemplateArgs))
+      return QualType();
+
+    if (TL.getNestedNameSpecifierLoc()) {
+      NewNestedNameSpec
+        = getDerived().TransformNestedNameSpecifierLoc(
+            TL.getNestedNameSpecifierLoc());
+      if (!NewNestedNameSpec)
+        return QualType();
+    }
+  }
+
+  QualType Result = TL.getType();
+  if (getDerived().AlwaysRebuild() || NewDeduced != OldDeduced ||
+      T->isDependentType()) {
+    llvm::SmallVector<TemplateArgument, 4> NewArgList;
+    NewArgList.reserve(NewArgList.size());
+    for (const auto &ArgLoc : NewTemplateArgs.arguments())
+      NewArgList.push_back(ArgLoc.getArgument());
+    Result = getDerived().RebuildAutoType(NewDeduced, T->getKeyword(), NewCD,
+                                          NewArgList);
+    if (Result.isNull())
+      return QualType();
+  }
+
+  AutoTypeLoc NewTL = TLB.push<AutoTypeLoc>(Result);
+  NewTL.setNameLoc(TL.getNameLoc());
+  NewTL.setNestedNameSpecifierLoc(NewNestedNameSpec);
+  NewTL.setTemplateKWLoc(TL.getTemplateKWLoc());
+  NewTL.setConceptNameLoc(TL.getConceptNameLoc());
+  NewTL.setFoundDecl(TL.getFoundDecl());
+  NewTL.setLAngleLoc(TL.getLAngleLoc());
+  NewTL.setRAngleLoc(TL.getRAngleLoc());
+  for (unsigned I = 0; I < TL.getNumArgs(); ++I)
+    NewTL.setArgLocInfo(I, NewTemplateArgs.arguments()[I].getLocInfo());
+
+  return Result;
+}
 
 template <typename Derived>
 QualType TreeTransform<Derived>::TransformTemplateSpecializationType(
@@ -11179,6 +11280,146 @@ TreeTransform<Derived>::TransformConceptSpecializationExpr(
       &TransArgs);
 }
 
+template<typename Derived>
+ExprResult
+TreeTransform<Derived>::TransformRequiresExpr(RequiresExpr *E) {
+  SmallVector<ParmVarDecl*, 4> TransParams;
+  SmallVector<QualType, 4> TransParamTypes;
+  Sema::ExtParameterInfoBuilder ExtParamInfos;
+
+  // C++2a [expr.prim.req]p2
+  // Expressions appearing within a requirement-body are unevaluated operands.
+  EnterExpressionEvaluationContext Ctx(
+      SemaRef, Sema::ExpressionEvaluationContext::Unevaluated);
+
+  RequiresExprBodyDecl *Body = RequiresExprBodyDecl::Create(
+      getSema().Context, E->getBody()->getDeclContext(),
+      E->getBody()->getBeginLoc());
+
+  Sema::ContextRAII SavedContext(getSema(), Body, /*NewThisContext*/false);
+
+  if (getDerived().TransformFunctionTypeParams(E->getRequiresKWLoc(),
+                                               E->getLocalParameters(),
+                                               /*ParamTypes=*/nullptr,
+                                               /*ParamInfos=*/nullptr,
+                                               TransParamTypes, &TransParams,
+                                               ExtParamInfos))
+    return ExprError();
+
+  for (ParmVarDecl *Param : TransParams)
+    Param->setDeclContext(Body);
+
+  SmallVector<concepts::Requirement *, 4> TransReqs;
+  if (getDerived().TransformRequiresExprRequirements(E->getRequirements(),
+                                                     TransReqs))
+    return ExprError();
+
+  for (concepts::Requirement *Req : TransReqs) {
+    if (auto *ER = dyn_cast<concepts::ExprRequirement>(Req)) {
+      if (ER->getReturnTypeRequirement().isTypeConstraint()) {
+        ER->getReturnTypeRequirement()
+                .getTypeConstraintTemplateParameterList()->getParam(0)
+                ->setDeclContext(Body);
+      }
+    }
+  }
+
+  return getDerived().RebuildRequiresExpr(E->getRequiresKWLoc(), Body,
+                                          TransParams, TransReqs,
+                                          E->getRBraceLoc());
+}
+
+template<typename Derived>
+bool TreeTransform<Derived>::TransformRequiresExprRequirements(
+    ArrayRef<concepts::Requirement *> Reqs,
+    SmallVectorImpl<concepts::Requirement *> &Transformed) {
+  for (concepts::Requirement *Req : Reqs) {
+    concepts::Requirement *TransReq = nullptr;
+    if (auto *TypeReq = dyn_cast<concepts::TypeRequirement>(Req))
+      TransReq = getDerived().TransformTypeRequirement(TypeReq);
+    else if (auto *ExprReq = dyn_cast<concepts::ExprRequirement>(Req))
+      TransReq = getDerived().TransformExprRequirement(ExprReq);
+    else
+      TransReq = getDerived().TransformNestedRequirement(
+                     cast<concepts::NestedRequirement>(Req));
+    if (!TransReq)
+      return true;
+    Transformed.push_back(TransReq);
+  }
+  return false;
+}
+
+template<typename Derived>
+concepts::TypeRequirement *
+TreeTransform<Derived>::TransformTypeRequirement(
+    concepts::TypeRequirement *Req) {
+  if (Req->isSubstitutionFailure()) {
+    if (getDerived().AlwaysRebuild())
+      return getDerived().RebuildTypeRequirement(
+              Req->getSubstitutionDiagnostic());
+    return Req;
+  }
+  TypeSourceInfo *TransType = getDerived().TransformType(Req->getType());
+  if (!TransType)
+    return nullptr;
+  return getDerived().RebuildTypeRequirement(TransType);
+}
+
+template<typename Derived>
+concepts::ExprRequirement *
+TreeTransform<Derived>::TransformExprRequirement(concepts::ExprRequirement *Req) {
+  llvm::PointerUnion<Expr *, concepts::Requirement::SubstitutionDiagnostic *> TransExpr;
+  if (Req->isExprSubstitutionFailure())
+    TransExpr = Req->getExprSubstitutionDiagnostic();
+  else {
+    ExprResult TransExprRes = getDerived().TransformExpr(Req->getExpr());
+    if (TransExprRes.isInvalid())
+      return nullptr;
+    TransExpr = TransExprRes.get();
+  }
+
+  llvm::Optional<concepts::ExprRequirement::ReturnTypeRequirement> TransRetReq;
+  const auto &RetReq = Req->getReturnTypeRequirement();
+  if (RetReq.isEmpty())
+    TransRetReq.emplace();
+  else if (RetReq.isSubstitutionFailure())
+    TransRetReq.emplace(RetReq.getSubstitutionDiagnostic());
+  else if (RetReq.isTypeConstraint()) {
+    TemplateParameterList *OrigTPL =
+        RetReq.getTypeConstraintTemplateParameterList();
+    TemplateParameterList *TPL =
+        getDerived().TransformTemplateParameterList(OrigTPL);
+    if (!TPL)
+      return nullptr;
+    TransRetReq.emplace(TPL);
+  }
+  assert(TransRetReq.hasValue() &&
+         "All code paths leading here must set TransRetReq");
+  if (Expr *E = TransExpr.dyn_cast<Expr *>())
+    return getDerived().RebuildExprRequirement(E, Req->isSimple(),
+                                               Req->getNoexceptLoc(),
+                                               std::move(*TransRetReq));
+  return getDerived().RebuildExprRequirement(
+      TransExpr.get<concepts::Requirement::SubstitutionDiagnostic *>(),
+      Req->isSimple(), Req->getNoexceptLoc(), std::move(*TransRetReq));
+}
+
+template<typename Derived>
+concepts::NestedRequirement *
+TreeTransform<Derived>::TransformNestedRequirement(
+    concepts::NestedRequirement *Req) {
+  if (Req->isSubstitutionFailure()) {
+    if (getDerived().AlwaysRebuild())
+      return getDerived().RebuildNestedRequirement(
+          Req->getSubstitutionDiagnostic());
+    return Req;
+  }
+  ExprResult TransConstraint =
+      getDerived().TransformExpr(Req->getConstraintExpr());
+  if (TransConstraint.isInvalid())
+    return nullptr;
+  return getDerived().RebuildNestedRequirement(TransConstraint.get());
+}
 
 template<typename Derived>
 ExprResult
diff --git a/clang/lib/Serialization/ASTCommon.cpp b/clang/lib/Serialization/ASTCommon.cpp
index cdb5b17022c2..f93f1f77405d 100644
--- a/clang/lib/Serialization/ASTCommon.cpp
+++ b/clang/lib/Serialization/ASTCommon.cpp
@@ -402,6 +402,7 @@ bool serialization::isRedeclarableDeclKind(unsigned Kind) {
   case Decl::Binding:
   case Decl::Concept:
   case Decl::LifetimeExtendedTemporary:
+  case Decl::RequiresExprBody:
     return false;
 
   // These indirectly derive from Redeclarable<T> but are not actually
diff --git a/clang/lib/Serialization/ASTReader.cpp b/clang/lib/Serialization/ASTReader.cpp
index 19e7ebe03a1f..8e8b04451fb1 100644
--- a/clang/lib/Serialization/ASTReader.cpp
+++ b/clang/lib/Serialization/ASTReader.cpp
@@ -6576,6 +6576,17 @@ void TypeLocReader::VisitUnaryTransformTypeLoc(UnaryTransformTypeLoc TL) {
 
 void TypeLocReader::VisitAutoTypeLoc(AutoTypeLoc TL) {
   TL.setNameLoc(readSourceLocation());
+  if (Reader.readBool()) {
+    TL.setNestedNameSpecifierLoc(ReadNestedNameSpecifierLoc());
+    TL.setTemplateKWLoc(readSourceLocation());
+    TL.setConceptNameLoc(readSourceLocation());
+    TL.setFoundDecl(Reader.readDeclAs<NamedDecl>());
+    TL.setLAngleLoc(readSourceLocation());
+    TL.setRAngleLoc(readSourceLocation());
+    for (unsigned i = 0, e = TL.getNumArgs(); i != e; ++i)
+      TL.setArgLocInfo(i, Reader.readTemplateArgumentLocInfo(
+                              TL.getTypePtr()->getArg(i).getKind()));
+  }
 }
 
 void TypeLocReader::VisitDeducedTemplateSpecializationTypeLoc(
diff --git a/clang/lib/Serialization/ASTReaderDecl.cpp b/clang/lib/Serialization/ASTReaderDecl.cpp
index 96a7d5ae0a31..093b69ab19d0 100644
--- a/clang/lib/Serialization/ASTReaderDecl.cpp
+++ b/clang/lib/Serialization/ASTReaderDecl.cpp
@@ -375,6 +375,7 @@ namespace clang {
     void VisitNonTypeTemplateParmDecl(NonTypeTemplateParmDecl *D);
     DeclID VisitTemplateDecl(TemplateDecl *D);
     void VisitConceptDecl(ConceptDecl *D);
+    void VisitRequiresExprBodyDecl(RequiresExprBodyDecl *D);
     RedeclarableResult VisitRedeclarableTemplateDecl(RedeclarableTemplateDecl *D);
     void VisitClassTemplateDecl(ClassTemplateDecl *D);
     void VisitBuiltinTemplateDecl(BuiltinTemplateDecl *D);
@@ -2037,6 +2038,9 @@ void ASTDeclReader::VisitConceptDecl(ConceptDecl *D) {
   mergeMergeable(D);
 }
 
