https://discordapp.com/channels/291054398077927425/291211035438874625/716098121721053185

DirectProduct.swift

struct DirectProduct<Left: Sequence, Right: Sequence>: Sequence {
    let left: Left
    let right: Right
    let iteratorType: Iterator.Type
    
    init(_ left: Left, _ right: Right) {
        self.left = left
        self.right = right
        iteratorType = Iterator1.self
    }
    
    func makeIterator() ->Iterator {
        iteratorType.init(left, right)
    }
    
    class Iterator: 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)? {
            fatalError("Override this method.")
        }
    }
    
    private class Iterator1: Iterator {
        private var leftIterator: Left.Iterator
        private var rightIterator: Right.Iterator?
        private var leftElement: Left.Element?
        
        required init(_ left: Left, _ right: Right) {
            leftIterator = left.makeIterator()
            rightIterator = right.makeIterator()
            leftElement = leftIterator.next()
            super.init(left, right)            
        }
        
        override func next() -> (Left.Element, Right.Element)? {
                guard let leftElement = self.leftElement else { return nil }
                
                if rightIterator == nil {
                    rightIterator = right.makeIterator()
                }
                
                guard let rightElement = rightIterator?.next() else {
                    rightIterator = nil
                    self.leftElement = leftIterator.next()
                    return next()
                }
                
                return (leftElement, rightElement)
        }
    }
}

extension DirectProduct where Left: Collection, Right: Collection, Left.Index == Int, Right.Index == Int {
    init(_ left: Left, _ right: Right) {
            self.left = left
            self.right = right
            iteratorType = Iterator2.self
    }

    private class Iterator2: Iterator {
        private var leftIndex: Int
        private var rightIndex: Int
        
        required init(_ left: Left, _ right: Right) {
            leftIndex = left.startIndex
            rightIndex = right.startIndex
            super.init(left, right)            
        }
        
        override func next() -> (Left.Element, Right.Element)? {
            guard leftIndex < left.endIndex else { return nil }
            guard rightIndex < right.endIndex else {
                leftIndex += 1
                rightIndex = right.startIndex
                return next()
            }
            
            defer { rightIndex += 1 }
            return (left[leftIndex], right[rightIndex])
        }
    }
}

do {
    let p = DirectProduct([2, 3, 5], ["X", "Y", "Z"] as Set<String>)
    for (l, r) in p { print("(\(l), \(r))") }
}

print()

do {
    let p = DirectProduct([2, 3, 5], ["X", "Y", "Z"])
    for (l, r) in p { print("(\(l), \(r))") }
}