[Serialization] Drop extensions whose base type can't be deserialized. #11323
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This shows up with swift_wrapper typedefs, which get imported into Swift as structs. If someone makes an extension of a swift_wrapper type, but the swift_wrapper is only applied in Swift 4 mode, that extension will break any Swift 3 clients. Recover by just dropping the extension entirely. There's still more complexity around extensions---what if a requirement can't be deserialized? what if something's depending on the protocol conformance provided by the extension?---but the missing base type case should be pretty safe. If you can't see the type at all, things that depend on its conformances are already in trouble. rdar://problem/33636733
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DougGregor
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Aug 3, 2017
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This shows up with
swift_wrappertypedefs, which get imported into Swift as structs. If someone makes an extension of aswift_wrappertype, but theswift_wrapperis only applied in Swift 4 mode, that extension will break any Swift 3 clients. Recover by just dropping the extension entirely.There's still more complexity around extensions—what if a requirement can't be deserialized? what if something's depending on the protocol conformance provided by the extension?—but the missing base type case should be pretty safe. If you can't see the type at all, things that depend on its conformances are already in trouble.
rdar://problem/33636733