| // 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 If more than one extension applies to a specific member |
| /// invocation, then we resort to a heuristic to choose one of the extensions to |
| /// apply. If exactly one of them is "more specific" than all the others, that |
| /// one is chosen. Otherwise it is a compile-time error. |
| /// |
| /// An extension with [on] type clause [T1] is more specific than another |
| /// extension with [on] type clause [T2] iff |
| /// |
| /// 1. The latter extension is declared in a platform library, and the former |
| /// extension is not |
| /// 2. they are both declared in platform libraries or both declared in |
| /// non-platform libraries, and |
| /// 3. the instantiated type (the type after applying type inference from the |
| /// receiver) of [T1] is a subtype of the instantiated type of [T2] and |
| /// either |
| /// 4. not vice versa, or |
| /// 5. the instantiate-to-bounds type of [T1] is a subtype of the |
| /// instantiate-to-bounds type of [T2] and not vice versa. |
| /// @description Check that: |
| /// For [x.best()], the most specific one is [BestList]. Because [List<int>] is a |
| /// proper subtype of both [iterable<int>] and [<List<num>], we expect BestList |
| /// to be the best implementation. The return type causes [v] to have type [int]. |
| /// If we had chosen [BestSpec] instead, the return type could only be [num], |
| /// which is one of the reasons why we choose the most specific instantiated type |
| /// as the winner. |
| /// For [y.best()], the most specific extension is [BestSpec]. The instantiated |
| /// on types that are compared are [Iterable<num>] for [BestCom] and [List<num>] |
| /// for the two other. Using the instantiate-to-bounds types as tie-breaker, we |
| /// find that [List<Object>] is less precise than [List<num>], so the code of |
| /// [BestSpec] has more precise information available for its method |
| /// implementation. The type of [w] becomes [num]. |
| /// @author iarkh@unipro.ru |
| |
| |
| import "../../Utils/expect.dart"; |
| |
| extension BestCom<T extends num> on Iterable<T> { |
| T best() { throw ""; } |
| } |
| extension BestList<T> on List<T> { |
| T best() { return this[0]; } |
| } |
| |
| extension BestSpec on List<num> { |
| num best() { return 0; } |
| } |
| |
| main() { |
| List<int> x = [1, 2, 3]; |
| Expect.equals(1, x.best()); |
| List<num> y = [1, 2, 3]; |
| Expect.equals(0, y.best()); |
| } |