1 files changed, 1 insertions, 1 deletions
diff --git a/src/library/scala/collection/generic/MutableMapBuilder.scala b/src/library/scala/collection/generic/MutableMapBuilder.scala
index 7e64543214..419eaef0a4 100644
--- a/src/library/scala/collection/generic/MutableMapBuilder.scala
+++ b/src/library/scala/collection/generic/MutableMapBuilder.scala
@@ -20,7 +20,7 @@ package scala.collection.generic
 class MutableMapBuilder[A, B, Coll <: mutable.Map[A, B] with MapTemplate[A, B, Coll]](empty: Coll)
 extends Builder[(A, B), Coll] {
   protected var elems: Coll = empty
-  def +=(x: (A, B)) { elems += x }
+  def +=(x: (A, B)): this.type = { elems += x; this }
   def clear() { elems = empty }
   def result: Coll = elems
 }