pkg/front_end/testcases/nnbd/issue42089.dart

// 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.

test<X>(X? x) {
  if (x is String?) {
    Object o = x;
    o = x;
  }
}

main() {}