sample.swift

protocol SpeakProtocol {
    // func speak() -> String
    func instinctSpeak() -> String
}

extension SpeakProtocol {
    func speak() -> String {
        return "I am anything"
    }
    
    func instinctSpeak() -> String {
        return speak()
    }
}

struct Human: SpeakProtocol {
    func speak() -> String {
        return "I am human"
    }
}

let human = Human()
human.speak()
human.instinctSpeak()