Make Stream.withFilter.{map,flatMap} run in constant stack space

The included test currently fails because `map` and `flatMap` do not
run in constant stack space on a stream returned by `Stream.withFilter`,
as I reported here:
https://groups.google.com/d/msg/scala-language/WqJR38REXnk/saaSiDdmyqoJ

Fix the problem and add a simple testcase.

Note that the stack space consumed when producing an element of this stream is
proportional to the number of elements failing the test before the next
success. The stack space consumed to produce the stream itself is the
space needed to produce the first element, that is, is proportional to
the number of failures before the first success.

1 files changed, 14 insertions, 0 deletions

diff --git a/test/files/run/stream-stack-overflow-filter-map.scala b/test/files/run/stream-stack-overflow-filter-map.scala
new file mode 100644
index 0000000000..e2bd47bfca
--- /dev/null
+++ b/test/files/run/stream-stack-overflow-filter-map.scala
@@ -0,0 +1,14 @@
+object Test extends App {
+  //This runs fine.
+  val resFMap1 = (1 to 10000).toStream filter (_ => false) flatMap (Seq(_))
+  val resMap1 = (1 to 10000).toStream filter (_ => false) map (_ + 1)
+  assert(resMap1.isEmpty)
+  assert(resFMap1.isEmpty)
+  println(resMap1)
+  println(resFMap1)
+  //This will cause a stack overflow 
+  val resFMap2 = (1 to 10000).toStream withFilter (_ => false) flatMap (Seq(_))
+  val resMap2 = (1 to 10000).toStream withFilter (_ => false) map (_ + 1)
+  assert(resMap1 == resMap2)
+  assert(resFMap1 == resFMap2)
+}