c++: implement C++20 Disambiguating Nested-Requirements (P2092R0)

The rule change in the title matches GCC's current behavior, so no change
was needed.  But the paper also makes 'typename' optional in a
requirement-parameter-list, so this implements that.

gcc/cp/ChangeLog
2020-02-28  Jason Merrill  <jason@redhat.com>

	Implement P2092R0, Disambiguating Nested-Requirements
	* parser.c (cp_parser_requirement_parameter_list): Pass
	CP_PARSER_FLAGS_TYPENAME_OPTIONAL.

gcc/cp/ChangeLog

 2020-02-28  Jason Merrill  <jason@redhat.com>
 
+	Implement P2092R0, Disambiguating Nested-Requirements
+	* parser.c (cp_parser_requirement_parameter_list): Pass
+	CP_PARSER_FLAGS_TYPENAME_OPTIONAL.
+
 	* call.c (build_user_type_conversion_1): Don't look at the second
 	conversion of a non-viable candidate.
 

gcc/cp/parser.c

@@ -27716,8 +27716,8 @@ cp_parser_requirement_parameter_list (cp_parser *parser)
   if (!parens.require_open (parser))
     return error_mark_node;
 
-  tree parms
-    = cp_parser_parameter_declaration_clause (parser, CP_PARSER_FLAGS_NONE);
+  tree parms = (cp_parser_parameter_declaration_clause
+		(parser, CP_PARSER_FLAGS_TYPENAME_OPTIONAL));
 
   if (!parens.require_close (parser))
     return error_mark_node;

gcc/testsuite/g++.dg/cpp2a/concepts-p2092.C

+// P2092R0
+// { dg-do compile { target concepts } }
+
+template <class T>
+concept Bar = requires (T::type t) { ++t; };
+
+template <class T>
+concept Foo = requires { requires { T(); }; }; // { dg-error "" }