I'm uncomfortable with the idea of hacking the compiler like this at this point. At the very least, please limit this to "synthesize overrides" rather than "derive a new conformance".

"It's no worse than NSCoding."

@belkadan I think I'm just using the wrong terminology here; what I'm looking to here is that just because a class inherits from a type that is Encodable or Decodable doesn't mean we shouldn't synthesize overrides on its behalf.

@belkadan In working on PR-11045 I understood what you meant about hacking the compiler like this. Yeah, it made me uncomfortable too, and even synthesizing overrides was a stretch. I've left things as they are, but added a note to improve the diagnostic that you get if you do something like this:

class Super : Codable {
    var value = 5
}

// Confusing to beginners: why does this not synthesize Codable for sub???
class Sub : Super {
    var superValue: Double
}

You now get a note suggesting you override init(from🙂 (and possibly encode(to🙂).

Unfortunately, we can't at the moment do much about cases where you add storage that has an initial value — without additional hackiness, we never really detect that case.