final ext.swift

public protocol P {
  func foo()
}

public protocol Q: P {
}
extension Q {
  final func foo() { … }
}

class Thingy: P {
  func foo() { … }  // OK!
}

class Dingy: Q {
  func foo() { … }  // Not allowed, because final
}

let p: P = …
p.foo()  // Needs dynamic dispatch, because P is public & other module could provide other impl

let q: Q = …
q.foo()  // Can use static dispatch iff extensions allow final