coroutines: Allow parameter packs in co_await/yield expressions [PR95345]

This corrects a pasto, where I copied the constraint on bare
parameter packs from the co_return to co_yield/await without
properly reviewing it.

gcc/cp/ChangeLog:

	PR c++/95345
	* coroutines.cc (finish_co_await_expr): Revise to allow for
	parameter packs.
	(finish_co_yield_expr): Likewise.

gcc/testsuite/ChangeLog:

	PR c++/95345
	* g++.dg/coroutines/pr95345.C: New test.

(cherry picked from commit cf7eac5805e714c7e71b699329e2c4f4a88addc1)

gcc/cp/coroutines.cc

@@ -852,19 +852,18 @@ finish_co_await_expr (location_t kw, tree expr)
   /* The current function has now become a coroutine, if it wasn't already.  */
   DECL_COROUTINE_P (current_function_decl) = 1;
 
-  if (processing_template_decl)
-    {
-      current_function_returns_value = 1;
-
-      if (check_for_bare_parameter_packs (expr))
-	return error_mark_node;
+  /* This function will appear to have no return statement, even if it
+     is declared to return non-void (most likely).  This is correct - we
+     synthesize the return for the ramp in the compiler.  So suppress any
+     extraneous warnings during substitution.  */
+  TREE_NO_WARNING (current_function_decl) = true;
 
-      /* If we don't know the promise type, we can't proceed.  */
-      tree functype = TREE_TYPE (current_function_decl);
-      if (dependent_type_p (functype) || type_dependent_expression_p (expr))
-	return build5_loc (kw, CO_AWAIT_EXPR, unknown_type_node, expr,
-			   NULL_TREE, NULL_TREE, NULL_TREE, integer_zero_node);
-    }
+  /* If we don't know the promise type, we can't proceed, build the
+     co_await with the expression unchanged.  */
+  tree functype = TREE_TYPE (current_function_decl);
+  if (dependent_type_p (functype) || type_dependent_expression_p (expr))
+    return build5_loc (kw, CO_AWAIT_EXPR, unknown_type_node, expr,
+		       NULL_TREE, NULL_TREE, NULL_TREE, integer_zero_node);
 
   /* We must be able to look up the "await_transform" method in the scope of
      the promise type, and obtain its return type.  */
@@ -930,19 +929,17 @@ finish_co_yield_expr (location_t kw, tree expr)
   /* The current function has now become a coroutine, if it wasn't already.  */
   DECL_COROUTINE_P (current_function_decl) = 1;
 
-  if (processing_template_decl)
-    {
-      current_function_returns_value = 1;
-
-      if (check_for_bare_parameter_packs (expr))
-	return error_mark_node;
+  /* This function will appear to have no return statement, even if it
+     is declared to return non-void (most likely).  This is correct - we
+     synthesize the return for the ramp in the compiler.  So suppress any
+     extraneous warnings during substitution.  */
+  TREE_NO_WARNING (current_function_decl) = true;
 
-      tree functype = TREE_TYPE (current_function_decl);
-      /* If we don't know the promise type, we can't proceed.  */
-      if (dependent_type_p (functype) || type_dependent_expression_p (expr))
-	return build2_loc (kw, CO_YIELD_EXPR, unknown_type_node, expr,
-			   NULL_TREE);
-    }
+  /* If we don't know the promise type, we can't proceed, build the
+     co_await with the expression unchanged.  */
+  tree functype = TREE_TYPE (current_function_decl);
+  if (dependent_type_p (functype) || type_dependent_expression_p (expr))
+    return build2_loc (kw, CO_YIELD_EXPR, unknown_type_node, expr, NULL_TREE);
 
   if (!coro_promise_type_found_p (current_function_decl, kw))
     /* We must be able to look up the "yield_value" method in the scope of

gcc/testsuite/g++.dg/coroutines/pr95345.C

+#if __has_include (<coroutine>)
+#include <coroutine>
+using namespace std;
+#elif defined (__clang__) && __has_include (<experimental/coroutine>)
+#include <experimental/coroutine>
+using namespace std::experimental;
+#endif
+
+struct dummy_coro
+{
+  using promise_type = dummy_coro;
+  bool await_ready() { return false; }
+  void await_suspend(std::coroutine_handle<>) { }
+  void await_resume() { }
+  dummy_coro get_return_object() { return {}; }
+  dummy_coro initial_suspend() { return {}; }
+  dummy_coro final_suspend() { return {}; }
+  void return_void() { }
+  void unhandled_exception() { }
+};
+
+template <int ...I>
+dummy_coro
+foo()
+{
+ ((co_await [](int){ return std::suspend_never{}; }(I)), ...);
+  co_return;
+}
+
+void bar() {
+  foo<1>();
+}