protocol P {
    func f()
}

extension P {
    func f() {}
}

class Foo: P {
//    func f() {}
}

class Bar: Foo {
    // Why no `override`? From what `Bar` sees its superclass `Foo` implements `P`'s interface, whether with explicit implementation of `f` in itself or borrowed implementation of `f` from `P`'s extension. Uncomment Foo.f() to see the contradictory behavior.
    override func f() {}
}