Jordan Rose 35f8e17e88 [Serialization] Drop extensions whose base type can't be deserialized. (#11323) ...

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

2017-08-03 16:02:16 -07:00

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