+void ASTDeclReader::VisitRequiresExprBodyDecl(RequiresExprBodyDecl *D) {
+}
+
 ASTDeclReader::RedeclarableResult
 ASTDeclReader::VisitRedeclarableTemplateDecl(RedeclarableTemplateDecl *D) {
   RedeclarableResult Redecl = VisitRedeclarable(D);
@@ -2313,12 +2317,12 @@ void ASTDeclReader::VisitTemplateTypeParmDecl(TemplateTypeParmDecl *D) {
 
   D->setDeclaredWithTypename(Record.readInt());
 
-  if (Record.readInt()) {
+  if (Record.readBool()) {
     NestedNameSpecifierLoc NNS = Record.readNestedNameSpecifierLoc();
     DeclarationNameInfo DN = Record.readDeclarationNameInfo();
-    ConceptDecl *NamedConcept = cast<ConceptDecl>(Record.readDecl());
+    ConceptDecl *NamedConcept = Record.readDeclAs<ConceptDecl>();
     const ASTTemplateArgumentListInfo *ArgsAsWritten = nullptr;
-    if (Record.readInt())
+    if (Record.readBool())
         ArgsAsWritten = Record.readASTTemplateArgumentListInfo();
     Expr *ImmediatelyDeclaredConstraint = Record.readExpr();
     D->setTypeConstraint(NNS, DN, /*FoundDecl=*/nullptr, NamedConcept,
@@ -2336,6 +2340,8 @@ void ASTDeclReader::VisitNonTypeTemplateParmDecl(NonTypeTemplateParmDecl *D) {
   // TemplateParmPosition.
   D->setDepth(Record.readInt());
   D->setPosition(Record.readInt());
+  if (D->hasPlaceholderTypeConstraint())
+    D->setPlaceholderTypeConstraint(Record.readExpr());
   if (D->isExpandedParameterPack()) {
     auto TypesAndInfos =
         D->getTrailingObjects<std::pair<QualType, TypeSourceInfo *>>();
@@ -3819,13 +3825,19 @@ Decl *ASTReader::ReadDeclRecord(DeclID ID) {
                                                  HasTypeConstraint);
     break;
   }
-  case DECL_NON_TYPE_TEMPLATE_PARM:
-    D = NonTypeTemplateParmDecl::CreateDeserialized(Context, ID);
+  case DECL_NON_TYPE_TEMPLATE_PARM: {
+    bool HasTypeConstraint = Record.readInt();
+    D = NonTypeTemplateParmDecl::CreateDeserialized(Context, ID,
+                                                    HasTypeConstraint);
     break;
-  case DECL_EXPANDED_NON_TYPE_TEMPLATE_PARM_PACK:
+  }
+  case DECL_EXPANDED_NON_TYPE_TEMPLATE_PARM_PACK: {
+    bool HasTypeConstraint = Record.readInt();
     D = NonTypeTemplateParmDecl::CreateDeserialized(Context, ID,
-                                                    Record.readInt());
+                                                    Record.readInt(),
+                                                    HasTypeConstraint);
     break;
+  }
   case DECL_TEMPLATE_TEMPLATE_PARM:
     D = TemplateTemplateParmDecl::CreateDeserialized(Context, ID);
     break;
@@ -3839,6 +3851,9 @@ Decl *ASTReader::ReadDeclRecord(DeclID ID) {
   case DECL_CONCEPT:
     D = ConceptDecl::CreateDeserialized(Context, ID);
     break;
+  case DECL_REQUIRES_EXPR_BODY:
+    D = RequiresExprBodyDecl::CreateDeserialized(Context, ID);
+    break;
   case DECL_STATIC_ASSERT:
     D = StaticAssertDecl::CreateDeserialized(Context, ID);
     break;
diff --git a/clang/lib/Serialization/ASTReaderStmt.cpp b/clang/lib/Serialization/ASTReaderStmt.cpp
index f558c26b5f1e..5dd0ef9d43c3 100644
--- a/clang/lib/Serialization/ASTReaderStmt.cpp
+++ b/clang/lib/Serialization/ASTReaderStmt.cpp
@@ -724,27 +724,15 @@ void ASTStmtReader::VisitUnaryExprOrTypeTraitExpr(UnaryExprOrTypeTraitExpr *E) {
   E->setRParenLoc(readSourceLocation());
 }
 
-void ASTStmtReader::VisitConceptSpecializationExpr(
-        ConceptSpecializationExpr *E) {
-  VisitExpr(E);
-  unsigned NumTemplateArgs = Record.readInt();
-  E->NestedNameSpec = Record.readNestedNameSpecifierLoc();
-  E->TemplateKWLoc = Record.readSourceLocation();
-  E->ConceptName = Record.readDeclarationNameInfo();
-  E->NamedConcept = readDeclAs<ConceptDecl>();
-  E->ArgsAsWritten = Record.readASTTemplateArgumentListInfo();
-  llvm::SmallVector<TemplateArgument, 4> Args;
-  for (unsigned I = 0; I < NumTemplateArgs; ++I)
-    Args.push_back(Record.readTemplateArgument());
-  E->setTemplateArguments(Args);
+static ConstraintSatisfaction
+readConstraintSatisfaction(ASTRecordReader &Record) {
   ConstraintSatisfaction Satisfaction;
   Satisfaction.IsSatisfied = Record.readInt();
   if (!Satisfaction.IsSatisfied) {
     unsigned NumDetailRecords = Record.readInt();
     for (unsigned i = 0; i != NumDetailRecords; ++i) {
       Expr *ConstraintExpr = Record.readExpr();
-      bool IsDiagnostic = Record.readInt();
-      if (IsDiagnostic) {
+      if (bool IsDiagnostic = Record.readInt()) {
         SourceLocation DiagLocation = Record.readSourceLocation();
         std::string DiagMessage = Record.readString();
         Satisfaction.Details.emplace_back(
@@ -755,8 +743,137 @@ void ASTStmtReader::VisitConceptSpecializationExpr(
         Satisfaction.Details.emplace_back(ConstraintExpr, Record.readExpr());
     }
   }
-  E->Satisfaction = ASTConstraintSatisfaction::Create(Record.getContext(),
-                                                      Satisfaction);
+  return Satisfaction;
+}
+
+void ASTStmtReader::VisitConceptSpecializationExpr(
+        ConceptSpecializationExpr *E) {
+  VisitExpr(E);
+  unsigned NumTemplateArgs = Record.readInt();
+  E->NestedNameSpec = Record.readNestedNameSpecifierLoc();
+  E->TemplateKWLoc = Record.readSourceLocation();
+  E->ConceptName = Record.readDeclarationNameInfo();
+  E->NamedConcept = readDeclAs<ConceptDecl>();
+  E->ArgsAsWritten = Record.readASTTemplateArgumentListInfo();
+  llvm::SmallVector<TemplateArgument, 4> Args;
+  for (unsigned I = 0; I < NumTemplateArgs; ++I)
+    Args.push_back(Record.readTemplateArgument());
+  E->setTemplateArguments(Args);
+  E->Satisfaction = E->isValueDependent() ? nullptr :
+      ASTConstraintSatisfaction::Create(Record.getContext(),
+                                        readConstraintSatisfaction(Record));
+}
+
+static concepts::Requirement::SubstitutionDiagnostic *
+readSubstitutionDiagnostic(ASTRecordReader &Record) {
+  std::string SubstitutedEntity = Record.readString();
+  SourceLocation DiagLoc = Record.readSourceLocation();
+  std::string DiagMessage = Record.readString();
+  return new (Record.getContext())
+      concepts::Requirement::SubstitutionDiagnostic{SubstitutedEntity, DiagLoc,
+                                                    DiagMessage};
+}
+
+void ASTStmtReader::VisitRequiresExpr(RequiresExpr *E) {
+  VisitExpr(E);
+  unsigned NumLocalParameters = Record.readInt();
+  unsigned NumRequirements = Record.readInt();
+  E->RequiresExprBits.RequiresKWLoc = Record.readSourceLocation();
+  E->RequiresExprBits.IsSatisfied = Record.readInt();
+  E->Body = Record.readDeclAs<RequiresExprBodyDecl>();
+  llvm::SmallVector<ParmVarDecl *, 4> LocalParameters;
+  for (unsigned i = 0; i < NumLocalParameters; ++i)
+    LocalParameters.push_back(cast<ParmVarDecl>(Record.readDecl()));
+  std::copy(LocalParameters.begin(), LocalParameters.end(),
+            E->getTrailingObjects<ParmVarDecl *>());
+  llvm::SmallVector<concepts::Requirement *, 4> Requirements;
+  for (unsigned i = 0; i < NumRequirements; ++i) {
+    auto RK =
+        static_cast<concepts::Requirement::RequirementKind>(Record.readInt());
+    concepts::Requirement *R = nullptr;
+    switch (RK) {
+      case concepts::Requirement::RK_Type: {
+        auto Status =
+            static_cast<concepts::TypeRequirement::SatisfactionStatus>(
+                Record.readInt());
+        if (Status == concepts::TypeRequirement::SS_SubstitutionFailure)
+          R = new (Record.getContext())
+              concepts::TypeRequirement(readSubstitutionDiagnostic(Record));
+        else
+          R = new (Record.getContext())
+              concepts::TypeRequirement(Record.readTypeSourceInfo());
+      } break;
+      case concepts::Requirement::RK_Simple:
+      case concepts::Requirement::RK_Compound: {
+        auto Status =
+            static_cast<concepts::ExprRequirement::SatisfactionStatus>(
+                Record.readInt());
+        llvm::PointerUnion<concepts::Requirement::SubstitutionDiagnostic *,
+                           Expr *> E;
+        if (Status == concepts::ExprRequirement::SS_ExprSubstitutionFailure) {
+          E = readSubstitutionDiagnostic(Record);
+        } else
+          E = Record.readExpr();
+
+        llvm::Optional<concepts::ExprRequirement::ReturnTypeRequirement> Req;
+        ConceptSpecializationExpr *SubstitutedConstraintExpr = nullptr;
+        SourceLocation NoexceptLoc;
+        if (RK == concepts::Requirement::RK_Simple) {
+          Req.emplace();
+        } else {
+          NoexceptLoc = Record.readSourceLocation();
+          switch (auto returnTypeRequirementKind = Record.readInt()) {
+            case 0:
+              // No return type requirement.
+              Req.emplace();
+              break;
+            case 1: {
+              // type-constraint
+              TemplateParameterList *TPL = Record.readTemplateParameterList();
+              if (Status >=
+                  concepts::ExprRequirement::SS_ConstraintsNotSatisfied)
+                SubstitutedConstraintExpr =
+                    cast<ConceptSpecializationExpr>(Record.readExpr());
+              Req.emplace(TPL);
+            } break;
+            case 2:
+              // Substitution failure
+              Req.emplace(readSubstitutionDiagnostic(Record));
+              break;
+          }
+        }
+        if (Expr *Ex = E.dyn_cast<Expr *>())
+          R = new (Record.getContext()) concepts::ExprRequirement(
+                  Ex, RK == concepts::Requirement::RK_Simple, NoexceptLoc,
+                  std::move(*Req), Status, SubstitutedConstraintExpr);
+        else
+          R = new (Record.getContext()) concepts::ExprRequirement(
+                  E.get<concepts::Requirement::SubstitutionDiagnostic *>(),
+                  RK == concepts::Requirement::RK_Simple, NoexceptLoc,
+                  std::move(*Req));
+      } break;
+      case concepts::Requirement::RK_Nested: {
+        if (bool IsSubstitutionDiagnostic = Record.readInt()) {
+          R = new (Record.getContext()) concepts::NestedRequirement(
+              readSubstitutionDiagnostic(Record));
+          break;
+        }
+        Expr *E = Record.readExpr();
+        if (E->isInstantiationDependent())
+          R = new (Record.getContext()) concepts::NestedRequirement(E);
+        else
+          R = new (Record.getContext())
+              concepts::NestedRequirement(Record.getContext(), E,
+                                          readConstraintSatisfaction(Record));
+      } break;
+    }
+    if (!R)
+      continue;
+    Requirements.push_back(R);
+  }
+  std::copy(Requirements.begin(), Requirements.end(),
+            E->getTrailingObjects<concepts::Requirement *>());
+  E->RBraceLoc = Record.readSourceLocation();
 }
 
 void ASTStmtReader::VisitArraySubscriptExpr(ArraySubscriptExpr *E) {
@@ -3566,11 +3683,18 @@ Stmt *ASTReader::ReadStmtFromStream(ModuleFile &F) {
       S = new (Context) DependentCoawaitExpr(Empty);
       break;
 
