Unusual Case of Generic Type-Inference

Jens · July 10, 2020, 9:06pm

[SR-10158] Associated type inference resolution ignores conditionality of extensions

opened 03:29PM - 23 Mar 19 UTC

closed 05:16PM - 25 Mar 19 UTC

bug Compiler

| | | |------------------|-----------------|… |Previous ID | SR-10158 | |Radar | None | |Original Reporter | @DevAndArtist | |Type | Bug | |Status | Closed | |Resolution | Won't Do | <details> <summary>Environment</summary> - Apple Swift version 4.2.1 (swiftlang-1000.11.42 clang-1000.11.45.1) - Also seems to be present in Swift 5 </details> <details> <summary>Additional Detail from JIRA</summary> | | | |------------------|-----------------| |Votes | 1 | |Component/s | Compiler | |Labels | Bug | |Assignee | None | |Priority | Medium | md5: 516ea1ec593c1b61178384afe9c7df85 </details> **relates to**: * [SR-5440](https://bugs.swift.org/browse/SR-5440) Typealias in constrained extension misinterprets the where clause **Issue Description:** This is a magical behavior, and it should be disallowed as it's a source of potential ambiguity issues in a more complex project. ``` java protocol Test { associatedtype Something var something: Something { get } } // This is a 'conditional' extension which should not be picked // by the compiler to satisfy protocol conformance automatically // when the conforming type does not provide any requirements // until the explicitly stated that `Something` should be of type `Int` extension Test where Something == Int { var something: Something { return 42 } } struct S: Test {} // THIS SHOULD ERROR OUT print(S().something) // prints 42 ``` Similar example where the default associated type is simply overriden/ignored: ``` java protocol Test { associatedtype Something = String var something: Something { get } } extension Test where Something == Int { var something: Something { return 42 } } struct S: Test {} print(S().something) // again an Int and not String ``` Discussion on the forums: <https://forums.swift.org/t/what-kind-of-magic-behavior-is-this/22014>

Not sure if related, only skimmed this thread.
Here is another place where I though it might be relevant.