GADT issue

[allanrenucci]

Found in the 2.13 std lib:

class Test {
  class Tree[A]
  class TreeSet[A](tree: Tree[A])

  def fromOrderedKeys[A](xs: Iterator[A]): Tree[A] = ???

  def from[E](it: Iterable[E]): TreeSet[E] =
    it match {
      case r: Range =>
        val it = r.iterator
        new TreeSet[E](fromOrderedKeys(it))
    }
}

-- [E007] Type Mismatch Error: Test.scala:11:39 --------------------
11 |        new TreeSet[E](fromOrderedKeys(it))
   |                                       ^^
   |                                       found:    Iterator[Int](it)
   |                                       required: Iterator[E]
   |                                       
one error found

[allanrenucci]

cc/ @AleksanderBG

[odersky]

The problem is that instantiation of covariant GADTs is unsound. So we can't implement that in Dotty. A cast is needed here.