swift - 将泛型添加到递归枚举时: compiler can not infer type

Question

这个问题已经有答案了:

In Swift, how to generically limit function to types that understand T + T (3 个回答)

已关闭 7 年前。

根据 Swift 手册中递归枚举下的示例，我尝试向枚举添加泛型类型，但编译器无法推断我传递给评估函数的类型。我可以做什么来更改要编译的代码？这是编译器的错误还是限制？

protocol Summable {
    func +(lhs: Self, rhs: Self) -> Self
}

protocol Multipliable {
    func *(lhs: Self, rhs: Self) -> Self
}

indirect enum ArithmeticExpression<T: protocol<Summable, Multipliable>> {
    case Number(T)
    case Addition(ArithmeticExpression, ArithmeticExpression)
    case Multiplication(ArithmeticExpression, ArithmeticExpression)
}

func evaluate<T>(expression: ArithmeticExpression<T>) -> T {
    switch expression {
    case .Number(let value):
        return value
    case .Addition(let left, let right):
        return evaluate(left) + evaluate(right)
    case .Multiplication(let left, let right):
        return evaluate(left) * evaluate(right)
    }
}


let five = ArithmeticExpression.Number(5)
let four = ArithmeticExpression.Number(4)
let sum = ArithmeticExpression.Addition(five, four)
let product = ArithmeticExpression.Multiplication(sum, ArithmeticExpression.Number(2))
print(evaluate(product))

Answer 1

我刚刚找到了答案，实际上非常简单:

extension Int: Summable, Multipliable {}