I guess fundamentally, this is the question I have about this situation: would you ever, in real code, want dispatch to go straight back to the default implementation when you have in hand a value with a custom implementation?
I may be having a failure of imagination; but I certainly haven't ever seen a case where I'd want that.
I’d imagine the issue lies in Swift classes utilizing only static dispatch. The protocol conformance creates a witness table populated by the methods of the class in question, but as Swift classes have no vtables except in ObjC compatability mode (right?) there’s no way to pass down the invocation from the protocol witness to a specific subclass as the method can not be resolved at runtime. This is the difference between the dynamic dispatch via witness table and the explicit static dispatch to the subclass when type information is available.
Thus it comes down to dynamic dispatch always (or at least for anything that has a protocol conformance at all) vs surprising behavior here.
Classes do get dynamic dispatch at least for methods/properties that can be overridden (i.e., `open`). I believe the value must also still have a reference to its class's method impls -- otherwise what happens when you cast it `if let noReallyLazyGreeter = lazyGreeter as? LazyGreeter {`? Not certain about these details, though.
I may be having a failure of imagination; but I certainly haven't ever seen a case where I'd want that.