closes #2788.

the added flexibility'll cost ya one ClassManifest

review by phaller

3 files changed, 8 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
   }
 
diff --git a/test/files/run/t2788.check b/test/files/run/t2788.check
new file mode 100644
index 0000000000..684f5f616c
--- /dev/null
+++ b/test/files/run/t2788.check
@@ -0,0 +1 @@
+List(1, 2)
\ No newline at end of file
diff --git a/test/files/run/t2788.scala b/test/files/run/t2788.scala
new file mode 100644
index 0000000000..7cc5031a59
--- /dev/null
+++ b/test/files/run/t2788.scala
@@ -0,0 +1,3 @@
+object Test extends Application {
+  println(Array(Some(1), None, Some(2)).flatten.toList)
+}
\ No newline at end of file