[4.1] [Sema] Disallow conditional conformances on objective-c generics. #14665
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There's no way to look up information about objective-c generic parameters, meaning the runtime cannot check same-type constraints or conformance requirements, so dynamic casts cannot work. We want to keep the static and dynamic systems the same, so we have to disable this functionality from the start (i.e. no conditional conformances on objective-c types :( ). Fixes rdar://problem/37524969.
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@swift-ci please test |
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@swift-ci please nominate |
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• Explanation: Dynamic casts to existentials (i.e. given
x: Any,x as SomeProtocol) requires looking up type information for conditional conformances to validate the requirements(i.e. ifxis aFoo<Bar>whereextension Foo: SomeProtocol where T == Int, then the cast above requires checking (and failing, in this case) thatT == Bar == Int), and these look-ups can only happen dynamically. Metadata about the types used in objective-C generics doesn't exist at runtime and so the look-ups are impossible. We want to keep the static and dynamic type systems the same, so we have to outlaw such conditional conformances.• Scope of Issue: Only code trying to conditionally conform a protocol to an objective-c generic type is affected, and 4.1 is the first release including conditional conformances.
• Origination: Only recently realised that this is a consequence of conditional conformances.
• Risk: Low: small change to unreleased feature.
• Reviewed By: @DougGregor
• Testing: Compiler regression tests
• Radar / SR: rdar://problem/37524969
#14628