Fixed bug in Iterator.iterate which would lead ...

Fixed bug in Iterator.iterate which would lead to a runtime exception
under some circumstances due to inadequate laziness in calculating the
next element. No review.

2 files changed, 12 insertions, 1 deletions

diff --git a/src/library/scala/collection/Iterator.scala b/src/library/scala/collection/Iterator.scala
index cbbaf0058c..5eca806933 100644
--- a/src/library/scala/collection/Iterator.scala
+++ b/src/library/scala/collection/Iterator.scala
@@ -109,9 +109,15 @@ object Iterator {
    *  @return      the iterator producing the infinite sequence of values `start, f(start), f(f(start)), ...`
    */
   def iterate[T](start: T)(f: T => T): Iterator[T] = new Iterator[T] {
+    private[this] var first = true
     private[this] var acc = start
     def hasNext: Boolean = true
-    def next(): T = { val res = acc ; acc = f(acc) ; res }
+    def next(): T = {
+      if (first) first = false
+      else acc = f(acc)
+
+      acc
+    }
   }
 
   /** Creates an infinite-length iterator which returns successive values from some start value.
diff --git a/test/files/run/iterator-iterate-lazy.scala b/test/files/run/iterator-iterate-lazy.scala
new file mode 100644
index 0000000000..73886f192b
--- /dev/null
+++ b/test/files/run/iterator-iterate-lazy.scala
@@ -0,0 +1,5 @@
+object Test {
+  def main(args: Array[String]): Unit = {
+    Iterator.iterate(1 to 5 toList)(_.tail).takeWhile(_.nonEmpty).map(_.head).toList
+  }
+}