operator fun <T> Pair<T, T>.iterator() = object : Iterator<T> {
    var i = 0
    
    override fun hasNext() = i < 2
    
    override fun next() = when (i++) {
        0 -> first
        1 -> second
        else -> throw NoSuchElementException()
    }
}

operator fun <T> Pair<T, T>.iterator() = sequence {
    yield(first)
    yield(second)
}.toIterator()

Pair

Iterable

Iterable

List

Iterator

Pair

iterator {}