LanguageFeatures/nnbd/future_value_type_A01_t04.dart

/*
 * Copyright (c) 2021, 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 Let f be an asynchronous non-generator function with declared
 * return type T. Then the future value type of f is futureValueType(T). The
 * function futureValueType is defined as follows:
 *
 *  futureValueType(S?) = futureValueType(S), for all S.
 *  futureValueType(S*) = futureValueType(S), for all S.
 *  futureValueType(Future<S>) = S, for all S.
 *  futureValueType(FutureOr<S>) = S, for all S.
 *  futureValueType(void) = void.
 *  futureValueType(dynamic) = dynamic.
 *  Otherwise, for all S, futureValueType(S) = Object?.
 *
 * @description Check that for asynchronous non-generator function with declared
 * return type futureValueType(FutureOr<S>?) = S, for all S. Check that it is an
 * error if type of return value is not assignable to the future
 *
 * @author sgrekhov@unipro.ru
 */

Future<num?>? foo() async {
  return new Future<dynamic>.value(42);
//       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
// [analyzer] unspecified
// [cfe] unspecified
}

main() {
  foo();
}