c++: Implement CWG 625: Use of auto as template-arg [PR97479]

This patch implements CWG 625 which prohibits using auto in a template
argument.  A few tests used this construction.  Since this usage was
allowed by the Concepts TS, we only give an error in C++20.

gcc/cp/ChangeLog:

	DR 625
	PR c++/97479
	* parser.c (cp_parser_type_id_1): Reject using auto as
	a template-argument in C++20.

gcc/testsuite/ChangeLog:

	DR 625
	PR c++/97479
	* g++.dg/cpp0x/auto3.C: Update dg-error.
	* g++.dg/cpp0x/auto9.C: Likewise.
	* g++.dg/cpp2a/concepts-pr84979-2.C: Likewise.
	* g++.dg/cpp2a/concepts-pr84979-3.C: Likewise.
	* g++.dg/cpp2a/concepts-pr84979.C: Likewise.
	* g++.dg/DRs/dr625.C: New test.

1 files changed, 16 insertions, 2 deletions

diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index d65b408..bd8c241 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -22419,9 +22419,20 @@ cp_parser_type_id_1 (cp_parser *parser, cp_parser_flags flags,
   if (!cp_parser_parse_definitely (parser))
     abstract_declarator = NULL;
 
+  bool auto_typeid_ok = false;
+  /* The concepts TS allows 'auto' as a type-id.  */
+  if (flag_concepts_ts)
+    auto_typeid_ok = !parser->in_type_id_in_expr_p;
+  /* DR 625 prohibits use of auto as a template-argument.  We allow 'auto'
+     outside the template-argument-list context here only for the sake of
+     diagnostic: grokdeclarator then can emit a better error message for
+     e.g. using T = auto.  */
+  else if (flag_concepts)
+    auto_typeid_ok = (!parser->in_type_id_in_expr_p
+		      && !parser->in_template_argument_list_p);
+
   if (type_specifier_seq.type
-      /* The concepts TS allows 'auto' as a type-id.  */
-      && (!flag_concepts || parser->in_type_id_in_expr_p)
+      && !auto_typeid_ok
       /* None of the valid uses of 'auto' in C++14 involve the type-id
 	 nonterminal, but it is valid in a trailing-return-type.  */
       && !(cxx_dialect >= cxx14 && is_trailing_return))
@@ -22448,6 +22459,9 @@ cp_parser_type_id_1 (cp_parser *parser, cp_parser_flags flags,
 		inform (DECL_SOURCE_LOCATION (tmpl), "%qD declared here",
 			tmpl);
 	      }
+	    else if (parser->in_template_argument_list_p)
+	      error_at (loc, "%qT not permitted in template argument",
+			auto_node);
 	    else
 	      error_at (loc, "invalid use of %qT", auto_node);
 	    return error_mark_node;