Fixes #3686 by special casing Stream transforme...

Fixes #3686 by special casing Stream transformers depending on target
type.

Review by extempore.

1 files changed, 39 insertions, 16 deletions

diff --git a/src/library/scala/collection/immutable/Stream.scala b/src/library/scala/collection/immutable/Stream.scala
index 02a17427ef..6a7d0535fc 100644
--- a/src/library/scala/collection/immutable/Stream.scala
+++ b/src/library/scala/collection/immutable/Stream.scala
@@ -120,6 +120,10 @@ self =>
    */
   @inline private def asThat[That](x: AnyRef): That     = x.asInstanceOf[That]
   @inline private def asStream[B](x: AnyRef): Stream[B] = x.asInstanceOf[Stream[B]]
+  @inline private def buildsThis[B, That](bf: CanBuildFrom[Stream[A], B, That]) = bf.isInstanceOf[Stream.StreamCanBuildFrom[_]]
+  @inline private def ifTargetThis[B, That](bf: CanBuildFrom[Stream[A], B, That])(ifIs: => Stream[B])(ifNot: => That): That =
+    if (buildsThis(bf)) ifIs.asInstanceOf[That]
+    else ifNot
 
   // Overridden methods from Traversable
 
@@ -139,10 +143,12 @@ self =>
    */
   override def ++[B >: A, That](that: TraversableOnce[B])(implicit bf: CanBuildFrom[Stream[A], B, That]): That =
     // we assume there is no other builder factory on streams and therefore know that That = Stream[A]
-    asThat[That](
+    ifTargetThis[B, That](bf) {
       if (isEmpty) that.toStream
       else new Stream.Cons(head, asStream[A](tail ++ that))
-    )
+    } {
+      super.++(that)(bf)
+    }
 
   /**
    * Create a new stream which contains all intermediate results of applying the operator
@@ -150,10 +156,12 @@ self =>
    * @note This works because the target type of the Builder That is a Stream.
    */
   override final def scanLeft[B, That](z: B)(op: (B, A) => B)(implicit bf: CanBuildFrom[Stream[A], B, That]): That =
-    asThat[That](
+    ifTargetThis[B, That](bf) {
       if (isEmpty) Stream(z)
       else new Stream.Cons(z, asStream[B](tail.scanLeft(op(z, head))(op)))
-    )
+    } {
+      super.scanLeft(z)(op)(bf)
+    }
 
   /** Returns the stream resulting from applying the given function
    *  `f` to each element of this stream.
@@ -162,11 +170,14 @@ self =>
    *  @return  <code>f(a<sub>0</sub>), ..., f(a<sub>n</sub>)</code> if this
    *           sequence is <code>a<sub>0</sub>, ..., a<sub>n</sub></code>.
    */
-  override final def map[B, That](f: A => B)(implicit bf: CanBuildFrom[Stream[A], B, That]): That =
-    asThat[That](
+  override final def map[B, That](f: A => B)(implicit bf: CanBuildFrom[Stream[A], B, That]): That = {
+    ifTargetThis[B, That](bf) {
       if (isEmpty) Stream.Empty
       else new Stream.Cons(f(head), asStream[B](tail map f))
-    )
+    } {
+      super.map(f)(bf)
+    }
+  }
 
   /** Applies the given function `f` to each element of
    *  this stream, then concatenates the results.
@@ -180,7 +191,7 @@ self =>
     // we assume there is no other builder factory on streams and therefore know that That = Stream[B]
     // optimisations are not for speed, but for functionality
     // see tickets #153, #498, #2147, and corresponding tests in run/ (as well as run/stream_flatmap_odds.scala)
-    asThat[That](
+    ifTargetThis[B, That](bf) {
       if (isEmpty) Stream.Empty
       else {
         // establish !prefix.isEmpty || nonEmptyPrefix.isEmpty
@@ -195,7 +206,9 @@ self =>
         if (nonEmptyPrefix.isEmpty) Stream.empty
         else prefix append asStream[B](nonEmptyPrefix.tail flatMap f)
       }
-    )
+    } {
+      super.flatMap(f)(bf)
+    }
 
   /** Returns all the elements of this stream that satisfy the
    *  predicate <code>p</code>. The order of the elements is preserved.
@@ -221,20 +234,24 @@ self =>
 
     override def map[B, That](f: A => B)(implicit bf: CanBuildFrom[Stream[A], B, That]): That = {
       def tailMap = asStream[B](tail withFilter p map f)
-      asThat[That](
+      ifTargetThis[B, That](bf) {
         if (isEmpty) Stream.Empty
         else if (p(head)) new Stream.Cons(f(head), tailMap)
         else tailMap
-      )
+      } {
+        super.map(f)(bf)
+      }
     }
 
     override def flatMap[B, That](f: A => Traversable[B])(implicit bf: CanBuildFrom[Stream[A], B, That]): That = {
       def tailFlatMap = asStream[B](tail withFilter p flatMap f)
-      asThat[That](
+      ifTargetThis[B, That](bf) {
         if (isEmpty) Stream.Empty
         else if (p(head)) f(head).toStream append tailFlatMap
         else tailFlatMap
-      )
+      } {
+        super.flatMap(f)(bf)
+      }
     }
 
     override def foreach[B](f: A => B) =
@@ -319,10 +336,12 @@ self =>
    */
   override final def zip[A1 >: A, B, That](that: Iterable[B])(implicit bf: CanBuildFrom[Stream[A], (A1, B), That]): That =
     // we assume there is no other builder factory on streams and therefore know that That = Stream[(A1, B)]
-    asThat[That](
+    ifTargetThis[(A1, B), That](bf) {
       if (this.isEmpty || that.isEmpty) Stream.Empty
       else new Stream.Cons((this.head, that.head), asStream[(A1, B)](this.tail zip that.tail))
-    )
+    } {
+      super.zip(that)(bf)
+    }
 
   /** Zips this iterable with its indices. `s.zipWithIndex` is equivalent to
    *  `s zip s.indices`
@@ -456,7 +475,11 @@ self =>
       if (these.isEmpty) Stream.fill(len)(elem)
       else new Stream.Cons(these.head, loop(len - 1, these.tail))
 
-    asThat[That](loop(len, this))
+    ifTargetThis[B, That](bf) {
+      loop(len, this)
+    } {
+      super.padTo(len, elem)(bf)
+    }
 // was:    if (bf.isInstanceOf[Stream.StreamCanBuildFrom[_]]) loop(len, this).asInstanceOf[That]
 //    else super.padTo(len, elem)
   }