試してみましたが extension ではできなさそうですね。 @swift-5.2.5
struct DirectProduct<Left: Sequence, Right: Sequence> { let left: Left let right: Right init(_ left: Left, _ right: Right) { self.left = left self.right = right } } extension DirectProduct: Sequence { func makeIterator() -> Iterator { print("makeIterator A") return Iterator(left, right) } class Iterator: IteratorProtocol { fileprivate let left: Left fileprivate let right: Right init(_ left: Left, _ right: Right) { self.left = left self.right = right } func next() -> (Left.Element, Right.Element)? { nil } } } extension DirectProduct where Left: Collection, Right: Collection, Left.Index == Int, Right.Index == Int { func makeIterator() -> Iterator { print("makeIterator B") return Iterator2(left, right) } class Iterator2: Iterator { override init(_ left: Left, _ right: Right) { super.init(left, right) } override func next() -> (Left.Element, Right.Element)? { nil } } } do { let p = DirectProduct([2, 3, 5], ["X", "Y", "Z"] as Set<String>) for (_, _) in p {} _ = p.makeIterator() } print() do { let p = DirectProduct([2, 3, 5], ["X", "Y", "Z"]) for (_, _) in p {} _ = p.makeIterator() }