SI-8627 make Stream.filterNot non-eager

The obvious fix, overriding `filterNot` in Stream, is not binary
compatible, see https://github.com/scala/scala/pull/3925

Instead, this makes `filterImpl` in TaversableLike private[scala],
which allows overriding it in Stream. The corresponding mima-failures
can be whitelisted, as the changes are only to private[scala].

In 2.12.x we can remove the override of `filter` in Stream, but in
2.11.x this is not binary compatible.

Eventually we'd also like to make filter / filterNot in
TraversableLike final, but that's not source compatible, so it cannot
be done in 2.12.x.

2 files changed, 14 insertions, 12 deletions

diff --git a/src/library/scala/collection/TraversableLike.scala b/src/library/scala/collection/TraversableLike.scala
index d3a7db6968..a8731a51b1 100644
--- a/src/library/scala/collection/TraversableLike.scala
+++ b/src/library/scala/collection/TraversableLike.scala
@@ -253,7 +253,7 @@ trait TraversableLike[+A, +Repr] extends Any
     b.result
   }
 
-  private def filterImpl(p: A => Boolean, isFlipped: Boolean): Repr = {
+  private[scala] def filterImpl(p: A => Boolean, isFlipped: Boolean): Repr = {
     val b = newBuilder
     for (x <- this)
       if (p(x) != isFlipped) b += x
diff --git a/src/library/scala/collection/immutable/Stream.scala b/src/library/scala/collection/immutable/Stream.scala
index d3ff5e8abf..f43f864e86 100644
--- a/src/library/scala/collection/immutable/Stream.scala
+++ b/src/library/scala/collection/immutable/Stream.scala
@@ -468,6 +468,16 @@ self =>
     )
     else super.flatMap(f)(bf)
 
+  override private[scala] def filterImpl(p: A => Boolean, isFlipped: Boolean): Stream[A] = {
+    // optimization: drop leading prefix of elems for which f returns false
+    // var rest = this dropWhile (!p(_)) - forget DRY principle - GC can't collect otherwise
+    var rest = this
+    while (!rest.isEmpty && p(rest.head) == isFlipped) rest = rest.tail
+    // private utility func to avoid `this` on stack (would be needed for the lazy arg)
+    if (rest.nonEmpty) Stream.filteredTail(rest, p, isFlipped)
+    else Stream.Empty
+  }
+
   /** Returns all the elements of this `Stream` that satisfy the predicate `p`
    * in a new `Stream` - i.e. it is still a lazy data structure. The order of
    * the elements is preserved
@@ -481,15 +491,7 @@ self =>
    * // produces
    * }}}
    */
-  override def filter(p: A => Boolean): Stream[A] = {
-    // optimization: drop leading prefix of elems for which f returns false
-    // var rest = this dropWhile (!p(_)) - forget DRY principle - GC can't collect otherwise
-    var rest = this
-    while (!rest.isEmpty && !p(rest.head)) rest = rest.tail
-    // private utility func to avoid `this` on stack (would be needed for the lazy arg)
-    if (rest.nonEmpty) Stream.filteredTail(rest, p)
-    else Stream.Empty
-  }
+  override def filter(p: A => Boolean): Stream[A] = filterImpl(p, isFlipped = false) // This override is only left in 2.11 because of binary compatibility, see PR #3925
 
   override final def withFilter(p: A => Boolean): StreamWithFilter = new StreamWithFilter(p)
 
@@ -1187,8 +1189,8 @@ object Stream extends SeqFactory[Stream] {
     else cons(start, range(start + step, end, step))
   }
 
-  private[immutable] def filteredTail[A](stream: Stream[A], p: A => Boolean) = {
-    cons(stream.head, stream.tail filter p)
+  private[immutable] def filteredTail[A](stream: Stream[A], p: A => Boolean, isFlipped: Boolean) = {
+    cons(stream.head, stream.tail.filterImpl(p, isFlipped))
   }
 
   private[immutable] def collectedTail[A, B, That](head: B, stream: Stream[A], pf: PartialFunction[A, B], bf: CanBuildFrom[Stream[A], B, That]) = {