a.swift

protocol P {
    func foo()
    func bar()
}

extension P {
    func foo() { print("foo() from P") }
    func bar() { print("bar() from P") }
}

class Base: P {
    func foo() {
        print("foo() from Base")
    }

    //func bar() { print("bar() from Base") }
}
class Refined: Base {
    override func foo() {
        print("foo() from Refined")
    }

    //override
    func bar() {
        print("bar() from Refined")
    }
}

let a = Refined()

a.foo()
(a as P).foo()
a.bar()
(a as P).bar()