swift

b1397703 - [ConformanceLookup] Just kidding, the compiler needs to prefer available

[ConformanceLookup] Just kidding, the compiler needs to prefer available Sendable conformances for source compatibility. If conformance lookup always prefers the conformance from the defining module, libraries introducing unavailable Sendable conformances can break source in clients that declare retroactive Sendable conformances. Instead, still prefer the available conformance, and always diagnose the client conformance as redundant (as a warning). Then, when the retroactive conformance is removed, the errors will surface, so the programmer has to take explicit action to experience the source break.