Refactor OnceCanBuildFrom to check the collection type.

The builder is now instantiated as an iterator builder only
if a generic builder cannot be found on the collection that
requested the builder.

Reason - we want this behaviour:

scala> scala.util.Random.shuffle(List(1, 2, 3): collection.TraversableOnce[Int])
res0: scala.collection.TraversableOnce[Int] = List(3, 1, 2)

instead of this one:

scala> scala.util.Random.shuffle(List(1, 2, 3): collection.TraversableOnce[Int])
res0: scala.collection.TraversableOnce[Int] = non-empty iterator

which may lead to nasty surprises.

Alternately, to avoid pattern-matching in OnceCanBuildFrom.apply, we
could mix in GenericTraversableTemplate-related functionaly into
TraversableOnce, but this may become too complicated.

1 files changed, 2 insertions, 1 deletions

diff --git a/src/library/scala/collection/Iterator.scala b/src/library/scala/collection/Iterator.scala
index af83291a46..3b2a7c7820 100644
--- a/src/library/scala/collection/Iterator.scala
+++ b/src/library/scala/collection/Iterator.scala
@@ -26,7 +26,8 @@ object Iterator {
    *  See `scala.util.Random.shuffle` for an example.
    */
   implicit def IteratorCanBuildFrom[A] = new TraversableOnce.BufferedCanBuildFrom[A, Iterator] {
-    def toColl[B](coll: ArrayBuffer[B]) = coll.iterator
+    def bufferToColl[B](coll: ArrayBuffer[B]) = coll.iterator
+    def traversableToColl[B](t: GenTraversable[B]) = t.toIterator
   }
 
   /** The iterator which produces no values. */