Why it is not a tail recursion?

问题

I have the following code, that I do not understand, why it is not a tail recursion:

override fun drop(n: Int): List<A> = if (n == 0) this else tail.drop(n - 1)

whereas this is a tail recursion:

fun drop(n: Int): List<A> {
  tailrec fun drop(n: Int, list: List<A>): List<A> =
    if (n <= 0) list else when (list) {
      is Cons -> drop(n - 1, list.tail)
      is Nil -> list
    }
  return drop(n, this)
}

Why is the first example not a tail recursion?

回答1:

It isn't tail recursion because Kotlin checks the recursive call is on the same receiver. In your case it's correct; drop is a virtual function (since you use override), so tail.drop could have a different implementation. For non-open functions there is the issue Tailrec optimization not being applied to tail recursive calls on a non-this receiver, but it doesn't seem to be actively worked on.

Note this bug as well: Recursive calls to open tailrec functions are generated incorrectly

来源：https://stackoverflow.com/questions/60202108/why-it-is-not-a-tail-recursion