为什么不能在Swift中创建协议一致性对象数组?

我创建了以下协议和符合协议的枚举,如下所示:

protocol HamburgerOption {

    static var all :[HamburgerOption] { get }
    var title :String { get }
}

enum Meat :HamburgerOption {

    private static var allCases :[Meat] = [.chicken, .beef]

    case chicken
    case beef

    static var all :[HamburgerOption] {
        return Meat.allCases
    }

    var title :String {
        switch self {
            case .chicken:
                return "Chicken"
            case .beef:
                return "Beef"
        }
    }
}

当我创建以下结构时,它会给我错误:

// Value of type 'Meat.Type' does not conform to expected element type //'HamburgerOption'

    struct HamburgerOptions  {

        var all :[HamburgerOption] {
            return [Meat, Sauces]
        }
    }