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.

4 files changed, 8 insertions, 6 deletions

diff --git a/src/library/scala/collection/generic/ImmutableSetFactory.scala b/src/library/scala/collection/generic/ImmutableSetFactory.scala
index f4d4e061bb..a72caf2633 100644
--- a/src/library/scala/collection/generic/ImmutableSetFactory.scala
+++ b/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])
 }
diff --git a/src/library/scala/collection/immutable/HashSet.scala b/src/library/scala/collection/immutable/HashSet.scala
index 82a1e471df..e808a966ab 100644
--- a/src/library/scala/collection/immutable/HashSet.scala
+++ b/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
diff --git a/src/library/scala/collection/immutable/ListSet.scala b/src/library/scala/collection/immutable/ListSet.scala
index 7ebaa26d26..1bb07eb02d 100644
--- a/src/library/scala/collection/immutable/ListSet.scala
+++ b/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
diff --git a/src/library/scala/collection/immutable/Set.scala b/src/library/scala/collection/immutable/Set.scala
index a888955bb2..e21a8dfa8a 100644
--- a/src/library/scala/collection/immutable/Set.scala
+++ b/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)