ListMap is reverse oriented, so use that for foldRight

Author: mkeskells

library/src/scala/collection/immutable/ListMap.scala

@@ -44,6 +44,11 @@ object ListMap extends ImmutableMapFactory[ListMap] {
 
   @SerialVersionUID(-8256686706655863282L)
   private object EmptyListMap extends ListMap[Any, Nothing]
+
+  @tailrec private def foldRightInternal[A, B, Z](map: ListMap[A, B], prevValue: Z, op: ((A, B), Z) => Z): Z = {
+    if (map.isEmpty) prevValue
+    else foldRightInternal(map.init, op(map.last, prevValue), op)
+  }
 }
 
 /**
@@ -127,6 +132,7 @@ sealed class ListMap[A, +B] extends AbstractMap[A, B]
   protected def value: B = throw new NoSuchElementException("value of empty map")
   protected def next: ListMap[A, B] = throw new NoSuchElementException("next of empty map")
 
+  override def foldRight[Z](z: Z)(op: ((A, B), Z) => Z): Z = ListMap.foldRightInternal(this, z, op)
   override def stringPrefix = "ListMap"
 
   /**