Conditional conformances and dynamic dispatch

ConditionalWrapper.swift

import Swift

struct ConditionalWrapper<Base> where Base: Sequence {
    fileprivate let base: Base
}

extension Sequence {
    func conditionalWrapper() -> ConditionalWrapper<Self> {
        return ConditionalWrapper(base: self)
    }
}

extension ConditionalWrapper: Sequence {
    typealias Element = Base.Element
    typealias Iterator = AnyIterator<Element>

    func makeIterator() -> AnyIterator<Element> {
        print("Sequence makeIterator")
        return AnyIterator(self.base.makeIterator())
    }
}

extension ConditionalWrapper: Collection where Base: Collection {
    var startIndex: Base.Index { return self.base.startIndex }
    var endIndex: Base.Index { return self.base.endIndex }
    func index(after index: Base.Index) -> Base.Index { return self.base.index(after: index) }
    subscript(position: Base.Index) -> Base.Element { return self.base[position] }

    func makeIterator() -> AnyIterator<Element> {
        print("Collection makeIterator")
        return AnyIterator(IndexingIterator(_elements: self))
    }
}

private func sequence<T>(_ x: T) where T: Sequence {
    _ = x.makeIterator()
}

private func unidirectional<T>(_ x: T) where T: Collection {
    _ = x.makeIterator()
}

func test_conditional() {
    let x = [1,2,3,4].conditionalWrapper()

    sequence(x) // prints "Sequence makeIterator"
    unidirectional(x) // prints "Sequence makeIterator" (unexpected)
    _ = x.makeIterator() // prints "Collection makeIterator"
}