c++: ICE with auto in template arg [PR110065]

Here we started crashing with r14-1659 because that removed the
auto checking in cp_parser_template_type_arg which seemed like
dead code.  But the attached test shows that the code can still
be reached because cp_parser_type_id_1 checks auto only when
auto_is_implicit_function_template_parm_p is on.

Then I noticed that we're still crashing in C++20, and that ICE
started with r12-4772.  So I changed the reemerged check to use
flag_concepts_ts rather than flag_concepts on the basis that
check_auto_in_tmpl_args also checks flag_concepts_ts.

	PR c++/110065

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_template_type_arg): Add auto checking.

gcc/testsuite/ChangeLog:

	* g++.dg/concepts/auto8.C: New test.
	* g++.dg/concepts/auto8a.C: New test.

1 files changed, 10 insertions, 2 deletions

diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index fd614ba..c7ad3ec 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -25063,12 +25063,20 @@ cp_parser_type_id (cp_parser *parser, cp_parser_flags flags,
 static tree
 cp_parser_template_type_arg (cp_parser *parser)
 {
-  tree r;
   const char *saved_message = parser->type_definition_forbidden_message;
   parser->type_definition_forbidden_message
     = G_("types may not be defined in template arguments");
-  r = cp_parser_type_id_1 (parser, CP_PARSER_FLAGS_NONE, true, false, NULL);
+  tree r = cp_parser_type_id_1 (parser, CP_PARSER_FLAGS_NONE,
+				/*is_template_arg=*/true,
+				/*is_trailing_return=*/false, nullptr);
   parser->type_definition_forbidden_message = saved_message;
+  /* cp_parser_type_id_1 checks for auto, but only for
+     ->auto_is_implicit_function_template_parm_p.  */
+  if (cxx_dialect >= cxx14 && !flag_concepts_ts && type_uses_auto (r))
+    {
+      error ("invalid use of %<auto%> in template argument");
+      r = error_mark_node;
+    }
   return r;
 }