-    case EXPR_CONCEPT_SPECIALIZATION:
+    case EXPR_CONCEPT_SPECIALIZATION: {
       unsigned numTemplateArgs = Record[ASTStmtReader::NumExprFields];
       S = ConceptSpecializationExpr::Create(Context, Empty, numTemplateArgs);
       break;
-      
+    }
+
+    case EXPR_REQUIRES:
+      unsigned numLocalParameters = Record[ASTStmtReader::NumExprFields];
+      unsigned numRequirement = Record[ASTStmtReader::NumExprFields + 1];
+      S = RequiresExpr::Create(Context, Empty, numLocalParameters,
+                               numRequirement);
+      break;
     }
 
     // We hit a STMT_STOP, so we're done with this expression.
diff --git a/clang/lib/Serialization/ASTWriter.cpp b/clang/lib/Serialization/ASTWriter.cpp
index 6eba48a1abe9..252853aad1f8 100644
--- a/clang/lib/Serialization/ASTWriter.cpp
+++ b/clang/lib/Serialization/ASTWriter.cpp
@@ -349,6 +349,18 @@ void TypeLocWriter::VisitUnaryTransformTypeLoc(UnaryTransformTypeLoc TL) {
 
 void TypeLocWriter::VisitAutoTypeLoc(AutoTypeLoc TL) {
   Record.AddSourceLocation(TL.getNameLoc());
+  Record.push_back(TL.isConstrained());
+  if (TL.isConstrained()) {
+    Record.AddNestedNameSpecifierLoc(TL.getNestedNameSpecifierLoc());
+    Record.AddSourceLocation(TL.getTemplateKWLoc());
+    Record.AddSourceLocation(TL.getConceptNameLoc());
+    Record.AddDeclRef(TL.getFoundDecl());
+    Record.AddSourceLocation(TL.getLAngleLoc());
+    Record.AddSourceLocation(TL.getRAngleLoc());
+    for (unsigned I = 0; I < TL.getNumArgs(); ++I)
+      Record.AddTemplateArgumentLocInfo(TL.getTypePtr()->getArg(I).getKind(),
+                                        TL.getArgLocInfo(I));
+  }
 }
 
 void TypeLocWriter::VisitDeducedTemplateSpecializationTypeLoc(
@@ -885,6 +897,7 @@ void ASTWriter::WriteBlockInfoBlock() {
   RECORD(DECL_NON_TYPE_TEMPLATE_PARM);
   RECORD(DECL_TEMPLATE_TEMPLATE_PARM);
   RECORD(DECL_CONCEPT);
+  RECORD(DECL_REQUIRES_EXPR_BODY);
   RECORD(DECL_TYPE_ALIAS_TEMPLATE);
   RECORD(DECL_STATIC_ASSERT);
   RECORD(DECL_CXX_BASE_SPECIFIERS);
diff --git a/clang/lib/Serialization/ASTWriterDecl.cpp b/clang/lib/Serialization/ASTWriterDecl.cpp
index b2a8c118d401..472136d99a13 100644
--- a/clang/lib/Serialization/ASTWriterDecl.cpp
+++ b/clang/lib/Serialization/ASTWriterDecl.cpp
@@ -104,6 +104,7 @@ namespace clang {
     void VisitNonTypeTemplateParmDecl(NonTypeTemplateParmDecl *D);
     void VisitTemplateDecl(TemplateDecl *D);
     void VisitConceptDecl(ConceptDecl *D);
+    void VisitRequiresExprBodyDecl(RequiresExprBodyDecl *D);
     void VisitRedeclarableTemplateDecl(RedeclarableTemplateDecl *D);
     void VisitClassTemplateDecl(ClassTemplateDecl *D);
     void VisitVarTemplateDecl(VarTemplateDecl *D);
@@ -1481,6 +1482,10 @@ void ASTDeclWriter::VisitConceptDecl(ConceptDecl *D) {
   Code = serialization::DECL_CONCEPT;
 }
 
+void ASTDeclWriter::VisitRequiresExprBodyDecl(RequiresExprBodyDecl *D) {
+  Code = serialization::DECL_REQUIRES_EXPR_BODY;
+}
+
 void ASTDeclWriter::VisitRedeclarableTemplateDecl(RedeclarableTemplateDecl *D) {
   VisitRedeclarable(D);
 
@@ -1670,6 +1675,8 @@ void ASTDeclWriter::VisitNonTypeTemplateParmDecl(NonTypeTemplateParmDecl *D) {
   // For an expanded parameter pack, record the number of expansion types here
   // so that it's easier for deserialization to allocate the right amount of
   // memory.
+  Expr *TypeConstraint = D->getPlaceholderTypeConstraint();
+  Record.push_back(!!TypeConstraint);
   if (D->isExpandedParameterPack())
     Record.push_back(D->getNumExpansionTypes());
 
@@ -1677,6 +1684,8 @@ void ASTDeclWriter::VisitNonTypeTemplateParmDecl(NonTypeTemplateParmDecl *D) {
   // TemplateParmPosition.
   Record.push_back(D->getDepth());
   Record.push_back(D->getPosition());
+  if (TypeConstraint)
+    Record.AddStmt(TypeConstraint);
 
   if (D->isExpandedParameterPack()) {
     for (unsigned I = 0, N = D->getNumExpansionTypes(); I != N; ++I) {
diff --git a/clang/lib/Serialization/ASTWriterStmt.cpp b/clang/lib/Serialization/ASTWriterStmt.cpp
index 9231f3b2b9ba..1b118c257a4c 100644
--- a/clang/lib/Serialization/ASTWriterStmt.cpp
+++ b/clang/lib/Serialization/ASTWriterStmt.cpp
@@ -12,6 +12,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "clang/Serialization/ASTRecordWriter.h"
+#include "clang/Sema/DeclSpec.h"
 #include "clang/AST/ASTContext.h"
 #include "clang/AST/DeclCXX.h"
 #include "clang/AST/DeclObjC.h"
@@ -388,19 +389,9 @@ void ASTStmtWriter::VisitDependentCoawaitExpr(DependentCoawaitExpr *E) {
   Code = serialization::EXPR_DEPENDENT_COAWAIT;
 }
 
-void ASTStmtWriter::VisitConceptSpecializationExpr(
-        ConceptSpecializationExpr *E) {
-  VisitExpr(E);
-  ArrayRef<TemplateArgument> TemplateArgs = E->getTemplateArguments();
-  Record.push_back(TemplateArgs.size());
-  Record.AddNestedNameSpecifierLoc(E->getNestedNameSpecifierLoc());
-  Record.AddSourceLocation(E->getTemplateKWLoc());
-  Record.AddDeclarationNameInfo(E->getConceptNameInfo());
-  Record.AddDeclRef(E->getNamedConcept());
-  Record.AddASTTemplateArgumentListInfo(E->getTemplateArgsAsWritten());
-  for (const TemplateArgument &Arg : TemplateArgs)
-    Record.AddTemplateArgument(Arg);
-  const ASTConstraintSatisfaction &Satisfaction = E->getSatisfaction();
+static void
+addConstraintSatisfaction(ASTRecordWriter &Record,
+                          const ASTConstraintSatisfaction &Satisfaction) {
   Record.push_back(Satisfaction.IsSatisfied);
   if (!Satisfaction.IsSatisfied) {
     Record.push_back(Satisfaction.NumRecords);
@@ -418,10 +409,98 @@ void ASTStmtWriter::VisitConceptSpecializationExpr(
       }
     }
   }
+}
+
+static void
+addSubstitutionDiagnostic(
+    ASTRecordWriter &Record,
+    const concepts::Requirement::SubstitutionDiagnostic *D) {
+  Record.AddString(D->SubstitutedEntity);
+  Record.AddSourceLocation(D->DiagLoc);
+  Record.AddString(D->DiagMessage);
+}
+
+void ASTStmtWriter::VisitConceptSpecializationExpr(
+        ConceptSpecializationExpr *E) {
+  VisitExpr(E);
+  ArrayRef<TemplateArgument> TemplateArgs = E->getTemplateArguments();
+  Record.push_back(TemplateArgs.size());
+  Record.AddNestedNameSpecifierLoc(E->getNestedNameSpecifierLoc());
+  Record.AddSourceLocation(E->getTemplateKWLoc());
+  Record.AddDeclarationNameInfo(E->getConceptNameInfo());
+  Record.AddDeclRef(E->getNamedConcept());
+  Record.AddASTTemplateArgumentListInfo(E->getTemplateArgsAsWritten());
+  for (const TemplateArgument &Arg : TemplateArgs)
+    Record.AddTemplateArgument(Arg);
+  if (!E->isValueDependent())
+    addConstraintSatisfaction(Record, E->getSatisfaction());
 
