:: needs to be invariant in this fork to Ycheck

Dotty upstream has custom fixes to make it covariant.

minicoll/collection/immutable/List.scala

@@ -436,7 +436,7 @@ case object Nil extends List[Nothing] {
  *  @since   2.8
  */
 @SerialVersionUID(509929039250432923L) // value computed by serialver for 2.11.2, annotation added in 2.11.4
-final case class ::[+B](override val head: B, private[scala] var tl: List[B]) extends List[B] {
+final case class ::[B](override val head: B, private[scala] var tl: List[B]) extends List[B] {
   override def tail : List[B] = tl
   override def isEmpty: Boolean = false
 }