Why does Swift favor protocol extension implementation over witness implementation?

[Slava_Pestov]

@Jumhyn

If a superclass A conforms to P but uses a default implementation for one of the requirements (echo), then there is no ‘slot’ in the conformance A: P for the echo requirement. As a subclass, B doesn’t get its own unique conformance to P—rather it has to reuse A’s conformance, including the ‘missing echo’ part of that conformance. So even if there is an echo method on B, it’s not registered as being part of the conformance.

I think you mean there's no slot in the class vtable, not in the protocol conformance. A protocol conformance always records an implementation for each protocol requirement. If the implementation is declared in a protocol extension, the corresponding entry points at the protocol extension method. If the implementation is declared inside a class, the entry points at a thunk which loads the entry from the class's vtable and calls it. Method overrides in a subclass are implemented by the subclass replacing the vtable entry with its own implementation.