make: changes to suffix substitution in macro expansion

The POSIX standard defines suffix substitution in macro expansion
as taking the form:

   $(string1 [: subst1 =[ subst2 ]])

Since 'subst1' isn't bracketed a value must be supplied.  Enforce
this in POSIX mode.

As a non-POSIX extension an empty 'subst1' is permitted with 'subst2'
being added to all words unconditionally.  If both 'subst1' and
'subst2' are empty the words are returned unchanged.

1 files changed, 11 insertions, 0 deletions

diff --git a/testsuite/make.tests b/testsuite/make.tests
index 8a0f1a7d2..7749bed52 100755
--- a/testsuite/make.tests
+++ b/testsuite/make.tests
@@ -450,6 +450,17 @@ test.b:
 '
 cd .. || exit 1; rm -rf make.tempdir 2>/dev/null
 
+# An empty original suffix indicates that every word should have
+# the new suffix added.  If neither suffix is provided the words
+# remain unchanged.
+testing "make macro expansion and suffix substitution 3" \
+        "make -f -" "src1.c src2.c\nsrc1 src2\n" "" '
+SRCS = src1 src2
+target:
+        @echo $(SRCS:=.c)
+        @echo $(SRCS:=)
+'
+
 # Skip duplicate entries in $? and $^
 mkdir make.tempdir && cd make.tempdir || exit 1
 touch -t 202206171200 file1 file3