New test, testing the behavior under scrutiny in SDK issue 49396

Change-Id: I1ebb2ea12f4d01c4077fca6fc380353f821bc68a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/267422
Commit-Queue: Erik Ernst <eernst@google.com>
Reviewed-by: Lasse Nielsen <lrn@google.com>

tests/language/async/regression_49396_test.dart

@@ -0,0 +1,78 @@
+// Copyright (c) 2022, 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.
+
+import 'dart:async';
+import 'package:expect/expect.dart';
+import '../static_type_helper.dart';
+
+// Regression test, exercising the behavior described in SDK issue 49396.
+
+class A {}
+
+abstract class B<X> extends A implements Future<X> {
+  Future<X> get fut;
+  asStream() => fut.asStream();
+  catchError(error, {test}) => fut.catchError(error, test: test);
+  then<R>(onValue, {onError}) => fut.then(onValue, onError: onError);
+  timeout(timeLimit, {onTimeout}) =>
+      fut.timeout(timeLimit, onTimeout: onTimeout);
+  whenComplete(action) => fut.whenComplete(action);
+}
+
+class C extends B<Object?> {
+  final Future<Object?> fut = Future.value(CompletelyUnrelated());
+}
+
+class CompletelyUnrelated {}
+
+class C1 extends A {}
+
+class C2 extends B<C1> {
+  final Future<C1> fut = Future.value(C1());
+}
+
+void main() async {
+  final Object o = Future<Object?>.value();
+  var o2 = await o; // Remains a future.
+  o2.expectStaticType<Exactly<Object>>();
+  Expect.type<Future<Object?>>(o2);
+  Expect.identical(o, o2);
+
+  final FutureOr<Object> x = Future<Object?>.value();
+  var x2 = await x; // Remains a future.
+  x2.expectStaticType<Exactly<Object>>();
+  Expect.type<Future<Object?>>(x2);
+  Expect.identical(x, x2);
+
+  final FutureOr<Future<int>> y = Future<int>.value(1);
+  var y2 = await y; // Remains a `Future<int>`.
+  y2.expectStaticType<Exactly<Future<int>>>();
+  Expect.type<Future<int>>(y2);
+  Expect.identical(y, y2);
+
+  A a = C();
+  var a2 = await a; // Remains a `C` and a `Future<Object?>`.
+  a2.expectStaticType<Exactly<A>>();
+  Expect.type<C>(a2);
+  Expect.identical(a, a2);
+
+  Future<void> f<X extends Object>(X x) async {
+    var x2 = await x; // Remains an `A` and a `Future<Object?>`.
+    x2.expectStaticType<Exactly<Object>>();
+    Expect.type<Future<Object?>>(x2);
+    Expect.identical(x, x2);
+  }
+
+  await f(Future<Object?>.value(null));
+
+  Future<void> g<X extends A>(X x) async {
+    var x2 = await x; // The future is awaited.
+    x2.expectStaticType<Exactly<A>>();
+    Expect.type<C1>(x2);
+    Expect.notType<C2>(x2);
+    Expect.notIdentical(x, x2);
+  }
+
+  await g(C2());
+}