coroutines: Improve diagnostics for one allocator case.

If the user provides operator new and that is noexcept, this
implies that it can fail with a null return.  At that point, we expect
to be able to call get_return_object_on_allocation_failure().

This diagnoses the case where such an operator new has been
provided, but the g-r-o-o-a-f is either missing or unusable.

gcc/cp/ChangeLog:

	* coroutines.cc (morph_fn_to_coro): Diagnose unavailable
	get_return_object_on_allocation_failure.

gcc/testsuite/ChangeLog:

	* g++.dg/coroutines/coro-bad-grooaf-01-grooaf-expected.C: New test.

(cherry picked from commit 9c5ca11a33fa91345fe813d449ddc4a821fc72d5)

gcc/cp/coroutines.cc

@@ -4053,6 +4053,10 @@ morph_fn_to_coro (tree orig, tree *resumer, tree *destroyer)
     else if (grooaf && !TYPE_NOTHROW_P (TREE_TYPE (func)))
       error_at (fn_start, "%qE is provided by %qT but %qE is not marked"
 		" %<throw()%> or %<noexcept%>", grooaf, promise_type, nwname);
+    else if (!grooaf && TYPE_NOTHROW_P (TREE_TYPE (func)))
+      warning_at (fn_start, 0, "%qE is marked %<throw()%> or %<noexcept%> but"
+		  " no usable %<get_return_object_on_allocation_failure%>"
+		  " is provided by %qT ", nwname, promise_type);
     }
   else /* No operator new in the promise.  */
     {

gcc/testsuite/g++.dg/coroutines/coro-bad-grooaf-01-grooaf-expected.C

+/* g-r-o-o-a-f would be expected, since we have a noexcept op new.  */
+
+#define USE_FAILING_OP_NEW
+#include "coro1-allocators.h"
+
+int used_grooaf = 0;
+
+struct coro1
+f () noexcept // { dg-warning {'operator new' is marked 'throw\(\)' or 'noexcept' but no usable 'get_return_object_on_allocation_failure' is provided by 'std::__n4861::__coroutine_traits_impl<coro1, void>::promise_type' \{aka 'coro1::promise_type'\}} }
+{
+  PRINT ("coro1: about to return");
+  co_return;
+} 
+