coroutines: Prevent repeated error messages for missing promise.

If the user's coroutine return type omits the mandatory promise
type then we will currently restate that error each time we see
a coroutine keyword, which doesn't provide any new information.
This suppresses all but the first instance in each coroutine.

gcc/cp/ChangeLog:

2020-02-04  Iain Sandoe  <iain@sandoe.co.uk>

	* coroutines.cc (find_promise_type): Delete unused forward
	declaration.
	(struct coroutine_info): Add a bool for no promise type error.
	(coro_promise_type_found_p): Only emit the error for a missing
	promise once in each affected coroutine.

gcc/testsuite/ChangeLog:

2020-02-04  Iain Sandoe  <iain@sandoe.co.uk>

	* g++.dg/coroutines/coro-missing-promise.C: New test.

gcc/cp/ChangeLog

+2020-02-04  Iain Sandoe  <iain@sandoe.co.uk>
+
+	* coroutines.cc (find_promise_type): Delete unused forward
+	declaration.
+	(struct coroutine_info): Add a bool for no promise type error.
+	(coro_promise_type_found_p): Only emit the error for a missing
+	promise once in each affected coroutine.
+
 2020-02-03  Jason Merrill  <jason@redhat.com>
 
 	PR c++/66477

gcc/cp/coroutines.cc

@@ -33,7 +33,6 @@ along with GCC; see the file COPYING3.  If not see
 #include "gcc-rich-location.h"
 #include "hash-map.h"
 
-static tree find_promise_type (tree);
 static bool coro_promise_type_found_p (tree, location_t);
 
 /* GCC C++ coroutines implementation.
@@ -93,6 +92,7 @@ struct GTY((for_user)) coroutine_info
 				    function into a coroutine.  */
   /* Flags to avoid repeated errors for per-function issues.  */
   bool coro_ret_type_error_emitted;
+  bool coro_promise_error_emitted;
 };
 
 struct coroutine_info_hasher : ggc_ptr_hash<coroutine_info>
@@ -460,7 +460,10 @@ coro_promise_type_found_p (tree fndecl, location_t loc)
       /* If we don't find it, punt on the rest.  */
       if (coro_info->promise_type == NULL_TREE)
 	{
-	  error_at (loc, "unable to find the promise type for this coroutine");
+	  if (!coro_info->coro_promise_error_emitted)
+	    error_at (loc, "unable to find the promise type for"
+		      " this coroutine");
+	  coro_info->coro_promise_error_emitted = true;
 	  return false;
 	}
 

gcc/testsuite/ChangeLog

+2020-02-04  Iain Sandoe  <iain@sandoe.co.uk>
+
+	* g++.dg/coroutines/coro-missing-promise.C: New test.
+
 2020-02-04  Richard Biener  <rguenther@suse.de>
 
 	PR tree-optimization/91123

gcc/testsuite/g++.dg/coroutines/coro-missing-promise.C

+//  { dg-additional-options "-fsyntax-only -w" }
+
+#include "coro.h"
+
+// Diagnose completely missing promise.
+
+// { dg-error {no type named 'promise_type' in 'struct NoPromiseHere'} "" { target *-*-* } 0 }
+
+struct NoPromiseHere {
+  coro::coroutine_handle<> handle;
+  NoPromiseHere () : handle (nullptr) {}
+  NoPromiseHere (coro::coroutine_handle<> handle) : handle (handle) {}
+};
+
+NoPromiseHere
+bar ()
+{
+  co_yield 22; // { dg-error {unable to find the promise type for this coroutine} }
+  co_return 0;
+}