For a custom Sequence protocol, but index don't change, but use while index change. why?

struct BoxedFibonacciSequence: Sequence, IteratorProtocol { typealias Element = Int var currentIndex = 0

mutating func next() -> Int? {
    defer { currentIndex += 1 }
    return  loadFibNumber(at: currentIndex)
}

func makeIterator() -> Self {
    return self
}

}

func loadFibNumber(at index: Int) -> Int { return fibNumber(at: index) }

var box = BoxedFibonacciSequence() for v in box { print("for index",box.currentIndex) if v < 20 { } else { break } }

let fib = BoxedFibonacciSequence() var iter = fib.makeIterator() while let v = iter.next() { print("while index",iter.currentIndex) if v < 20 { } else { break } }

why use for index always 0, but while index is normal? I'm confused.