coroutines: Wrap co_await in a target expr where needed [PR95050]

Since the co_await expression is mostly opaque to the existing
machinery, we were hiding the details of the await_resume return
value.  If that needs to be wrapped in a target expression, then
emulate this with the whole co_await.  Similarly, if the await
expression we build in response to co_await p.yield_value (e)
is wrapped in a target expression, then we need to transfer that
wrapper to the resultant CO_YIELD_EXPR (which is, itself, just
a proxy for the underlying co_await).

gcc/cp/ChangeLog:

	PR c++/95050
	* coroutines.cc (build_co_await): Wrap the co_await expression
	in a TARGET_EXPR, where needed.
	(finish_co_yield_expr): Likewise.

gcc/testsuite/ChangeLog:

	PR c++/95050
	* g++.dg/coroutines/pr95050.C: New test.

gcc/cp/coroutines.cc

@@ -816,6 +816,12 @@ build_co_await (location_t loc, tree a, suspend_point_kind suspend_kind)
   tree awaiter_calls = make_tree_vec (3);
   TREE_VEC_ELT (awaiter_calls, 0) = awrd_call; /* await_ready().  */
   TREE_VEC_ELT (awaiter_calls, 1) = awsp_call; /* await_suspend().  */
+  tree te = NULL_TREE;
+  if (TREE_CODE (awrs_call) == TARGET_EXPR)
+    {
+      te = awrs_call;
+      awrs_call = TREE_OPERAND (awrs_call, 1);
+    }
   TREE_VEC_ELT (awaiter_calls, 2) = awrs_call; /* await_resume().  */
 
   tree await_expr = build5_loc (loc, CO_AWAIT_EXPR,
@@ -823,7 +829,13 @@ build_co_await (location_t loc, tree a, suspend_point_kind suspend_kind)
 				a, e_proxy, o, awaiter_calls,
 				build_int_cst (integer_type_node,
 					       (int) suspend_kind));
-  return convert_from_reference (await_expr);
+  if (te)
+    {
+      TREE_OPERAND (te, 1) = await_expr;
+      await_expr = te;
+    }
+  tree t = convert_from_reference (await_expr);
+  return t;
 }
 
 tree
@@ -960,8 +972,21 @@ finish_co_yield_expr (location_t kw, tree expr)
   tree op = build_co_await (kw, yield_call, CO_YIELD_SUSPEND_POINT);
   if (op != error_mark_node)
     {
-      op = build2_loc (kw, CO_YIELD_EXPR, TREE_TYPE (op), expr, op);
+      if (REFERENCE_REF_P (op))
+	op = TREE_OPERAND (op, 0);
+      /* If the await expression is wrapped in a TARGET_EXPR, then transfer
+	 that wrapper to the CO_YIELD_EXPR, since this is just a proxy for
+	 its contained await.  Otherwise, just build the CO_YIELD_EXPR.  */
+      if (TREE_CODE (op) == TARGET_EXPR)
+	{
+	  tree t = TREE_OPERAND (op, 1);
+	  t = build2_loc (kw, CO_YIELD_EXPR, TREE_TYPE (t), expr, t);
+	  TREE_OPERAND (op, 1) = t;
+	}
+      else
+	op = build2_loc (kw, CO_YIELD_EXPR, TREE_TYPE (op), expr, op);
       TREE_SIDE_EFFECTS (op) = 1;
+      op = convert_from_reference (op);
     }
 
   return op;

gcc/testsuite/g++.dg/coroutines/pr95050.C

@@ -0,0 +1,49 @@
+#if __has_include (<coroutine>)
+#include <coroutine>
+using namespace std;
+#elif defined (__clang__) && __has_include (<experimental/coroutine>)
+#include <experimental/coroutine>
+using namespace std::experimental;
+#endif
+#include <utility>
+
+struct ret_type 
+{
+  ret_type () = default;
+  ret_type (const ret_type&) = delete;
+  //ret_type (ret_type&&) = default;
+  ~ret_type() {}
+};
+
+struct task
+{
+  struct promise_type
+  {
+    auto get_return_object () -> task  { return {}; }
+    auto initial_suspend () -> suspend_always { return {}; }
+    auto final_suspend () -> suspend_always { return {}; }
+    void return_void () {} 
+    void unhandled_exception () { }
+    void thing (ret_type x) {} 
+  };
+};
+
+struct awaiter
+{
+  bool await_ready() const { return true; }
+  void await_suspend (coroutine_handle<>) {}
+  ret_type await_resume() { return {}; }
+};
+
+task
+my_coro ()
+{
+  ret_type r2{co_await awaiter{}};
+  //ret_type r3 (std::move(r2));
+}
+
+int main()
+{
+ auto x = my_coro ();
+ return 0;
+}