   Code = serialization::EXPR_CONCEPT_SPECIALIZATION;
 }
 
+void ASTStmtWriter::VisitRequiresExpr(RequiresExpr *E) {
+  VisitExpr(E);
+  Record.push_back(E->getLocalParameters().size());
+  Record.push_back(E->getRequirements().size());
+  Record.AddSourceLocation(E->RequiresExprBits.RequiresKWLoc);
+  Record.push_back(E->RequiresExprBits.IsSatisfied);
+  Record.AddDeclRef(E->getBody());
+  for (ParmVarDecl *P : E->getLocalParameters())
+    Record.AddDeclRef(P);
+  for (concepts::Requirement *R : E->getRequirements()) {
+    if (auto *TypeReq = dyn_cast<concepts::TypeRequirement>(R)) {
+      Record.push_back(concepts::Requirement::RK_Type);
+      Record.push_back(TypeReq->Status);
+      if (TypeReq->Status == concepts::TypeRequirement::SS_SubstitutionFailure)
+        addSubstitutionDiagnostic(Record, TypeReq->getSubstitutionDiagnostic());
+      else
+        Record.AddTypeSourceInfo(TypeReq->getType());
+    } else if (auto *ExprReq = dyn_cast<concepts::ExprRequirement>(R)) {
+      Record.push_back(ExprReq->getKind());
+      Record.push_back(ExprReq->Status);
+      if (ExprReq->isExprSubstitutionFailure()) {
+        addSubstitutionDiagnostic(Record,
+         ExprReq->Value.get<concepts::Requirement::SubstitutionDiagnostic *>());
+      } else
+        Record.AddStmt(ExprReq->Value.get<Expr *>());
+      if (ExprReq->getKind() == concepts::Requirement::RK_Compound) {
+        Record.AddSourceLocation(ExprReq->NoexceptLoc);
+        const auto &RetReq = ExprReq->getReturnTypeRequirement();
+        if (RetReq.isSubstitutionFailure()) {
+          Record.push_back(2);
+          addSubstitutionDiagnostic(Record, RetReq.getSubstitutionDiagnostic());
+        } else if (RetReq.isTypeConstraint()) {
+          Record.push_back(1);
+          Record.AddTemplateParameterList(
+              RetReq.getTypeConstraintTemplateParameterList());
+          if (ExprReq->Status >=
+              concepts::ExprRequirement::SS_ConstraintsNotSatisfied)
+            Record.AddStmt(
+                ExprReq->getReturnTypeRequirementSubstitutedConstraintExpr());
+        } else {
+          assert(RetReq.isEmpty());
+          Record.push_back(0);
+        }
+      }
+    } else {
+      auto *NestedReq = cast<concepts::NestedRequirement>(R);
+      Record.push_back(concepts::Requirement::RK_Nested);
+      Record.push_back(NestedReq->isSubstitutionFailure());
+      if (NestedReq->isSubstitutionFailure()){
+        addSubstitutionDiagnostic(Record,
+                                  NestedReq->getSubstitutionDiagnostic());
+      } else {
+        Record.AddStmt(NestedReq->Value.get<Expr *>());
+        if (!NestedReq->isDependent())
+          addConstraintSatisfaction(Record, *NestedReq->Satisfaction);
+      }
+    }
+  }
+  Record.AddSourceLocation(E->getEndLoc());
+
+  Code = serialization::EXPR_REQUIRES;
+}
+
 
 void ASTStmtWriter::VisitCapturedStmt(CapturedStmt *S) {
   VisitStmt(S);
diff --git a/clang/lib/StaticAnalyzer/Core/ExprEngine.cpp b/clang/lib/StaticAnalyzer/Core/ExprEngine.cpp
index f917a4c8637b..b542cf2c0303 100644
--- a/clang/lib/StaticAnalyzer/Core/ExprEngine.cpp
+++ b/clang/lib/StaticAnalyzer/Core/ExprEngine.cpp
@@ -1386,6 +1386,7 @@ void ExprEngine::Visit(const Stmt *S, ExplodedNode *Pred,
     case Stmt::AsTypeExprClass:
     case Stmt::ConceptSpecializationExprClass:
     case Stmt::CXXRewrittenBinaryOperatorClass:
+    case Stmt::RequiresExprClass:
       // Fall through.
 
     // Cases we intentionally don't evaluate, since they don't need
diff --git a/clang/tools/driver/driver.cpp b/clang/tools/driver/driver.cpp
index 39f6c7f62be4..4457e40ff04b 100644
--- a/clang/tools/driver/driver.cpp
+++ b/clang/tools/driver/driver.cpp
@@ -241,8 +241,6 @@ static void getCLEnvVarOptions(std::string &EnvValue, llvm::StringSaver &Saver,
       *NumberSignPtr = '=';
 }
 
-static int ExecuteCC1Tool(ArrayRef<const char *> argv);
-
 static void SetBackdoorDriverOutputsFromEnvVars(Driver &TheDriver) {
   // Handle CC_PRINT_OPTIONS and CC_PRINT_OPTIONS_FILE.
   TheDriver.CCPrintOptions = !!::getenv("CC_PRINT_OPTIONS");
@@ -313,21 +311,27 @@ static void SetInstallDir(SmallVectorImpl<const char *> &argv,
     TheDriver.setInstalledDir(InstalledPathParent);
 }
 
-static int ExecuteCC1Tool(ArrayRef<const char *> argv) {
+static int ExecuteCC1Tool(SmallVectorImpl<const char *> &ArgV) {
   // If we call the cc1 tool from the clangDriver library (through
   // Driver::CC1Main), we need to clean up the options usage count. The options
   // are currently global, and they might have been used previously by the
   // driver.
   llvm::cl::ResetAllOptionOccurrences();
-  StringRef Tool = argv[1];
-  void *GetExecutablePathVP = (void *)(intptr_t) GetExecutablePath;
+
+  llvm::BumpPtrAllocator A;
+  llvm::StringSaver Saver(A);
+  llvm::cl::ExpandResponseFiles(Saver, &llvm::cl::TokenizeGNUCommandLine, ArgV,
+                                /*MarkEOLs=*/false);
+  StringRef Tool = ArgV[1];
+  void *GetExecutablePathVP = (void *)(intptr_t)GetExecutablePath;
   if (Tool == "-cc1")
-    return cc1_main(argv.slice(2), argv[0], GetExecutablePathVP);
+    return cc1_main(makeArrayRef(ArgV).slice(2), ArgV[0], GetExecutablePathVP);
   if (Tool == "-cc1as")
-    return cc1as_main(argv.slice(2), argv[0], GetExecutablePathVP);
+    return cc1as_main(makeArrayRef(ArgV).slice(2), ArgV[0],
+                      GetExecutablePathVP);
   if (Tool == "-cc1gen-reproducer")
-    return cc1gen_reproducer_main(argv.slice(2), argv[0], GetExecutablePathVP);
-
+    return cc1gen_reproducer_main(makeArrayRef(ArgV).slice(2), ArgV[0],
+                                  GetExecutablePathVP);
   // Reject unknown tools.
   llvm::errs() << "error: unknown integrated tool '" << Tool << "'. "
                << "Valid tools include '-cc1' and '-cc1as'.\n";
diff --git a/libcxx/include/__config b/libcxx/include/__config
index ac4549802ef2..8f48f16c2364 100644
--- a/libcxx/include/__config
+++ b/libcxx/include/__config
@@ -1350,10 +1350,9 @@ _LIBCPP_FUNC_VIS extern "C" void __sanitizer_annotate_contiguous_container(
 #define _LIBCPP_HAS_NO_COROUTINES
 #endif
 
-// FIXME: Correct this macro when either (A) a feature test macro for the
-// spaceship operator is provided, or (B) a compiler provides a complete
-// implementation.
+#if !defined(__cpp_impl_three_way_comparison) || __cpp_impl_three_way_comparison < 201907L
 #define _LIBCPP_HAS_NO_SPACESHIP_OPERATOR
+#endif
 
 // Decide whether to use availability macros.
 #if !defined(_LIBCPP_BUILDING_LIBRARY) &&                                      \
diff --git a/libcxx/include/__threading_support b/libcxx/include/__threading_support
index 026429f6a8ec..dbf313a1bf22 100644
--- a/libcxx/include/__threading_support
+++ b/libcxx/include/__threading_support
@@ -445,7 +445,7 @@ int __libcpp_tls_set(__libcpp_tls_key __key, void *__p)
 
 int __libcpp_recursive_mutex_init(__libcpp_recursive_mutex_t *__m)
 {
-  return mtx_init(__m, mtx_recursive) == thrd_success ? 0 : EINVAL;
+  return mtx_init(__m, mtx_plain | mtx_recursive) == thrd_success ? 0 : EINVAL;
 }
 
 int __libcpp_recursive_mutex_lock(__libcpp_recursive_mutex_t *__m)
diff --git a/lld/COFF/InputFiles.cpp b/lld/COFF/InputFiles.cpp
index d884201ba31b..272f9c0717c0 100644
--- a/lld/COFF/InputFiles.cpp
+++ b/lld/COFF/InputFiles.cpp
@@ -500,6 +500,17 @@ void ObjFile::handleComdatSelection(COFFSymbolRef sym, COMDATType &selection,
     leaderSelection = selection = IMAGE_COMDAT_SELECT_LARGEST;
   }
 
+  // GCCs __declspec(selectany) doesn't actually pick "any" but "same size as".
+  // Clang on the other hand picks "any". To be able to link two object files
+  // with a __declspec(selectany) declaration, one compiled with gcc and the
+  // other with clang, we merge them as proper "same size as"
+  if (config->mingw && ((selection == IMAGE_COMDAT_SELECT_ANY &&
+                         leaderSelection == IMAGE_COMDAT_SELECT_SAME_SIZE) ||
+                        (selection == IMAGE_COMDAT_SELECT_SAME_SIZE &&
+                         leaderSelection == IMAGE_COMDAT_SELECT_ANY))) {
+    leaderSelection = selection = IMAGE_COMDAT_SELECT_SAME_SIZE;
+  }
+
   // Other than that, comdat selections must match.  This is a bit more
   // strict than link.exe which allows merging "any" and "largest" if "any"
   // is the first symbol the linker sees, and it allows merging "largest"
diff --git a/lld/ELF/Arch/PPC.cpp b/lld/ELF/Arch/PPC.cpp
index 3c0b0c290b58..1d4e80184dcd 100644
--- a/lld/ELF/Arch/PPC.cpp
+++ b/lld/ELF/Arch/PPC.cpp
@@ -136,6 +136,7 @@ void writePPC32GlinkSection(uint8_t *buf, size_t numEntries) {
 }
 
 PPC::PPC() {
+  copyRel = R_PPC_COPY;
   gotRel = R_PPC_GLOB_DAT;
   noneRel = R_PPC_NONE;
   pltRel = R_PPC_JMP_SLOT;
diff --git a/lld/ELF/Arch/PPC64.cpp b/lld/ELF/Arch/PPC64.cpp
index da77a4ddaddf..e48a184c9db2 100644
--- a/lld/ELF/Arch/PPC64.cpp
+++ b/lld/ELF/Arch/PPC64.cpp
@@ -293,6 +293,7 @@ static uint32_t readFromHalf16(const uint8_t *loc) {
 }
 
 PPC64::PPC64() {
+  copyRel = R_PPC64_COPY;
   gotRel = R_PPC64_GLOB_DAT;
   noneRel = R_PPC64_NONE;
   pltRel = R_PPC64_JMP_SLOT;
diff --git a/lld/ELF/Arch/RISCV.cpp b/lld/ELF/Arch/RISCV.cpp
index 42db8e08162d..527f9db0ef29 100644
--- a/lld/ELF/Arch/RISCV.cpp
+++ b/lld/ELF/Arch/RISCV.cpp
@@ -236,9 +236,15 @@ RelExpr RISCV::getRelExpr(const RelType type, const Symbol &s,
   case R_RISCV_TPREL_LO12_S:
     return R_TLS;
   case R_RISCV_RELAX:
-  case R_RISCV_ALIGN:
   case R_RISCV_TPREL_ADD:
     return R_NONE;
+  case R_RISCV_ALIGN:
+    // Not just a hint; always padded to the worst-case number of NOPs, so may
+    // not currently be aligned, and without linker relaxation support we can't
+    // delete NOPs to realign.
+    errorOrWarn(getErrorLocation(loc) + "relocation R_RISCV_ALIGN requires "
+                "unimplemented linker relaxation; recompile with -mno-relax");
+    return R_NONE;
   default:
     error(getErrorLocation(loc) + "unknown relocation (" + Twine(type) +
           ") against symbol " + toString(s));
@@ -431,7 +437,6 @@ void RISCV::relocateOne(uint8_t *loc, const RelType type,
     write64le(loc, val - dtpOffset);
     break;
 
