Allow Deriving Type Witnesses Across Files #33187
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A recent change to witness matching in swiftlang#32578 suddenly made the following construction illegal: // File1.swift enum MyEnumInAnotherFile { /**/ } // File2.swift extension MyEnumInAnotherFile { static var allCases: [MyEnumInAnotherFile] { /**/ } } Because it was no longer possible to derive the type witness for `AllCases`. This is because, when inference ran before synthesis, we would use the value witness to pick out the type witness and thus had no need for synthesis. Now that we run synthesis first, we ought to just allow deriving type witnesses across files, but still ban deriving value witnesses. In general, if you can utter a type in a different file to extend it, you should be able to see the requirements necessary to derive a default type witness. rdar://66279278, rdar://66279375, rdar://66279384, rdar://66279415, rdar://66279503
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slavapestov
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Jul 30, 2020
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A recent change to witness matching in #32578 suddenly made the
following construction illegal:
Because it was no longer possible to derive the type witness for
AllCases. This is because, when inference ran before synthesis, wewould use the value witness to pick out the type witness and thus had no
need for synthesis. Now that we run synthesis first, we ought to just
allow deriving type witnesses across files, but still ban deriving value
witnesses. In general, if you can utter a type in a different file to
extend it, you should be able to see the requirements necessary to
derive a default type witness.
rdar://66279278, rdar://66279375, rdar://66279384, rdar://66279415, rdar://66279503, rdar://65943338