在Swift协议中 - 为什么我被迫使用关键字＆＃39; Final＆＃39;在我的羊班之前？

Question

protocol Cloner {
    func cloneSelf(thingBeingCloned t: Self) -> Self
}

final class Sheep : Cloner {
    func cloneSelf(thingBeingCloned t: Sheep) -> Sheep {
        let newSheep = Sheep.init()
        return newSheep
    }
}

如果我使用关键字＆＃39; final＆＃39;在课前羊离开时，编译器对我大吼大叫。我收到错误：

method 'cloneSelf(thingBeingCloned:)' in non-final class 'Sheep' must return `Self` to conform to protocol 'Cloner'

好的..然后我听到了，我将返回值更改为Self而不是Sheep，然后我又得到了另一个错误：

error: cannot convert return expression of type 'Sheep' to return type 'Self'