-  case R_RISCV_ALIGN:
   case R_RISCV_RELAX:
     return; // Ignored (for now)
 
diff --git a/lld/ELF/Config.h b/lld/ELF/Config.h
index 06ba88a83dd4..ef1edbcd1992 100644
--- a/lld/ELF/Config.h
+++ b/lld/ELF/Config.h
@@ -169,6 +169,7 @@ struct Configuration {
   bool mipsN32Abi = false;
   bool mmapOutputFile;
   bool nmagic;
+  bool noDynamicLinker = false;
   bool noinhibitExec;
   bool nostdlib;
   bool oFormatBinary;
diff --git a/lld/ELF/Driver.cpp b/lld/ELF/Driver.cpp
index 23da749d3078..25330832339c 100644
--- a/lld/ELF/Driver.cpp
+++ b/lld/ELF/Driver.cpp
@@ -602,8 +602,13 @@ static DiscardPolicy getDiscard(opt::InputArgList &args) {
 
 static StringRef getDynamicLinker(opt::InputArgList &args) {
   auto *arg = args.getLastArg(OPT_dynamic_linker, OPT_no_dynamic_linker);
-  if (!arg || arg->getOption().getID() == OPT_no_dynamic_linker)
+  if (!arg)
+    return "";
+  if (arg->getOption().getID() == OPT_no_dynamic_linker) {
+    // --no-dynamic-linker suppresses undefined weak symbols in .dynsym
+    config->noDynamicLinker = true;
     return "";
+  }
   return arg->getValue();
 }
 
diff --git a/lld/ELF/Relocations.cpp b/lld/ELF/Relocations.cpp
index 4731554e0c0d..ced9991f2003 100644
--- a/lld/ELF/Relocations.cpp
+++ b/lld/ELF/Relocations.cpp
@@ -408,6 +408,14 @@ static bool isStaticLinkTimeConstant(RelExpr e, RelType type, const Symbol &sym,
 
   assert(absVal && relE);
 
+  // Allow R_PLT_PC (optimized to R_PC here) to a hidden undefined weak symbol
+  // in PIC mode. This is a little strange, but it allows us to link function
+  // calls to such symbols (e.g. glibc/stdlib/exit.c:__run_exit_handlers).
+  // Normally such a call will be guarded with a comparison, which will load a
+  // zero from the GOT.
+  if (sym.isUndefWeak())
+    return true;
+
   // We set the final symbols values for linker script defined symbols later.
   // They always can be computed as a link time constant.
   if (sym.scriptDefined)
diff --git a/lld/ELF/Symbols.cpp b/lld/ELF/Symbols.cpp
index f0f6121009a5..0dcf34722d33 100644
--- a/lld/ELF/Symbols.cpp
+++ b/lld/ELF/Symbols.cpp
@@ -278,7 +278,11 @@ bool Symbol::includeInDynsym() const {
   if (computeBinding() == STB_LOCAL)
     return false;
   if (!isDefined() && !isCommon())
-    return true;
+    // This should unconditionally return true, unfortunately glibc -static-pie
+    // expects undefined weak symbols not to exist in .dynsym, e.g.
+    // __pthread_mutex_lock reference in _dl_add_to_namespace_list,
+    // __pthread_initialize_minimal reference in csu/libc-start.c.
+    return !(config->noDynamicLinker && isUndefWeak());
 
   return exportDynamic || inDynamicList;
 }
diff --git a/lld/ELF/Target.cpp b/lld/ELF/Target.cpp
index 70a68fd8db9e..d3899d0f18f1 100644
--- a/lld/ELF/Target.cpp
+++ b/lld/ELF/Target.cpp
@@ -95,7 +95,7 @@ template <class ELFT> static ErrorPlace getErrPlace(const uint8_t *loc) {
   assert(loc != nullptr);
   for (InputSectionBase *d : inputSections) {
     auto *isec = cast<InputSection>(d);
-    if (!isec->getParent())
+    if (!isec->getParent() || (isec->type & SHT_NOBITS))
       continue;
 
     const uint8_t *isecLoc =
diff --git a/lld/ELF/Writer.cpp b/lld/ELF/Writer.cpp
index 6373044d8804..ac332de2a057 100644
--- a/lld/ELF/Writer.cpp
+++ b/lld/ELF/Writer.cpp
@@ -135,8 +135,8 @@ StringRef getOutputSectionName(const InputSectionBase *s) {
 }
 
 static bool needsInterpSection() {
-  return !config->shared && !config->dynamicLinker.empty() &&
-         script->needsInterpSection();
+  return !config->relocatable && !config->shared &&
+         !config->dynamicLinker.empty() && script->needsInterpSection();
 }
 
 template <class ELFT> void writeResult() { Writer<ELFT>().run(); }
diff --git a/llvm/include/llvm/BinaryFormat/ELFRelocs/PowerPC64.def b/llvm/include/llvm/BinaryFormat/ELFRelocs/PowerPC64.def
index 8c5b482f0511..719d0c9c36ac 100644
--- a/llvm/include/llvm/BinaryFormat/ELFRelocs/PowerPC64.def
+++ b/llvm/include/llvm/BinaryFormat/ELFRelocs/PowerPC64.def
@@ -26,6 +26,7 @@
 #undef R_PPC64_GOT16_LO
 #undef R_PPC64_GOT16_HI
 #undef R_PPC64_GOT16_HA
+#undef R_PPC64_COPY
 #undef R_PPC64_GLOB_DAT
 #undef R_PPC64_JMP_SLOT
 #undef R_PPC64_RELATIVE
@@ -119,6 +120,7 @@ ELF_RELOC(R_PPC64_GOT16,                14)
 ELF_RELOC(R_PPC64_GOT16_LO,             15)
 ELF_RELOC(R_PPC64_GOT16_HI,             16)
 ELF_RELOC(R_PPC64_GOT16_HA,             17)
+ELF_RELOC(R_PPC64_COPY,                 19)
 ELF_RELOC(R_PPC64_GLOB_DAT,             20)
 ELF_RELOC(R_PPC64_JMP_SLOT,             21)
 ELF_RELOC(R_PPC64_RELATIVE,             22)
diff --git a/llvm/include/llvm/CodeGen/AsmPrinter.h b/llvm/include/llvm/CodeGen/AsmPrinter.h
index b374fd3d80af..a860ce2773e1 100644
--- a/llvm/include/llvm/CodeGen/AsmPrinter.h
+++ b/llvm/include/llvm/CodeGen/AsmPrinter.h
@@ -113,6 +113,9 @@ public:
 
   ProfileSummaryInfo *PSI;
 
+  /// The symbol for the entry in __patchable_function_entires.
+  MCSymbol *CurrentPatchableFunctionEntrySym = nullptr;
+
   /// The symbol for the current function. This is recalculated at the beginning
   /// of each call to runOnMachineFunction().
   MCSymbol *CurrentFnSym = nullptr;
@@ -449,6 +452,9 @@ public:
   /// instructions in verbose mode.
   virtual void emitImplicitDef(const MachineInstr *MI) const;
 
+  /// Emit N NOP instructions.
+  void emitNops(unsigned N);
+
   //===------------------------------------------------------------------===//
   // Symbol Lowering Routines.
   //===------------------------------------------------------------------===//
diff --git a/llvm/include/llvm/MC/MCAsmBackend.h b/llvm/include/llvm/MC/MCAsmBackend.h
index ed7d5c7f01f4..bf41420f2a5a 100644
--- a/llvm/include/llvm/MC/MCAsmBackend.h
+++ b/llvm/include/llvm/MC/MCAsmBackend.h
@@ -108,6 +108,14 @@ public:
     return false;
   }
 
+  virtual bool evaluateTargetFixup(const MCAssembler &Asm,
+                                   const MCAsmLayout &Layout,
+                                   const MCFixup &Fixup, const MCFragment *DF,
+                                   const MCValue &Target, uint64_t &Value,
+                                   bool &WasForced) {
+    llvm_unreachable("Need to implement hook if target has custom fixups");
+  }
+
   /// Apply the \p Value for given \p Fixup into the provided data fragment, at
   /// the offset specified by the fixup and following the fixup kind as
   /// appropriate. Errors (such as an out of range fixup value) should be
diff --git a/llvm/include/llvm/MC/MCFixupKindInfo.h b/llvm/include/llvm/MC/MCFixupKindInfo.h
index 0ea34866db6a..0d57441ce0dc 100644
--- a/llvm/include/llvm/MC/MCFixupKindInfo.h
+++ b/llvm/include/llvm/MC/MCFixupKindInfo.h
@@ -19,7 +19,10 @@ struct MCFixupKindInfo {
     FKF_IsPCRel = (1 << 0),
 
     /// Should this fixup kind force a 4-byte aligned effective PC value?
-    FKF_IsAlignedDownTo32Bits = (1 << 1)
+    FKF_IsAlignedDownTo32Bits = (1 << 1),
+
+    /// Should this fixup be evaluated in a target dependent manner?
+    FKF_IsTarget = (1 << 2)
   };
 
   /// A target specific name for the fixup kind. The names will be unique for
diff --git a/llvm/include/llvm/Target/Target.td b/llvm/include/llvm/Target/Target.td
index b122b51bb169..cdc9b640e74e 100644
--- a/llvm/include/llvm/Target/Target.td
+++ b/llvm/include/llvm/Target/Target.td
@@ -1182,7 +1182,7 @@ def PATCHABLE_FUNCTION_ENTER : StandardPseudoInstruction {
   let InOperandList = (ins);
   let AsmString = "# XRay Function Enter.";
   let usesCustomInserter = 1;
-  let hasSideEffects = 0;
+  let hasSideEffects = 1;
 }
 def PATCHABLE_RET : StandardPseudoInstruction {
   let OutOperandList = (outs);
@@ -1198,7 +1198,7 @@ def PATCHABLE_FUNCTION_EXIT : StandardPseudoInstruction {
   let InOperandList = (ins);
   let AsmString = "# XRay Function Exit.";
   let usesCustomInserter = 1;
-  let hasSideEffects = 0; // FIXME: is this correct?
+  let hasSideEffects = 1;
   let isReturn = 0; // Original return instruction will follow
 }
 def PATCHABLE_TAIL_CALL : StandardPseudoInstruction {
diff --git a/llvm/include/llvm/Transforms/Utils/SizeOpts.h b/llvm/include/llvm/Transforms/Utils/SizeOpts.h
index ba0f86c45263..1137c89195a6 100644
--- a/llvm/include/llvm/Transforms/Utils/SizeOpts.h
+++ b/llvm/include/llvm/Transforms/Utils/SizeOpts.h
@@ -17,15 +17,13 @@
 #include "llvm/Analysis/ProfileSummaryInfo.h"
 #include "llvm/Support/CommandLine.h"
 
