Fixes for Dotty compatibility

- `CollisionProofHashMap` was missing overrides for `fromSpecific`,
  `newSpecificBuilder` and `empty`
- `TreeSet.from` exploited a GADT unsoundness in scalac

Author: szeiger

src/library/scala/collection/mutable/CollisionProofHashMap.scala

@@ -15,6 +15,7 @@ package mutable
 
 import scala.annotation.meta.{getter, setter}
 import scala.annotation.tailrec
+import scala.annotation.unchecked.uncheckedVariance
 import scala.collection.generic.DefaultSerializationProxy
 import scala.collection.mutable
 import scala.runtime.Statics
@@ -63,6 +64,11 @@ final class CollisionProofHashMap[K, V](initialCapacity: Int, loadFactor: Double
 
   @`inline` private[this] final def index(hash: Int) = hash & (table.length - 1)
 
+  override protected def fromSpecific(coll: IterableOnce[(K, V)] @uncheckedVariance): CollisionProofHashMap[K, V] @uncheckedVariance = CollisionProofHashMap.from(coll)
+  override protected def newSpecificBuilder: Builder[(K, V), CollisionProofHashMap[K, V]] @uncheckedVariance = CollisionProofHashMap.newBuilder[K, V]
+
+  override def empty: CollisionProofHashMap[K, V] = new CollisionProofHashMap[K, V]
+
   override def contains(key: K): Boolean = findNode(key) ne null
 
   def get(key: K): Option[V] = findNode(key) match {

src/library/scala/collection/mutable/TreeSet.scala

@@ -194,7 +194,7 @@ 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))
+        new TreeSet[E](RB.fromOrderedKeys(it.asInstanceOf[Iterator[E]], r.size))
       case _ =>
         val t: RB.Tree[E, Null] = RB.Tree.empty
         val i = it.iterator