1 files changed, 4 insertions, 3 deletions
diff --git a/src/library/scala/collection/mutable/ArrayOps.scala b/src/library/scala/collection/mutable/ArrayOps.scala
index 98d635ecee..a465e9152c 100644
--- a/src/library/scala/collection/mutable/ArrayOps.scala
+++ b/src/library/scala/collection/mutable/ArrayOps.scala
@@ -64,10 +64,11 @@ abstract class ArrayOps[T] extends ArrayLike[T, Array[T]] with Parallelizable[Pa
    *  @param asArray   A function that converts elements of this array to rows - arrays of type `U`.
    *  @return          An array obtained by concatenating rows of this array.
    */
-  def flatten[U](implicit asArray: T => /*<:<!!!*/ Array[U]): Array[U] = {
-    val b = rowBuilder[U]
+  def flatten[U, To](implicit asTrav: T => collection.Traversable[U], m: ClassManifest[U]): Array[U] = {
+    val b = Array.newBuilder[U]
+    b.sizeHint(map{case is: IndexedSeq[_] => is.size case _ => 0} sum)
     for (xs <- this)
-      b ++= asArray(xs)
+      b ++= asTrav(xs)
     b.result
   }