Main.scala

object Main {
  def head[A](list: List[A]): A = list.head

  def x1: List[Int] | List[String] = List(3)

  // this conversion is ok. the opposite doesn't work.
  def x2: List[Int | String] = x1

  def x3 = head(x1)

  def x4 = head(x2)

  x3 : (Int | String) // compilation error with Matchable
  x4 : (Int | String) // this succeeds
}