In the added test case, the `typealias` refers to the `HiddenStruct` type in the private module, which is imported as `@_implementationOnly`. Because the import is `@_implementationOnly`, during deserialization, we don’t import the private module and hence any reference to the `HiddenStruct` type fails. In the common deserialization code path, this causes us to skip over the `typealias` member. However, when creating the protocol conformance, we assume that we can resolve the type to which the `typealias` refers and thus we are crashing.
If `LangOpts.EnableDeserializationRecovery` is set to `true`, we should do our best to recover from such failures so this patch makes the deserialization failure handling more graceful and resolve the right-hand side of the `typealias` as an `ErrorType`.
Fixes rdar://72891807
