Workaround scala/scala3#5418

Author: allanrenucci

src/library/scala/collection/immutable/TreeSet.scala

@@ -201,7 +201,10 @@ object TreeSet extends SortedIterableFactory[TreeSet] {
         new TreeSet[E](RB.fromOrderedKeys(ss.iterator, ss.size))
       case r: Range if (ordering eq Ordering.Int) || (ordering eq Ordering.Int.reverse) =>
         val it = if((ordering eq Ordering.Int) == (r.step > 0)) r.iterator else r.reverseIterator
-        new TreeSet[E](RB.fromOrderedKeys(it, r.size))
+        val tree = RB.fromOrderedKeys(it.asInstanceOf[Iterator[E]], r.size)
+          // The cast is needed to compile with Dotty:
+          // Dotty doesn't infer that E =:= Int, since instantiation of covariant GADTs is unsound
+        new TreeSet[E](tree)
       case _ =>
         var t: RB.Tree[E, Null] = null
         val i = it.iterator