-using namespace llvm;
-
-extern cl::opt<bool> EnablePGSO;
-extern cl::opt<bool> PGSOLargeWorkingSetSizeOnly;
-extern cl::opt<bool> PGSOIRPassOrTestOnly;
-extern cl::opt<bool> PGSOColdCodeOnly;
-extern cl::opt<bool> ForcePGSO;
-extern cl::opt<int> PgsoCutoffInstrProf;
-extern cl::opt<int> PgsoCutoffSampleProf;
+extern llvm::cl::opt<bool> EnablePGSO;
+extern llvm::cl::opt<bool> PGSOLargeWorkingSetSizeOnly;
+extern llvm::cl::opt<bool> PGSOIRPassOrTestOnly;
+extern llvm::cl::opt<bool> PGSOColdCodeOnly;
+extern llvm::cl::opt<bool> ForcePGSO;
+extern llvm::cl::opt<int> PgsoCutoffInstrProf;
+extern llvm::cl::opt<int> PgsoCutoffSampleProf;
 
 namespace llvm {
 
diff --git a/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp b/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
index 6f9aa4dd79fd..3516f4a7b370 100644
--- a/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
@@ -706,6 +706,21 @@ void AsmPrinter::EmitFunctionHeader() {
     }
   }
 
+  // Emit M NOPs for -fpatchable-function-entry=N,M where M>0. We arbitrarily
+  // place prefix data before NOPs.
+  unsigned PatchableFunctionPrefix = 0;
+  (void)F.getFnAttribute("patchable-function-prefix")
+      .getValueAsString()
+      .getAsInteger(10, PatchableFunctionPrefix);
+  if (PatchableFunctionPrefix) {
+    CurrentPatchableFunctionEntrySym =
+        OutContext.createLinkerPrivateTempSymbol();
+    OutStreamer->EmitLabel(CurrentPatchableFunctionEntrySym);
+    emitNops(PatchableFunctionPrefix);
+  } else {
+    CurrentPatchableFunctionEntrySym = CurrentFnBegin;
+  }
+
   // Emit the function descriptor. This is a virtual function to allow targets
   // to emit their specific function descriptor.
   if (MAI->needsFunctionDescriptors())
@@ -1167,7 +1182,7 @@ void AsmPrinter::EmitFunctionBody() {
     // unspecified.
     if (Noop.getOpcode()) {
       OutStreamer->AddComment("avoids zero-length function");
-      OutStreamer->EmitInstruction(Noop, getSubtargetInfo());
+      emitNops(1);
     }
   }
 
@@ -2797,6 +2812,13 @@ void AsmPrinter::printOffset(int64_t Offset, raw_ostream &OS) const {
     OS << Offset;
 }
 
+void AsmPrinter::emitNops(unsigned N) {
+  MCInst Nop;
+  MF->getSubtarget().getInstrInfo()->getNoop(Nop);
+  for (; N; --N)
+    EmitToStreamer(*OutStreamer, Nop);
+}
+
 //===----------------------------------------------------------------------===//
 // Symbol Lowering Routines.
 //===----------------------------------------------------------------------===//
@@ -3199,7 +3221,14 @@ void AsmPrinter::recordSled(MCSymbol *Sled, const MachineInstr &MI,
 
 void AsmPrinter::emitPatchableFunctionEntries() {
   const Function &F = MF->getFunction();
-  if (!F.hasFnAttribute("patchable-function-entry"))
+  unsigned PatchableFunctionPrefix = 0, PatchableFunctionEntry = 0;
+  (void)F.getFnAttribute("patchable-function-prefix")
+      .getValueAsString()
+      .getAsInteger(10, PatchableFunctionPrefix);
+  (void)F.getFnAttribute("patchable-function-entry")
+      .getValueAsString()
+      .getAsInteger(10, PatchableFunctionEntry);
+  if (!PatchableFunctionPrefix && !PatchableFunctionEntry)
     return;
   const unsigned PointerSize = getPointerSize();
   if (TM.getTargetTriple().isOSBinFormatELF()) {
@@ -3228,7 +3257,7 @@ void AsmPrinter::emitPatchableFunctionEntries() {
           "__patchable_function_entries", ELF::SHT_PROGBITS, Flags));
     }
     EmitAlignment(Align(PointerSize));
-    OutStreamer->EmitSymbolValue(CurrentFnBegin, PointerSize);
+    OutStreamer->EmitSymbolValue(CurrentPatchableFunctionEntrySym, PointerSize);
   }
 }
 
diff --git a/llvm/lib/CodeGen/CodeGenPrepare.cpp b/llvm/lib/CodeGen/CodeGenPrepare.cpp
index f05afd058746..003db39fe5f9 100644
--- a/llvm/lib/CodeGen/CodeGenPrepare.cpp
+++ b/llvm/lib/CodeGen/CodeGenPrepare.cpp
@@ -6122,6 +6122,7 @@ bool CodeGenPrepare::optimizeSelectInst(SelectInst *SI) {
   BasicBlock *StartBlock = SI->getParent();
   BasicBlock::iterator SplitPt = ++(BasicBlock::iterator(LastSI));
   BasicBlock *EndBlock = StartBlock->splitBasicBlock(SplitPt, "select.end");
+  BFI->setBlockFreq(EndBlock, BFI->getBlockFreq(StartBlock).getFrequency());
 
   // Delete the unconditional branch that was just created by the split.
   StartBlock->getTerminator()->eraseFromParent();
diff --git a/llvm/lib/CodeGen/PatchableFunction.cpp b/llvm/lib/CodeGen/PatchableFunction.cpp
index 1d6069c50554..a8466396f9b8 100644
--- a/llvm/lib/CodeGen/PatchableFunction.cpp
+++ b/llvm/lib/CodeGen/PatchableFunction.cpp
@@ -57,10 +57,15 @@ static bool doesNotGeneratecode(const MachineInstr &MI) {
 bool PatchableFunction::runOnMachineFunction(MachineFunction &MF) {
   if (MF.getFunction().hasFnAttribute("patchable-function-entry")) {
     MachineBasicBlock &FirstMBB = *MF.begin();
-    MachineInstr &FirstMI = *FirstMBB.begin();
     const TargetInstrInfo *TII = MF.getSubtarget().getInstrInfo();
-    BuildMI(FirstMBB, FirstMI, FirstMI.getDebugLoc(),
-            TII->get(TargetOpcode::PATCHABLE_FUNCTION_ENTER));
+    if (FirstMBB.empty()) {
+      BuildMI(&FirstMBB, DebugLoc(),
+              TII->get(TargetOpcode::PATCHABLE_FUNCTION_ENTER));
+    } else {
+      MachineInstr &FirstMI = *FirstMBB.begin();
+      BuildMI(FirstMBB, FirstMI, FirstMI.getDebugLoc(),
+              TII->get(TargetOpcode::PATCHABLE_FUNCTION_ENTER));
+    }
     return true;
   }
 
diff --git a/llvm/lib/CodeGen/StackColoring.cpp b/llvm/lib/CodeGen/StackColoring.cpp
index b6e81116286f..40bc36c3030b 100644
--- a/llvm/lib/CodeGen/StackColoring.cpp
+++ b/llvm/lib/CodeGen/StackColoring.cpp
@@ -960,6 +960,7 @@ void StackColoring::remapInstructions(DenseMap<int, int> &SlotRemap) {
   }
 
   // Remap all instructions to the new stack slots.
+  std::vector<std::vector<MachineMemOperand *>> SSRefs(MFI->getObjectIndexEnd());
   for (MachineBasicBlock &BB : *MF)
     for (MachineInstr &I : BB) {
       // Skip lifetime markers. We'll remove them soon.
@@ -1025,6 +1026,16 @@ void StackColoring::remapInstructions(DenseMap<int, int> &SlotRemap) {
       SmallVector<MachineMemOperand *, 2> NewMMOs;
       bool ReplaceMemOps = false;
       for (MachineMemOperand *MMO : I.memoperands()) {
+        // Collect MachineMemOperands which reference
+        // FixedStackPseudoSourceValues with old frame indices.
+        if (const auto *FSV = dyn_cast_or_null<FixedStackPseudoSourceValue>(
+                MMO->getPseudoValue())) {
+          int FI = FSV->getFrameIndex();
+          auto To = SlotRemap.find(FI);
+          if (To != SlotRemap.end())
+            SSRefs[FI].push_back(MMO);
+        }
+
         // If this memory location can be a slot remapped here,
         // we remove AA information.
         bool MayHaveConflictingAAMD = false;
@@ -1062,6 +1073,14 @@ void StackColoring::remapInstructions(DenseMap<int, int> &SlotRemap) {
         I.setMemRefs(*MF, NewMMOs);
     }
 
+  // Rewrite MachineMemOperands that reference old frame indices.
+  for (auto E : enumerate(SSRefs)) {
+    const PseudoSourceValue *NewSV =
+        MF->getPSVManager().getFixedStack(SlotRemap[E.index()]);
+    for (MachineMemOperand *Ref : E.value())
+      Ref->setValue(NewSV);
+  }
+
   // Update the location of C++ catch objects for the MSVC personality routine.
   if (WinEHFuncInfo *EHInfo = MF->getWinEHFuncInfo())
     for (WinEHTryBlockMapEntry &TBME : EHInfo->TryBlockMap)
diff --git a/llvm/lib/CodeGen/TargetPassConfig.cpp b/llvm/lib/CodeGen/TargetPassConfig.cpp
index 41cb511ad9b4..d08d05d4b2ed 100644
--- a/llvm/lib/CodeGen/TargetPassConfig.cpp
+++ b/llvm/lib/CodeGen/TargetPassConfig.cpp
@@ -956,6 +956,12 @@ void TargetPassConfig::addMachinePasses() {
   if (getOptLevel() != CodeGenOpt::None)
     addBlockPlacement();
 
+  // Insert before XRay Instrumentation.
+  addPass(&FEntryInserterID, false);
+
+  addPass(&XRayInstrumentationID, false);
+  addPass(&PatchableFunctionID, false);
+
   addPreEmitPass();
 
   if (TM->Options.EnableIPRA)
@@ -968,12 +974,6 @@ void TargetPassConfig::addMachinePasses() {
   addPass(&StackMapLivenessID, false);
   addPass(&LiveDebugValuesID, false);
 
-  // Insert before XRay Instrumentation.
-  addPass(&FEntryInserterID, false);
-
-  addPass(&XRayInstrumentationID, false);
-  addPass(&PatchableFunctionID, false);
-
   if (TM->Options.EnableMachineOutliner && getOptLevel() != CodeGenOpt::None &&
       EnableMachineOutliner != NeverOutline) {
     bool RunOnAllFunctions = (EnableMachineOutliner == AlwaysOutline);
diff --git a/llvm/lib/IR/Verifier.cpp b/llvm/lib/IR/Verifier.cpp
index d15b70d71b47..61707cc83504 100644
--- a/llvm/lib/IR/Verifier.cpp
+++ b/llvm/lib/IR/Verifier.cpp
@@ -1852,16 +1852,25 @@ void Verifier::verifyFunctionAttrs(FunctionType *FT, AttributeList Attrs,
       CheckFailed("invalid value for 'frame-pointer' attribute: " + FP, V);
   }
 
