| // Copyright (c) 2019, 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 requirement that satisfaction of the bounds on the formal type |
| /// parameters of a generic type alias [D] must imply satisfaction of all bounds |
| /// pertaining to every type that occurs in the body of [D] limits the expressive |
| /// power of generic type aliases. However, it would require the constraints on |
| /// formal type parameters to be expressed in a much more powerful language if we |
| /// were to allow a significantly larger set of types to be expressed using a |
| /// generic type alias. |
| /// For example, consider the following code: |
| /// class A<X extends void Function(num)> {} |
| /// typedef F<Y> = A<void Function(Y)> Function(); // compile-time error |
| /// There is no way to specify a bound on [Y] in the declaration of [F] which |
| /// will ensure that all bounds on the right hand side are respected. This is |
| /// because the actual requirement is that [Y] must be a supertype of [num], but |
| /// Dart does not support lower bounds for type parameters. The type [A<void |
| /// Function(U)> Function()] can still be specified explicitly for every [U] |
| /// which satisfies the bounds declared by [A]. So the types can be expressed, |
| /// they just cannot be abbreviated using a generic type alias. |
| /// @description Checks that example from the Spec throws compile error as |
| /// expected. |
| /// @Issue 42436 |
| /// @author iarkh@unipro.ru |
| |
| class A<X extends void Function(num)> {} |
| typedef F<Y> = A<void Function(Y)> Function(); |
| // ^ |
| // [analyzer] unspecified |
| // [cfe] unspecified |
| |
| main() { |
| } |