c++: diagnose auto in template arg

We were failing to diagnose this Concepts TS feature that didn't make it
into C++20 because the 'auto' was getting converted to a template parameter
before we checked for it.  So also check in cp_parser_simple_type_specifier.

The code in cp_parser_template_type_arg that I initially expected to
diagnose this seems unreachable because cp_parser_type_id_1 already checks
auto.

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_simple_type_specifier): Check for auto
	in template argument.
	(cp_parser_template_type_arg): Remove auto checking.

gcc/testsuite/ChangeLog:

	* g++.dg/concepts/auto7.C: New test.
	* g++.dg/concepts/auto7a.C: New test.

3 files changed, 25 insertions, 9 deletions

diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index d77fbd2..09cba71 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -19823,15 +19823,19 @@ cp_parser_simple_type_specifier (cp_parser* parser,
 			 "only available with "
 			 "%<-std=c++14%> or %<-std=gnu++14%>");
 	    }
+	  else if (!flag_concepts_ts && parser->in_template_argument_list_p)
+	    pedwarn (token->location, 0,
+		     "use of %<auto%> in template argument "
+		     "only available with %<-fconcepts-ts%>");
+	  else if (!flag_concepts)
+	    pedwarn (token->location, 0,
+		     "use of %<auto%> in parameter declaration "
+		     "only available with %<-std=c++20%> or %<-fconcepts%>");
 	  else if (cxx_dialect < cxx14)
 	    error_at (token->location,
 		     "use of %<auto%> in parameter declaration "
 		     "only available with "
 		     "%<-std=c++14%> or %<-std=gnu++14%>");
-	  else if (!flag_concepts)
-	    pedwarn (token->location, 0,
-		     "use of %<auto%> in parameter declaration "
-		     "only available with %<-std=c++20%> or %<-fconcepts%>");
 	}
       else
 	type = make_auto ();
@@ -24522,11 +24526,6 @@ cp_parser_template_type_arg (cp_parser *parser)
     = G_("types may not be defined in template arguments");
   r = cp_parser_type_id_1 (parser, CP_PARSER_FLAGS_NONE, true, false, NULL);
   parser->type_definition_forbidden_message = saved_message;
-  if (cxx_dialect >= cxx14 && !flag_concepts && type_uses_auto (r))
-    {
-      error ("invalid use of %<auto%> in template argument");
-      r = error_mark_node;
-    }
   return r;
 }
 
diff --git a/gcc/testsuite/g++.dg/concepts/auto7.C b/gcc/testsuite/g++.dg/concepts/auto7.C
new file mode 100644
index 0000000..3cbf5dd
--- /dev/null
+++ b/gcc/testsuite/g++.dg/concepts/auto7.C
@@ -0,0 +1,9 @@
+// { dg-do compile { target c++14 } }
+// { dg-additional-options -fconcepts-ts }
+
+template <class T> struct A { };
+void f(A<auto> a) { }
+int main()
+{
+  f(A<int>());
+}
diff --git a/gcc/testsuite/g++.dg/concepts/auto7a.C b/gcc/testsuite/g++.dg/concepts/auto7a.C
new file mode 100644
index 0000000..88868f4
--- /dev/null
+++ b/gcc/testsuite/g++.dg/concepts/auto7a.C
@@ -0,0 +1,8 @@
+// { dg-do compile { target c++14 } }
+
+template <class T> struct A { };
+void f(A<auto> a) { }		// { dg-error "auto. in template argument" }
+int main()
+{
+  f(A<int>());
+}