+  if (Attrs.hasFnAttribute("patchable-function-prefix")) {
+    StringRef S = Attrs
+                      .getAttribute(AttributeList::FunctionIndex,
+                                    "patchable-function-prefix")
+                      .getValueAsString();
+    unsigned N;
+    if (S.getAsInteger(10, N))
+      CheckFailed(
+          "\"patchable-function-prefix\" takes an unsigned integer: " + S, V);
+  }
   if (Attrs.hasFnAttribute("patchable-function-entry")) {
-    StringRef S0 = Attrs
-                       .getAttribute(AttributeList::FunctionIndex,
-                                     "patchable-function-entry")
-                       .getValueAsString();
-    StringRef S = S0;
+    StringRef S = Attrs
+                      .getAttribute(AttributeList::FunctionIndex,
+                                    "patchable-function-entry")
+                      .getValueAsString();
     unsigned N;
     if (S.getAsInteger(10, N))
       CheckFailed(
-          "\"patchable-function-entry\" takes an unsigned integer: " + S0, V);
+          "\"patchable-function-entry\" takes an unsigned integer: " + S, V);
   }
 }
 
diff --git a/llvm/lib/MC/MCAssembler.cpp b/llvm/lib/MC/MCAssembler.cpp
index b30137aafb8d..75ec27975564 100644
--- a/llvm/lib/MC/MCAssembler.cpp
+++ b/llvm/lib/MC/MCAssembler.cpp
@@ -217,6 +217,13 @@ bool MCAssembler::evaluateFixup(const MCAsmLayout &Layout,
   }
 
   assert(getBackendPtr() && "Expected assembler backend");
+  bool IsTarget = getBackendPtr()->getFixupKindInfo(Fixup.getKind()).Flags &
+                  MCFixupKindInfo::FKF_IsTarget;
+
+  if (IsTarget)
+    return getBackend().evaluateTargetFixup(*this, Layout, Fixup, DF, Target,
+                                            Value, WasForced);
+
   bool IsPCRel = getBackendPtr()->getFixupKindInfo(Fixup.getKind()).Flags &
                  MCFixupKindInfo::FKF_IsPCRel;
 
diff --git a/llvm/lib/Target/AArch64/AArch64AsmPrinter.cpp b/llvm/lib/Target/AArch64/AArch64AsmPrinter.cpp
index 00e321f9b850..b8953583a310 100644
--- a/llvm/lib/Target/AArch64/AArch64AsmPrinter.cpp
+++ b/llvm/lib/Target/AArch64/AArch64AsmPrinter.cpp
@@ -250,8 +250,7 @@ void AArch64AsmPrinter::LowerPATCHABLE_FUNCTION_ENTER(const MachineInstr &MI)
             .getValueAsString()
             .getAsInteger(10, Num))
       return;
-    for (; Num; --Num)
-      EmitToStreamer(*OutStreamer, MCInstBuilder(AArch64::HINT).addImm(0));
+    emitNops(Num);
     return;
   }
 
diff --git a/llvm/lib/Target/AArch64/AArch64LoadStoreOptimizer.cpp b/llvm/lib/Target/AArch64/AArch64LoadStoreOptimizer.cpp
index 3156bb446963..bc91d628f0b4 100644
--- a/llvm/lib/Target/AArch64/AArch64LoadStoreOptimizer.cpp
+++ b/llvm/lib/Target/AArch64/AArch64LoadStoreOptimizer.cpp
@@ -1325,6 +1325,19 @@ canRenameUpToDef(MachineInstr &FirstMI, LiveRegUnits &UsedInBetween,
 
     // For defs, check if we can rename the first def of RegToRename.
     if (FoundDef) {
+      // For some pseudo instructions, we might not generate code in the end
+      // (e.g. KILL) and we would end up without a correct def for the rename
+      // register.
+      // TODO: This might be overly conservative and we could handle those cases
+      // in multiple ways:
+      //       1. Insert an extra copy, to materialize the def.
+      //       2. Skip pseudo-defs until we find an non-pseudo def.
+      if (MI.isPseudo()) {
+        LLVM_DEBUG(dbgs() << "  Cannot rename pseudo instruction " << MI
+                          << "\n");
+        return false;
+      }
+
       for (auto &MOP : MI.operands()) {
         if (!MOP.isReg() || !MOP.isDef() || MOP.isDebug() || !MOP.getReg() ||
             !TRI->regsOverlap(MOP.getReg(), RegToRename))
diff --git a/llvm/lib/Target/ARM/ARMMCInstLower.cpp b/llvm/lib/Target/ARM/ARMMCInstLower.cpp
index c92689f4942e..8e01b998d900 100644
--- a/llvm/lib/Target/ARM/ARMMCInstLower.cpp
+++ b/llvm/lib/Target/ARM/ARMMCInstLower.cpp
@@ -207,10 +207,7 @@ void ARMAsmPrinter::EmitSled(const MachineInstr &MI, SledKind Kind)
   EmitToStreamer(*OutStreamer, MCInstBuilder(ARM::Bcc).addImm(20)
     .addImm(ARMCC::AL).addReg(0));
 
-  MCInst Noop;
-  Subtarget->getInstrInfo()->getNoop(Noop);
-  for (int8_t I = 0; I < NoopsInSledCount; I++)
-    OutStreamer->EmitInstruction(Noop, getSubtargetInfo());
+  emitNops(NoopsInSledCount);
 
   OutStreamer->EmitLabel(Target);
   recordSled(CurSled, MI, Kind);
diff --git a/llvm/lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.cpp b/llvm/lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.cpp
index 5881a0a86ef7..373d0ccb1857 100644
--- a/llvm/lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.cpp
+++ b/llvm/lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.cpp
@@ -9,6 +9,7 @@
 #include "RISCVAsmBackend.h"
 #include "RISCVMCExpr.h"
 #include "llvm/ADT/APInt.h"
+#include "llvm/MC/MCAsmLayout.h"
 #include "llvm/MC/MCAssembler.h"
 #include "llvm/MC/MCContext.h"
 #include "llvm/MC/MCDirectives.h"
@@ -28,8 +29,6 @@ using namespace llvm;
 bool RISCVAsmBackend::shouldForceRelocation(const MCAssembler &Asm,
                                             const MCFixup &Fixup,
                                             const MCValue &Target) {
-  bool ShouldForce = false;
-
   switch (Fixup.getTargetKind()) {
   default:
     break;
@@ -44,40 +43,9 @@ bool RISCVAsmBackend::shouldForceRelocation(const MCAssembler &Asm,
   case RISCV::fixup_riscv_tls_got_hi20:
   case RISCV::fixup_riscv_tls_gd_hi20:
     return true;
-  case RISCV::fixup_riscv_pcrel_lo12_i:
-  case RISCV::fixup_riscv_pcrel_lo12_s:
-    // For pcrel_lo12, force a relocation if the target of the corresponding
-    // pcrel_hi20 is not in the same fragment.
-    const MCFixup *T = cast<RISCVMCExpr>(Fixup.getValue())->getPCRelHiFixup();
-    if (!T) {
-      Asm.getContext().reportError(Fixup.getLoc(),
-                                   "could not find corresponding %pcrel_hi");
-      return false;
-    }
-
-    switch (T->getTargetKind()) {
-    default:
-      llvm_unreachable("Unexpected fixup kind for pcrel_lo12");
-      break;
-    case RISCV::fixup_riscv_got_hi20:
-    case RISCV::fixup_riscv_tls_got_hi20:
-    case RISCV::fixup_riscv_tls_gd_hi20:
-      ShouldForce = true;
-      break;
-    case RISCV::fixup_riscv_pcrel_hi20: {
-      MCFragment *TFragment = T->getValue()->findAssociatedFragment();
-      MCFragment *FixupFragment = Fixup.getValue()->findAssociatedFragment();
-      assert(FixupFragment && "We should have a fragment for this fixup");
-      ShouldForce =
-          !TFragment || TFragment->getParent() != FixupFragment->getParent();
-      break;
-    }
-    }
-    break;
   }
 
-  return ShouldForce || STI.getFeatureBits()[RISCV::FeatureRelax] ||
-         ForceRelocs;
+  return STI.getFeatureBits()[RISCV::FeatureRelax] || ForceRelocs;
 }
 
 bool RISCVAsmBackend::fixupNeedsRelaxationAdvanced(const MCFixup &Fixup,
@@ -284,6 +252,67 @@ static uint64_t adjustFixupValue(const MCFixup &Fixup, uint64_t Value,
   }
 }
 
+bool RISCVAsmBackend::evaluateTargetFixup(
+    const MCAssembler &Asm, const MCAsmLayout &Layout, const MCFixup &Fixup,
+    const MCFragment *DF, const MCValue &Target, uint64_t &Value,
+    bool &WasForced) {
+  const MCFixup *AUIPCFixup;
+  const MCFragment *AUIPCDF;
+  MCValue AUIPCTarget;
+  switch (Fixup.getTargetKind()) {
+  default:
+    llvm_unreachable("Unexpected fixup kind!");
+  case RISCV::fixup_riscv_pcrel_hi20:
+    AUIPCFixup = &Fixup;
+    AUIPCDF = DF;
+    AUIPCTarget = Target;
+    break;
+  case RISCV::fixup_riscv_pcrel_lo12_i:
+  case RISCV::fixup_riscv_pcrel_lo12_s: {
+    AUIPCFixup = cast<RISCVMCExpr>(Fixup.getValue())->getPCRelHiFixup(&AUIPCDF);
+    if (!AUIPCFixup) {
+      Asm.getContext().reportError(Fixup.getLoc(),
+                                   "could not find corresponding %pcrel_hi");
+      return true;
+    }
+
+    // MCAssembler::evaluateFixup will emit an error for this case when it sees
+    // the %pcrel_hi, so don't duplicate it when also seeing the %pcrel_lo.
+    const MCExpr *AUIPCExpr = AUIPCFixup->getValue();
+    if (!AUIPCExpr->evaluateAsRelocatable(AUIPCTarget, &Layout, AUIPCFixup))
+      return true;
+    break;
+  }
+  }
+
+  if (!AUIPCTarget.getSymA() || AUIPCTarget.getSymB())
+    return false;
+
+  const MCSymbolRefExpr *A = AUIPCTarget.getSymA();
+  const MCSymbol &SA = A->getSymbol();
+  if (A->getKind() != MCSymbolRefExpr::VK_None || SA.isUndefined())
+    return false;
+
+  auto *Writer = Asm.getWriterPtr();
+  if (!Writer)
+    return false;
+
+  bool IsResolved = Writer->isSymbolRefDifferenceFullyResolvedImpl(
+      Asm, SA, *AUIPCDF, false, true);
+  if (!IsResolved)
+    return false;
+
+  Value = Layout.getSymbolOffset(SA) + AUIPCTarget.getConstant();
+  Value -= Layout.getFragmentOffset(AUIPCDF) + AUIPCFixup->getOffset();
+
+  if (shouldForceRelocation(Asm, *AUIPCFixup, AUIPCTarget)) {
+    WasForced = true;
+    return false;
+  }
+
+  return true;
+}
+
 void RISCVAsmBackend::applyFixup(const MCAssembler &Asm, const MCFixup &Fixup,
                                  const MCValue &Target,
                                  MutableArrayRef<char> Data, uint64_t Value,
diff --git a/llvm/lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.h b/llvm/lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.h
index 254249c87dc8..1c3c587355a2 100644
--- a/llvm/lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.h
+++ b/llvm/lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.h
@@ -65,6 +65,11 @@ public:
                                      const MCAsmLayout &Layout,
                                      MCAlignFragment &AF) override;
 
