c, c++: Support musttail attribute even using __attribute__ form [PR116545]

Apparently some programs in the wild use
 #if __has_attribute(musttail)
   __attribute__((musttail)) return foo ();
 #else
   return foo ();
 #endif
clang supports musttail both as a standard attribute ([[clang::musttail]]
which we also support for compatibility) and the above worked just
fine with GCC 14 which had __has_attribute(musttail) 0.  Now that it is
0, this doesn't compile anymore.
So, either we need to ensure that __has_attribute(musttail) is 0
and just __has_c{,pp}_attribute({gnu,clang}::musttail) are non-zero,
or IMHO better we just make it work in the attribute form, especially for
C < C23 I can see why some projects would prefer that form.
While [[gnu::musttail]] is rejected as an error in C11 etc. before GCC 15,
rather than just handled as an unknown attribute.
I view this as both a regression and compatibility issue.
The patch handles it in similar spots to fallthrough/assume attributes
inside of __attribute__ for C, and for C++ enables mixing of standard [[]]
and GNU __attribute__(()) attributes at the start of statements in any order.

While working on it, I've noticed we weren't diagnosing arguments to the
clang::musttail attribute (fixed by the c-attribs.cc hunk) and newly
on the __attribute__ form attribute (in that case the arguments aren't just
skipped, they are always parsed and because we don't call decl_attributes
etc., it wouldn't be diagnosed without a manual check).

2025-03-18  Jakub Jelinek  <jakub@redhat.com>

	PR c/116545
gcc/
	* doc/extend.texi (musttail statement attribute): Document
	that musttail GNU attribute can be used as well.
gcc/c-family/
	* c-attribs.cc (c_common_clang_attributes): Add musttail.
gcc/c/
	* c-parser.cc (c_parser_declaration_or_fndef): Parse
	__attribute__((musttail)) return.
	(c_parser_handle_musttail): Diagnose attribute arguments.
	(c_parser_statement_after_labels): Parse
	__attribute__((musttail)) return.
gcc/cp/
	* parser.cc (cp_parser_statement): Call cp_parser_attributes_opt
	rather than cp_parser_std_attribute_spec_seq.
	(cp_parser_jump_statement): Diagnose gnu::musttail attributes
	with no arguments.
gcc/testsuite/
	* c-c++-common/attr-fallthrough-2.c: Adjust expected diagnostics
	for C++.
	* c-c++-common/musttail15.c: New test.
	* c-c++-common/musttail16.c: New test.
	* c-c++-common/musttail17.c: New test.
	* c-c++-common/musttail18.c: New test.
	* c-c++-common/musttail19.c: New test.
	* c-c++-common/musttail20.c: New test.
	* c-c++-common/musttail21.c: New test.
	* c-c++-common/musttail22.c: New test.
	* c-c++-common/musttail23.c: New test.
	* c-c++-common/musttail24.c: New test.
	* g++.dg/musttail7.C: New test.
	* g++.dg/musttail8.C: New test.
	* g++.dg/musttail12.C: New test.
	* g++.dg/musttail13.C: New test.
	* g++.dg/musttail14.C: New test.
	* g++.dg/ext/pr116545.C: New test.

0 files changed, 0 insertions, 0 deletions