SI-6457 ImmutableSetFactory.empty results in StackOverflowError

ImmutableSetFactory relies upon empty to create its builder, which in turn relies upon its builder to create an empty.  Added an emptyInstance method to hold the actual empty set (which also removes duplicated functionality for empty sets).

No test, as the original pattern:

  object Factory extends collection.generic.ImmutableSetFactory[collection.immutable.Set] {}

no longer compiles.  This should be verified by whoever checks this commit, but this kind of change is hard to revert by accident.  No reason to waste resources checking it forevermore.

src/library/scala/collection/generic/ImmutableSetFactory.scala

@@ -15,6 +15,7 @@ import scala.language.higherKinds
 
 abstract class ImmutableSetFactory[CC[X] <: immutable.Set[X] with SetLike[X, CC[X]]]
   extends SetFactory[CC] {
-
+  private[collection] def emptyInstance: CC[Any]
+  override def empty[A] = emptyInstance.asInstanceOf[CC[A]]
   def newBuilder[A]: Builder[A, CC[A]] = new SetBuilder[A, CC[A]](empty[A])
 }

src/library/scala/collection/immutable/HashSet.scala

@@ -99,10 +99,10 @@ object HashSet extends ImmutableSetFactory[HashSet] {
 
   /** $setCanBuildFromInfo */
   implicit def canBuildFrom[A]: CanBuildFrom[Coll, A, HashSet[A]] = setCanBuildFrom[A]
-  override def empty[A]: HashSet[A] = EmptyHashSet.asInstanceOf[HashSet[A]]
 
   private object EmptyHashSet extends HashSet[Any] { }
-
+  private[collection] def emptyInstance: HashSet[Any] = EmptyHashSet
+
   // utility method to create a HashTrieSet from two leaf HashSets (HashSet1 or HashSetCollision1) with non-colliding hash code)
   private def makeHashTrieSet[A](hash0:Int, elem0:HashSet[A], hash1:Int, elem1:HashSet[A], level:Int) : HashTrieSet[A] = {
     val index0 = (hash0 >>> level) & 0x1f

src/library/scala/collection/immutable/ListSet.scala

@@ -22,10 +22,11 @@ import mutable.{ ListBuffer, Builder }
 object ListSet extends ImmutableSetFactory[ListSet] {
   /** setCanBuildFromInfo */
   implicit def canBuildFrom[A]: CanBuildFrom[Coll, A, ListSet[A]] = setCanBuildFrom[A]
-  override def empty[A] = EmptyListSet.asInstanceOf[ListSet[A]]
+
   override def newBuilder[A]: Builder[A, ListSet[A]] = new ListSetBuilder[A]
 
   private object EmptyListSet extends ListSet[Any] { }
+  private[collection] def emptyInstance: ListSet[Any] = EmptyListSet
 
   /** A custom builder because forgetfully adding elements one at
    *  a time to a list backed set puts the "squared" in N^2.  There is a

src/library/scala/collection/immutable/Set.scala

@@ -53,8 +53,7 @@ trait Set[A] extends Iterable[A]
 object Set extends ImmutableSetFactory[Set] {
   /** $setCanBuildFromInfo */
   implicit def canBuildFrom[A]: CanBuildFrom[Coll, A, Set[A]] = setCanBuildFrom[A]
-  override def empty[A]: Set[A] = EmptySet.asInstanceOf[Set[A]]
-
+
   /** An optimized representation for immutable empty sets */
   private object EmptySet extends AbstractSet[Any] with Set[Any] with Serializable {
     override def size: Int = 0
@@ -64,6 +63,7 @@ object Set extends ImmutableSetFactory[Set] {
     def iterator: Iterator[Any] = Iterator.empty
     override def foreach[U](f: Any =>  U): Unit = {}
   }
+  private[collection] def emptyInstance: Set[Any] = EmptySet
 
   /** An optimized representation for immutable sets of size 1 */
   @SerialVersionUID(1233385750652442003L)