+  bool evaluateTargetFixup(const MCAssembler &Asm, const MCAsmLayout &Layout,
+                           const MCFixup &Fixup, const MCFragment *DF,
+                           const MCValue &Target, uint64_t &Value,
+                           bool &WasForced) override;
+
   void applyFixup(const MCAssembler &Asm, const MCFixup &Fixup,
                   const MCValue &Target, MutableArrayRef<char> Data,
                   uint64_t Value, bool IsResolved,
@@ -101,9 +106,12 @@ public:
       { "fixup_riscv_hi20",         12,     20,  0 },
       { "fixup_riscv_lo12_i",       20,     12,  0 },
       { "fixup_riscv_lo12_s",        0,     32,  0 },
-      { "fixup_riscv_pcrel_hi20",   12,     20,  MCFixupKindInfo::FKF_IsPCRel },
-      { "fixup_riscv_pcrel_lo12_i", 20,     12,  MCFixupKindInfo::FKF_IsPCRel },
-      { "fixup_riscv_pcrel_lo12_s",  0,     32,  MCFixupKindInfo::FKF_IsPCRel },
+      { "fixup_riscv_pcrel_hi20",   12,     20,
+        MCFixupKindInfo::FKF_IsPCRel | MCFixupKindInfo::FKF_IsTarget },
+      { "fixup_riscv_pcrel_lo12_i", 20,     12,
+        MCFixupKindInfo::FKF_IsPCRel | MCFixupKindInfo::FKF_IsTarget },
+      { "fixup_riscv_pcrel_lo12_s",  0,     32,
+        MCFixupKindInfo::FKF_IsPCRel | MCFixupKindInfo::FKF_IsTarget },
       { "fixup_riscv_got_hi20",     12,     20,  MCFixupKindInfo::FKF_IsPCRel },
       { "fixup_riscv_tprel_hi20",   12,     20,  0 },
       { "fixup_riscv_tprel_lo12_i", 20,     12,  0 },
diff --git a/llvm/lib/Target/RISCV/MCTargetDesc/RISCVMCExpr.cpp b/llvm/lib/Target/RISCV/MCTargetDesc/RISCVMCExpr.cpp
index 7aa9b5e7d683..2a6f372e50be 100644
--- a/llvm/lib/Target/RISCV/MCTargetDesc/RISCVMCExpr.cpp
+++ b/llvm/lib/Target/RISCV/MCTargetDesc/RISCVMCExpr.cpp
@@ -47,7 +47,7 @@ void RISCVMCExpr::printImpl(raw_ostream &OS, const MCAsmInfo *MAI) const {
     OS << ')';
 }
 
-const MCFixup *RISCVMCExpr::getPCRelHiFixup() const {
+const MCFixup *RISCVMCExpr::getPCRelHiFixup(const MCFragment **DFOut) const {
   MCValue AUIPCLoc;
   if (!getSubExpr()->evaluateAsRelocatable(AUIPCLoc, nullptr, nullptr))
     return nullptr;
@@ -81,6 +81,8 @@ const MCFixup *RISCVMCExpr::getPCRelHiFixup() const {
     case RISCV::fixup_riscv_tls_got_hi20:
     case RISCV::fixup_riscv_tls_gd_hi20:
     case RISCV::fixup_riscv_pcrel_hi20:
+      if (DFOut)
+        *DFOut = DF;
       return &F;
     }
   }
@@ -88,74 +90,9 @@ const MCFixup *RISCVMCExpr::getPCRelHiFixup() const {
   return nullptr;
 }
 
-bool RISCVMCExpr::evaluatePCRelLo(MCValue &Res, const MCAsmLayout *Layout,
-                                  const MCFixup *Fixup) const {
-  // VK_RISCV_PCREL_LO has to be handled specially.  The MCExpr inside is
-  // actually the location of a auipc instruction with a VK_RISCV_PCREL_HI fixup
-  // pointing to the real target.  We need to generate an MCValue in the form of
-  // (<real target> + <offset from this fixup to the auipc fixup>).  The Fixup
-  // is pcrel relative to the VK_RISCV_PCREL_LO fixup, so we need to add the
-  // offset to the VK_RISCV_PCREL_HI Fixup from VK_RISCV_PCREL_LO to correct.
-
-  // Don't try to evaluate if the fixup will be forced as a relocation (e.g.
-  // as linker relaxation is enabled). If we evaluated pcrel_lo in this case,
-  // the modified fixup will be converted into a relocation that no longer
-  // points to the pcrel_hi as the linker requires.
-  auto &RAB =
-      static_cast<RISCVAsmBackend &>(Layout->getAssembler().getBackend());
-  if (RAB.willForceRelocations())
-    return false;
-
-  MCValue AUIPCLoc;
-  if (!getSubExpr()->evaluateAsValue(AUIPCLoc, *Layout))
-    return false;
-
-  const MCSymbolRefExpr *AUIPCSRE = AUIPCLoc.getSymA();
-  // Don't try to evaluate %pcrel_hi/%pcrel_lo pairs that cross fragment
-  // boundries.
-  if (!AUIPCSRE ||
-      findAssociatedFragment() != AUIPCSRE->findAssociatedFragment())
-    return false;
-
-  const MCSymbol *AUIPCSymbol = &AUIPCSRE->getSymbol();
-  if (!AUIPCSymbol)
-    return false;
-
-  const MCFixup *TargetFixup = getPCRelHiFixup();
-  if (!TargetFixup)
-    return false;
-
-  if ((unsigned)TargetFixup->getKind() != RISCV::fixup_riscv_pcrel_hi20)
-    return false;
-
-  MCValue Target;
-  if (!TargetFixup->getValue()->evaluateAsValue(Target, *Layout))
-    return false;
-
-  if (!Target.getSymA() || !Target.getSymA()->getSymbol().isInSection())
-    return false;
-
-  if (&Target.getSymA()->getSymbol().getSection() !=
-      findAssociatedFragment()->getParent())
-    return false;
-
-  // We must use TargetFixup rather than AUIPCSymbol here. They will almost
-  // always have the same offset, except for the case when AUIPCSymbol is at
-  // the end of a fragment and the fixup comes from offset 0 in the next
-  // fragment.
-  uint64_t AUIPCOffset = TargetFixup->getOffset();
-
-  Res = MCValue::get(Target.getSymA(), nullptr,
-                     Target.getConstant() + (Fixup->getOffset() - AUIPCOffset));
-  return true;
-}
-
 bool RISCVMCExpr::evaluateAsRelocatableImpl(MCValue &Res,
                                             const MCAsmLayout *Layout,
                                             const MCFixup *Fixup) const {
-  if (Kind == VK_RISCV_PCREL_LO && evaluatePCRelLo(Res, Layout, Fixup))
-    return true;
-
   if (!getSubExpr()->evaluateAsRelocatable(Res, Layout, Fixup))
     return false;
 
diff --git a/llvm/lib/Target/RISCV/MCTargetDesc/RISCVMCExpr.h b/llvm/lib/Target/RISCV/MCTargetDesc/RISCVMCExpr.h
index 921df376f3df..167e7d553e7d 100644
--- a/llvm/lib/Target/RISCV/MCTargetDesc/RISCVMCExpr.h
+++ b/llvm/lib/Target/RISCV/MCTargetDesc/RISCVMCExpr.h
@@ -46,9 +46,6 @@ private:
 
   int64_t evaluateAsInt64(int64_t Value) const;
 
-  bool evaluatePCRelLo(MCValue &Res, const MCAsmLayout *Layout,
-                       const MCFixup *Fixup) const;
-
   explicit RISCVMCExpr(const MCExpr *Expr, VariantKind Kind)
       : Expr(Expr), Kind(Kind) {}
 
@@ -61,11 +58,11 @@ public:
   const MCExpr *getSubExpr() const { return Expr; }
 
   /// Get the corresponding PC-relative HI fixup that a VK_RISCV_PCREL_LO
-  /// points to.
+  /// points to, and optionally the fragment containing it.
   ///
   /// \returns nullptr if this isn't a VK_RISCV_PCREL_LO pointing to a
   /// known PC-relative HI fixup.
-  const MCFixup *getPCRelHiFixup() const;
+  const MCFixup *getPCRelHiFixup(const MCFragment **DFOut) const;
 
   void printImpl(raw_ostream &OS, const MCAsmInfo *MAI) const override;
   bool evaluateAsRelocatableImpl(MCValue &Res, const MCAsmLayout *Layout,
diff --git a/llvm/lib/Target/RISCV/RISCVInstrInfo.cpp b/llvm/lib/Target/RISCV/RISCVInstrInfo.cpp
index 3b416ce3d3f4..f6bc52968a33 100644
--- a/llvm/lib/Target/RISCV/RISCVInstrInfo.cpp
+++ b/llvm/lib/Target/RISCV/RISCVInstrInfo.cpp
@@ -24,14 +24,14 @@
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/TargetRegistry.h"
 
+using namespace llvm;
+
 #define GEN_CHECK_COMPRESS_INSTR
 #include "RISCVGenCompressInstEmitter.inc"
 
 #define GET_INSTRINFO_CTOR_DTOR
 #include "RISCVGenInstrInfo.inc"
 
-using namespace llvm;
-
 RISCVInstrInfo::RISCVInstrInfo(RISCVSubtarget &STI)
     : RISCVGenInstrInfo(RISCV::ADJCALLSTACKDOWN, RISCV::ADJCALLSTACKUP),
       STI(STI) {}
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp b/llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
index ebf9d24eecc4..c288a7d8d403 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
@@ -1439,9 +1439,12 @@ Instruction *InstCombiner::visitStoreInst(StoreInst &SI) {
       if (PrevSI->isUnordered() && equivalentAddressValues(PrevSI->getOperand(1),
                                                         SI.getOperand(1))) {
         ++NumDeadStore;
-        ++BBI;
+        // Manually add back the original store to the worklist now, so it will
+        // be processed after the operands of the removed store, as this may
+        // expose additional DSE opportunities.
+        Worklist.Add(&SI);
         eraseInstFromFunction(*PrevSI);
-        continue;
+        return nullptr;
       }
       break;
     }
diff --git a/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp b/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
index aabd974cd73e..479bca83b51e 100644
--- a/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
+++ b/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
@@ -377,6 +377,18 @@ static Value *isOneOf(const InstructionsState &S, Value *Op) {
   return S.OpValue;
 }
 
+/// \returns true if \p Opcode is allowed as part of of the main/alternate
+/// instruction for SLP vectorization.
+///
+/// Example of unsupported opcode is SDIV that can potentially cause UB if the
+/// "shuffled out" lane would result in division by zero.
+static bool isValidForAlternation(unsigned Opcode) {
+  if (Instruction::isIntDivRem(Opcode))
+    return false;
+
+  return true;
+}
+
 /// \returns analysis of the Instructions in \p VL described in
 /// InstructionsState, the Opcode that we suppose the whole list
 /// could be vectorized even if its structure is diverse.
@@ -399,7 +411,8 @@ static InstructionsState getSameOpcode(ArrayRef<Value *> VL,
     if (IsBinOp && isa<BinaryOperator>(VL[Cnt])) {
       if (InstOpcode == Opcode || InstOpcode == AltOpcode)
         continue;
-      if (Opcode == AltOpcode) {
+      if (Opcode == AltOpcode && isValidForAlternation(InstOpcode) &&
+          isValidForAlternation(Opcode)) {
         AltOpcode = InstOpcode;
         AltIndex = Cnt;
         continue;
@@ -411,6 +424,9 @@ static InstructionsState getSameOpcode(ArrayRef<Value *> VL,
         if (InstOpcode == Opcode || InstOpcode == AltOpcode)
           continue;
         if (Opcode == AltOpcode) {
+          assert(isValidForAlternation(Opcode) &&
+                 isValidForAlternation(InstOpcode) &&
+                 "Cast isn't safe for alternation, logic needs to be updated!");
           AltOpcode = InstOpcode;
           AltIndex = Cnt;
           continue;