object test {
  class List[+a] {
    def ::[b >: a](x: b): List[b] = new Cons(x, this)
  }
  case class Cons[a, b <: a](x: a, xs: List[b]) extends List[a]
  case object Nil extends List[Nothing]
  def nil[n]: List[n] = Nil
  def cons[a](x: a, xs: List[a]): List[a] = null
  val x: List[Int] = Nil.::(1)
  val y: List[Int] = nil.::(1)
}