SI-8428 Fix regression in iterator concatenation

Regressed in e3ddb2d7, which introduced `ConcatIterator` to
avoid super-linear runtime on chains of concatenated iterators.

`ConcatIterator` maintains a queue of thunks for the remaining
iterators. Both `next` and `hasNext` delegate to `advance`, which
evaluates the thunks and discards any empty iterators from the
start of the queue. The first non-empty iterator is stored in
the var `current`.

It also overrides `++`, and creates a new `ConcatIterator` with
the given `that` as an extra element in the queue. However, it
failed to copy `current` across, which led to data loss.

2 files changed, 13 insertions, 1 deletions

diff --git a/src/library/scala/collection/Iterator.scala b/src/library/scala/collection/Iterator.scala
index 01a0aa3b51..3d379bfa95 100644
--- a/src/library/scala/collection/Iterator.scala
+++ b/src/library/scala/collection/Iterator.scala
@@ -185,7 +185,7 @@ object Iterator {
     def next()  = if (hasNext) current.next else Iterator.empty.next
 
     override def ++[B >: A](that: => GenTraversableOnce[B]): Iterator[B] =
-      new ConcatIterator(queue :+ (() => that.toIterator))
+      new ConcatIterator((() => current) +: (queue :+ (() => that.toIterator)))
   }
 
   private[scala] final class JoinIterator[+A](lhs: Iterator[A], that: => GenTraversableOnce[A]) extends Iterator[A] {
diff --git a/test/files/run/t8428.scala b/test/files/run/t8428.scala
new file mode 100644
index 0000000000..7da1207b7b
--- /dev/null
+++ b/test/files/run/t8428.scala
@@ -0,0 +1,12 @@
+object Test extends App {
+  val xs = List.tabulate(4)(List(_))
+  val i = xs.map(_.iterator).reduce { (a,b) =>
+    a.hasNext
+    a ++ b
+  }
+
+  val r1 = i.toList
+  val r2 = xs.flatten.toList
+
+  assert(r1 == r2, r1)
+}