Fix for Iterator flattening regression.

but my theory is that ++ takes a by name argument, but doing a foldLeft
and using ++ to join creates a closure which loses the by-nameness. If
this theory is correct that's an ugly trap.

Not sure how I write a test against this sort of thing? Will take
pointers. For now, closes #4582, no review.

1 files changed, 6 insertions, 1 deletions

diff --git a/src/library/scala/collection/TraversableOnce.scala b/src/library/scala/collection/TraversableOnce.scala
index 90f1583c58..84fe4bdf4c 100644
--- a/src/library/scala/collection/TraversableOnce.scala
+++ b/src/library/scala/collection/TraversableOnce.scala
@@ -346,7 +346,12 @@ object TraversableOnce {
   }
 
   class FlattenOps[A](travs: TraversableOnce[TraversableOnce[A]]) {
-    def flatten: Iterator[A] = travs.foldLeft(Iterator.empty: Iterator[A])(_ ++ _)
+    def flatten: Iterator[A] = new Iterator[A] {
+      val its = travs.toIterator
+      private var it: Iterator[A] = Iterator.empty
+      def hasNext: Boolean = it.hasNext || its.hasNext && { it = its.next.toIterator; hasNext }
+      def next(): A = if (hasNext) it.next() else Iterator.empty.next()
+    }
   }
 
   class MonadOps[+A](trav: TraversableOnce[A]) {