c++: CTAD within template argument [PR102933]

Here when checking for erroneous occurrences of 'auto' inside a template
argument (which is allowed by the concepts TS for class templates),
extract_autos_r picks up the CTAD placeholder for X{T{0}} which causes
check_auto_in_tmpl_args to reject this valid template argument.  This
patch fixes this by making extract_autos_r ignore CTAD placeholders.

However, it seems we don't need to call check_auto_in_tmpl_args at all
outside of the concepts TS since using 'auto' as a type-id is otherwise
rejected more generally at parse time.  So this patch makes the function
just exit early if !flag_concepts_ts.

Similarly, I think the concepts code paths in do_auto_deduction and
type_uses_auto are only necessary for the concepts TS, so this patch
also restricts these code paths accordingly.

	PR c++/102933

gcc/cp/ChangeLog:

	* parser.c (cp_parser_simple_type_specifier): Adjust diagnostic
	for using auto in parameter declaration.
	* pt.c (extract_autos_r): Ignore CTAD placeholders.
	(extract_autos): Use range-based for.
	(do_auto_deduction): Use extract_autos only for the concepts TS
	and not also for standard concepts.
	(type_uses_auto): Likewise with for_each_template_parm.
	(check_auto_in_tmpl_args): Just return false outside of the
	concepts TS.  Simplify.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/nontype-class50.C: New test.
	* g++.dg/cpp2a/nontype-class50a.C: New test.

1 files changed, 1 insertions, 1 deletions

diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index 93335c8..4c20757 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -19513,7 +19513,7 @@ cp_parser_simple_type_specifier (cp_parser* parser,
 	  else if (!flag_concepts)
 	    pedwarn (token->location, 0,
 		     "use of %<auto%> in parameter declaration "
-		     "only available with %<-fconcepts-ts%>");
+		     "only available with %<-std=c++20%> or %<-fconcepts%>");
 	}
       else
 	type = make_auto ();