@swift-5.2.5
struct DirectProduct<Left: Collection, Right: Collection> { let left: Left let right: Right let iteratorType: DirectProductIterator<Left, Right>.Type init(_ left: Left, _ right: Right) { self.left = left self.right = right iteratorType = DirectProductIterator.self } } extension DirectProduct where Left.Index == Int, Right.Index == Int { init(_ left: Left, _ right: Right) { self.left = left self.right = right iteratorType = DirectProductIterator2.self } } extension DirectProduct: Sequence { func makeIterator() -> DirectProductIterator<Left, Right> { iteratorType.init(left, right) } } class DirectProductIterator<Left: Collection, Right: Collection>: IteratorProtocol { fileprivate let left: Left fileprivate let right: Right required init(_ left: Left, _ right: Right) { self.left = left self.right = right } func next() -> (Left.Element, Right.Element)? { print("DirectProductIterator") return nil } } class DirectProductIterator2<Left: Collection, Right: Collection>: DirectProductIterator<Left, Right> where Left.Index == Int, Right.Index == Int { required init(_ left: Left, _ right: Right) { super.init(left, right) } override func next() -> (Left.Element, Right.Element)? { print("DirectProductIterator2") return nil } } do { let p = DirectProduct([2, 3, 5], ["X", "Y", "Z"] as Set<String>) for (_, _) in p {} } print() do { let p = DirectProduct([2, 3, 5], ["X", "Y", "Z"]) for (_, _) in p {} }
