SI-9134 Verify Stream#withFilter#map lazy in tail

As discussed in comments on [the issue](https://issues.scala-lang.org/browse/SI-9134),
it appears that the fix for SI-8990 also fixed this issue.

These tests demonstrate that it is fixed and will serve
as regression tests. Also, if back-ported to 2.11.x, they
could drive a potentially different fix there.

Squashed:

  - incorporate review comments from adriaanm

1 files changed, 25 insertions, 0 deletions

diff --git a/test/junit/scala/collection/immutable/StreamTest.scala b/test/junit/scala/collection/immutable/StreamTest.scala
index 36732fef2c..437cbc8926 100644
--- a/test/junit/scala/collection/immutable/StreamTest.scala
+++ b/test/junit/scala/collection/immutable/StreamTest.scala
@@ -80,4 +80,29 @@ class StreamTest {
     assertTrue( wf.map(identity).length == 5 )       // success instead of NPE
   }
 
+  /** Test helper to verify that the given Stream operation is properly lazy in the tail */
+  def assertStreamOpLazyInTail(op: (=> Stream[Int]) => Stream[Int], expectedEvaluated: List[Int]): Unit = {
+    // mutable state to record every strict evaluation
+    var evaluated: List[Int] = Nil
+
+    def trackEffectsOnNaturals: Stream[Int] = {
+      def loop(i: Int): Stream[Int] = { evaluated ++= List(i); i #:: loop(i + 1) }
+      loop(1)
+    }
+
+    // call op on a stream which records every strict evaluation
+    val result = op(trackEffectsOnNaturals)
+
+    assertTrue( evaluated == expectedEvaluated )
+  }
+
+  @Test // SI-9134
+  def filter_map_properly_lazy_in_tail: Unit = {
+    assertStreamOpLazyInTail(_.filter(_ % 2 == 0).map(identity), List(1, 2))
+  }
+
+  @Test // SI-9134
+  def withFilter_map_properly_lazy_in_tail: Unit = {
+    assertStreamOpLazyInTail(_.withFilter(_ % 2 == 0).map(identity), List(1, 2))
+  }
 }