1 files changed, 145 insertions, 3 deletions
diff --git a/unit-tests/cond-func-empty.mk b/unit-tests/cond-func-empty.mk
index 737403f945254..f93b45895e6eb 100644
--- a/unit-tests/cond-func-empty.mk
+++ b/unit-tests/cond-func-empty.mk
@@ -1,8 +1,150 @@
-# $NetBSD: cond-func-empty.mk,v 1.2 2020/08/16 14:25:16 rillig Exp $
+# $NetBSD: cond-func-empty.mk,v 1.8 2020/09/23 08:11:28 rillig Exp $
 #
-# Tests for the empty() function in .if conditions.
+# Tests for the empty() function in .if conditions, which tests a variable
+# expression for emptiness.
+#
+# Note that the argument in the parentheses is indeed a variable name,
+# optionally followed by variable modifiers.  This is like the defined()
+# function.
+#
+
+.undef UNDEF
+EMPTY=	# empty
+SPACE=	${:U }
+WORD=	word
+
+# An undefined variable is empty.
+.if !empty(UNDEF)
+.  error
+.endif
+
+# An undefined variable has the empty string as the value, and the :M
+# variable modifier does not change that.
+#
+.if !empty(UNDEF:M*)
+.  error
+.endif
+
+# The :S modifier replaces the empty value with an actual word, and
+# after that the expression is no longer empty.  Because the variable
+# was undefined in the first place, the expression has the flag VAR_JUNK
+# but not VAR_KEEP, therefore it is still considered undefined.
+# Only very few variable modifiers turn an undefined variable expression
+# into a defined variable expression.  The :U and :D modifiers belong to
+# that group, but :S doesn't (see VAR_KEEP).
+#
+# XXX: This is hard to explain to someone who doesn't know these
+# implementation details.
+#
+.if !empty(UNDEF:S,^$,value,W)
+.  error
+.endif
+
+# The :U modifier modifies expressions based on undefined variables
+# (VAR_JUNK) by adding the VAR_KEEP flag, which marks the expression
+# as "being interesting enough to be further processed".
+#
+.if empty(UNDEF:S,^$,value,W:Ufallback)
+.  error
+.endif
+
+# And now to the surprising part.  Applying the following :S modifier to the
+# undefined variable makes it non-empty, but the marker VAR_JUNK is preserved
+# nevertheless.  The :U modifier that follows only looks at VAR_JUNK to decide
+# whether the variable is defined or not.  This kind of makes sense since the
+# :U modifier tests the _variable_, not the _expression_.
+#
+# But since the variable was undefined to begin with, the fallback value is
+# used in this expression.
+#
+.if ${UNDEF:S,^$,value,W:Ufallback} != "fallback"
+.  error
+.endif
+
+# The variable EMPTY is completely empty (0 characters).
+.if !empty(EMPTY)
+.  error
+.endif
+
+# The variable SPACE has a single space, which counts as being empty.
+.if !empty(SPACE)
+.  error
+.endif
+
+# The variable .newline has a single newline, which counts as being empty.
+.if !empty(.newline)
+.  error
+.endif
+
+# The empty variable named "" gets a fallback value of " ", which counts as
+# empty.
+#
+# Contrary to the other functions in conditionals, the trailing space is not
+# stripped off, as can be seen in the -dv debug log.  If the space had been
+# stripped, it wouldn't make a difference in this case.
+#
+.if !empty(:U )
+.  error
+.endif
+
+# Now the variable named " " gets a non-empty value, which demonstrates that
+# neither leading nor trailing spaces are trimmed in the argument of the
+# function.  If the spaces were trimmed, the variable name would be "" and
+# that variable is indeed undefined.  Since ParseEmptyArg calls Var_Parse
+# without VARE_UNDEFERR, the value of the undefined variable is returned as
+# an empty string.
+${:U }=	space
+.if empty( )
+.  error
+.endif
+
+# The value of the following expression is " word", which is not empty.
+.if empty(:U word)
+.  error
+.endif
+
+# The :L modifier creates a variable expression that has the same value as
+# its name, which both are "VAR" in this case.  The value is therefore not
+# empty.
+.if empty(VAR:L)
+.  error
+.endif
+
+# The variable WORD has the value "word", which does not count as empty.
+.if empty(WORD)
+.  error
+.endif
+
+# The expression ${} for a variable with the empty name always evaluates
+# to an empty string (see Var_Parse, varUndefined).
+.if !empty()
+.  error
+.endif
+
+# Ensure that variable expressions that appear as part of the argument are
+# properly parsed.  Typical use cases for this are .for loops, which are
+# expanded to exactly these ${:U} expressions.
+#
+# If everything goes well, the argument expands to "WORD", and that variable
+# is defined at the beginning of this file.  The surrounding 'W' and 'D'
+# ensure that the parser in ParseEmptyArg has the correct position, both
+# before and after the call to Var_ParsePP.
+.if empty(W${:UOR}D)
+.  error
+.endif
+
+# There may be spaces at the outside of the parentheses.
+# Spaces inside the parentheses are interpreted as part of the variable name.
+.if ! empty ( WORD )
+.  error
+.endif
+
+${:U WORD }=	variable name with spaces
 
-# TODO: Implementation
+# Now there is a variable named " WORD ", and it is not empty.
+.if empty ( WORD )
+.  error
+.endif
 
 all:
 	@:;