blob: 5ffe0910398f11a9e93d682dc9bdd201655a0045 [file] [log] [blame]
* 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.
* @compile-error
* @Issue 42436
* @author
class A<X extends num> {}
typedef A<Y> Testme<Y extends String>();
main() {