Fixes and closes #4294.

No review.

2 files changed, 29 insertions, 0 deletions

diff --git a/src/library/scala/collection/immutable/Stream.scala b/src/library/scala/collection/immutable/Stream.scala
index 5988780943..771b945dec 100644
--- a/src/library/scala/collection/immutable/Stream.scala
+++ b/src/library/scala/collection/immutable/Stream.scala
@@ -174,6 +174,22 @@ self =>
     else super.map(f)(bf)
   }
 
+  override final def collect[B, That](pf: PartialFunction[A, B])(implicit bf: CanBuildFrom[Stream[A], B, That]): That = {
+    if (!isStreamBuilder(bf)) super.collect(pf)(bf)
+    else {
+      // this implementation avoids:
+      // 1) stackoverflows (could be achieved with tailrec, too)
+      // 2) out of memory errors for big streams (`this` reference can be eliminated from the stack)
+      var rest: Stream[A] = this
+      while (rest.nonEmpty && !pf.isDefinedAt(rest.head)) rest = rest.tail
+
+      //  without the call to the companion object, a thunk is created for the tail of the new stream,
+      //  and the closure of the thunk will reference `this`
+      if (rest.isEmpty) Stream.Empty.asInstanceOf[That]
+      else Stream.collectedTail(rest, pf, bf).asInstanceOf[That]
+    }
+  }
+
   /** Applies the given function `f` to each element of
    *  this stream, then concatenates the results.
    *
@@ -673,6 +689,10 @@ object Stream extends SeqFactory[Stream] {
     new Stream.Cons(stream.head, stream.tail filter p)
   }
 
+  private[immutable] def collectedTail[A, B, That](stream: Stream[A], pf: PartialFunction[A, B], bf: CanBuildFrom[Stream[A], B, That]) = {
+    new Stream.Cons(stream.head, stream.tail.collect(pf)(bf).asInstanceOf[Stream[B]])
+  }
+
   /** A stream containing all elements of a given iterator, in the order they are produced.
    *  @param it   The iterator producing the stream's elements
    */
diff --git a/test/files/run/t4294.scala b/test/files/run/t4294.scala
new file mode 100644
index 0000000000..588fa90f76
--- /dev/null
+++ b/test/files/run/t4294.scala
@@ -0,0 +1,9 @@
+
+
+
+
+object Test {
+  def main(args: Array[String]) {
+    (Stream.from(1).collect{case x if x > 5000000 => x}: Stream[Int])
+  }
+}