c++: diagnose usage of co_await and co_yield in default args [PR115906]

This is a partial fix for PR115906.  Per [expr.await] 2s3, "An
await-expression shall not appear in a default argument
([dcl.fct.default])".  This patch introduces the diagnostic in that
case, and in the case of a co_yield (as co_yield is defined in terms of
co_await, so prerequisites of co_await hold).

PR c++/115906 - [coroutines] missing diagnostic and ICE when co_await used as default argument in function declaration

gcc/cp/ChangeLog:

	PR c++/115906
	* parser.cc (cp_parser_unary_expression): Reject await
	expressions if use of local variables is currently forbidden.
	(cp_parser_yield_expression): Reject yield expressions if use of
	local variables is currently forbidden.

gcc/testsuite/ChangeLog:

	PR c++/115906
	* g++.dg/coroutines/pr115906-yield.C: New test.
	* g++.dg/coroutines/pr115906.C: New test.
	* g++.dg/coroutines/co-await-syntax-02-outside-fn.C: Don't rely
	on default arguments.
	* g++.dg/coroutines/co-yield-syntax-01-outside-fn.C: Ditto.

1 files changed, 17 insertions, 0 deletions

diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index e46cdfd..eb102de 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -9242,6 +9242,14 @@ cp_parser_unary_expression (cp_parser *parser, cp_id_kind * pidk,
 	    if (expr == error_mark_node)
 	      return error_mark_node;
 
+	    /* ... but, we cannot use co_await in default arguments.  */
+	    if (parser->local_variables_forbidden_p & LOCAL_VARS_FORBIDDEN)
+	      {
+		error_at (kw_loc,
+			  "%<co_await%> cannot be used in default arguments");
+		return error_mark_node;
+	      }
+
 	    /* Handle [expr.await].  */
 	    return cp_expr (finish_co_await_expr (kw_loc, expr));
 	  }
@@ -29651,6 +29659,15 @@ cp_parser_yield_expression (cp_parser* parser)
   else
     expr = cp_parser_assignment_expression (parser);
 
+  /* Similar to co_await, we cannot use co_yield in default arguments (as
+     co_awaits underlie co_yield).  */
+  if (parser->local_variables_forbidden_p & LOCAL_VARS_FORBIDDEN)
+    {
+      error_at (kw_loc,
+		"%<co_yield%> cannot be used in default arguments");
+      return error_mark_node;
+    }
+
   if (expr == error_mark_node)
     return expr;