// note, P does not declare greet as requirement

protocol P {}

// ... but this does provide a "default" implementation

extension P {
    func greet() {print("hello")}
}

// ... so if you have a class that overrides this

class C : P {
    func greet() {print("hello from C")
    }
}

// but then use the protocol, rather than the class

func greeter(_ obj: P) {
    obj.greet()
}

// then this will, confusingly IMHO, print "hello", not "hello from C"

greeter(C())

// ----

// However, if we define a protocol with this as a requirement

protocol Q {
    func greet()
}

// ... and provide a default implementation

extension Q {
    func greet() {print("hello")}
}

// ... and then override this default implementation

class D : Q {
    func greet() {print("hello from D")}
}

// ... now this Q will honor any overrides that the class may implement

func anotherGreeter(_ obj: Q) {
    obj.greet()
}

// ... this will now print "hello from D", as you'd expect it to

anotherGreeter(D())