LanguageFeatures/nnbd/Least-greatest-closure/nonfunction_typedef/nonfunction_static_t05.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.
 */
/**
 * @assertion The definitions of least and greatest closure are changed in null
 * safe libraries to substitute [Never] in positions where previously [Null
 * would have been substituted, and [Object?] in positions where previously
 * [Object] or [dynamic] would have been substituted.
 *
 * @description Check the [class A<X extends A<X>>; typedef C<X extends A<X>> =
 * A<X>] case.
 * The analyzer explicitly states that [Object?] is the inferred (but
 * non-working) value of [X].
 *
 * This matches the specified approach for inference of the return type of a
 * function literal (here () => captureTypeArgument()): The type R is the
 * greatest closure of C<_> which is C<Object?>, so the function literal gets
 * the inferred return type C<Object?>, and inference concludes that X gets the
 * value Object?.
 *
 * Also, X == Never would satisfy X <: C<X>, but other possible values of X
 * would be classes implementing C (such as class D extends C<D> {}), and it is
 * not plausible that inference could (or should) choose any such solution. In
 * other words, it is justified for the static analysis to fail to find a
 * suitable value for X.
 *
 * But it is not true that Object? <: C<Object?>, and this means that the
 * inferred type does not satisfy the declared bound. A generic function
 * invocation cannot be super-bounded, so the choice must be rejected.
 * Created #44092 to report the basic issue here (which is expressible without
 * any F-bounds).
 *
 * @note Read more about the least and greatest closure test template:
 * https://github.com/dart-lang/co19/issues/575#issuecomment-613542349
 *
 * @Issue 44073, 44092
 * @author iarkh@unipro.ru
 */
// SharedOptions=--enable-experiment=nonfunction-type-aliases

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

abstract class A<X extends A<X>> {}
typedef C<X extends A<X>> = A<X>;

void main() {
  void f<X extends C<X>>(C<X> Function() g) => g();

  // Verify that the captured type has an `x`.
  f(() => captureTypeArgument());
//^
// [analyzer] unspecified
// [cfe] unspecified
}