假设我想在swift中用面向协议的编程来建模一个幻想游戏。下面的代码会产生以下错误：
协议“Character”只能用作泛型约束，因为它具有Self或关联的类型要求
这有什么问题，怎么做才对？

protocol Character: Identifiable {
    var name: String {get}
    var maxHealt: Int {get}
    var healt: Int { get set }
}
extension Character {
    var id: String {return name}
    mutating func setHealth(newValue: Int) {
        healt = newValue
    }
}

protocol Fighter {
    var attackPower: Int { get }
}
extension Fighter {
    func attack(enemy: Character ) -> Character {
        var enemy = enemy
        enemy.setHealth(newValue: enemy.maxHealt - self.attackPower)
        return enemy
    }
}