将Range和StrideTo转换为序列时出现异常错误

Question

以下函数产生编译器错误 ：

func iterateColumnsAlongGravity<S: Sequence>(using block: (_ indexes: S) -> ())
    where S.Element == Int {

        switch gravityDirection {
        case .bot:
            for i in stride(from: 0, to: w * h, by: h) {
                // 'Range<Int>' is not convertible to 'S'
                block(i..<(i + h))
            }
        case .top:
            for i in stride(from: 0, to: w * h, by: h) {
                // 'ReversedCollection<(Range<Int>)>' is not convertible to 'S'
                block((i..<(i + h)).reversed())
            }
        case .left:
            for y in 0..<h {
                let indexes = stride(from: y, to: w * h, by: h).reversed()
                // '([Int]) -> ()' is not convertible to '(S) -> ()'
                block(indexes)
            }
        case .right:
            for y in 0..<h {
                // '(StrideTo<Int>) -> ()' is not convertible to '(S) -> ()'
                let indexes = stride(from: y, to: w * h, by: h)
                block(indexes)
            }
        }
}

我不明白为什么Range显然符合Sequence且元素为Int为什么编译器不将 Range<Int> 转换为S （以及其他类型）。

更奇怪的是 ，如果我用具有相似签名的类方法替换block ， 就不会有任何错误 ：

func printIntSequence<S: Sequence>(_ s: S) where S.Element == Int {
    for i in s {
        print(i)
    }
}

func typeConversionTest() {
    switch gravityDirection {
    case .bot:
        for i in stride(from: 0, to: w * h, by: h) {
            printIntSequence(i..<(i + h))
        }
    case .top:
        for i in stride(from: 0, to: w * h, by: h) {
            printIntSequence((i..<(i + h)).reversed())
        }
    case .left:
        for y in 0..<h {
            let indexes = stride(from: y, to: w * h, by: h).reversed()
            printIntSequence(indexes)
        }
    case .right:
        for y in 0..<h {
            let indexes = stride(from: y, to: w * h, by: h)
            printIntSequence(indexes)
        }
    }
}

iterateColumnsAlongGravity(using:)和typeConversionTest()之间的唯一区别是第一个将块作为参数。

Answer 1

在讨论中对雨燕论坛以下信息透露：

1. 我尝试实现的功能称为高级多态性，而Swift尚未添加此重大功能。

2. 可能最简单的解决方案是丢弃泛型并在此处使用StrideTo<Int> 。

    func iterateColumnsAlongGravity(using block: (_ indexes: StrideTo<Int>) -> ()) { switch gravityDirection { case .bot: for i in stride(from: 0, to: w * h, by: h) { block(stride(from: i, to: i + h, by: 1)) } case .top: for i in stride(from: 0, to: w * h, by: h) { block(stride(from: i + h - h, to: i - h, by: -1)) } case .left: for y in 0..<h { block(stride(from: (w - 1) * h + y, to: -1, by: -h)) } case .right: for y in 0..<h { block(stride(from: y, to: w * h, by: h)) } } }