こんな感じのことをやりたいですか？

struct DirectProduct<Iterator, Element, Left: Sequence, Right: Sequence> where Left.Element == Element, Right.Element == Element, Left.Iterator == Iterator, Right.Iterator == Iterator { let left: Left let right: Right } extension DirectProduct: Sequence where Left: Collection, Right: Collection { func makeIterator() -> Iterator { if left.startIndex is Int, right.startIndex is Int { // return iterator } else { // return another iterator } } }