// Copyright (c) 2020, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

/// @assertion The flatten function is modified as follows:
///
/// [flatten(T)] is defined by cases on [T]:
///
///   if [T] is [S?] then [flatten(T) = flatten(S)?]
///   otherwise if [T] is [S*] then [flatten(T) = flatten(S)*]
///   otherwise if [T] is [FutureOr<S>] then [flatten(T) = S]
///   otherwise [if T <: Future] then let [S] be a type such that [T <:
///       Future<S>] and for all [R], if [T <: Future<R>] then [S <: R]; then
///       [flatten(T) = S]
///   otherwise [flatten(T) = T]
///
/// @description Check that async function with `Future<FutureOr<Object>>` return
/// type can return `null` in the weak mode.
///
/// Here is a situation where sound and non-sound null checking produce different
/// results. The `null` value is a result of `getNull()`, it's tested using `null
/// as FutureOr<Object>`, and such a cast succeeds. So `test()` does not throw
/// error  and returns `Future<FutureOr<Object>>` that will be completed with
/// `null`. No dynamic error here.
///
/// @Issue https://github.com/dart-lang/language/pull/941
/// @Issue https://github.com/dart-lang/co19/issues/703
/// @Issue 41266,41437,42236,42237
///
/// @author iarkh@unipro.ru

// Requirements=nnbd-weak

import "dart:async";
import "../../../Utils/expect.dart";

dynamic getNull() => null;

Future<FutureOr<Object>> test() async => await getNull();

main() {
  asyncStart();
  test().then((value) => asyncEnd());
}