Fixes #4761.

This changes the signature of flatten - I do not see how to use a @bridge annotation here, since after erasure both the bridge and the original method have the same signature.

Review by extempore.

5 files changed, 33 insertions, 8 deletions

diff --git a/src/library/scala/collection/Traversable.scala b/src/library/scala/collection/Traversable.scala
index 3cdc4cbf82..3f550adeed 100644
--- a/src/library/scala/collection/Traversable.scala
+++ b/src/library/scala/collection/Traversable.scala
@@ -29,10 +29,10 @@ trait Traversable[+A] extends TraversableLike[A, Traversable[A]]
   override def seq: Traversable[A] = this
 
   @bridge
-  def flatten[B](implicit asTraversable: A => /*<:<!!!*/ TraversableOnce[B]): Traversable[B] = super.flatten(asTraversable)
+  def flatten[B](implicit asTraversable: A => /*<:<!!!*/ GenTraversableOnce[B]): Traversable[B] = super.flatten(asTraversable)
 
   @bridge
-  def transpose[B](implicit asTraversable: A => /*<:<!!!*/ TraversableOnce[B]): Traversable[Traversable[B]] = super.transpose(asTraversable)
+  def transpose[B](implicit asTraversable: A => /*<:<!!!*/ GenTraversableOnce[B]): Traversable[Traversable[B]] = super.transpose(asTraversable)
 
   /* The following methods are inherited from TraversableLike
    *
diff --git a/src/library/scala/collection/generic/GenericTraversableTemplate.scala b/src/library/scala/collection/generic/GenericTraversableTemplate.scala
index 805009be77..a1b10dc9e3 100644
--- a/src/library/scala/collection/generic/GenericTraversableTemplate.scala
+++ b/src/library/scala/collection/generic/GenericTraversableTemplate.scala
@@ -13,6 +13,7 @@ package generic
 
 import mutable.Builder
 import annotation.migration
+import annotation.bridge
 import annotation.unchecked.uncheckedVariance
 
 /** A template class for companion objects of ``regular`` collection classes
@@ -120,17 +121,22 @@ trait GenericTraversableTemplate[+A, +CC[X] <: GenTraversable[X]] extends HasNew
    *
    *  @tparam B the type of the elements of each traversable collection.
    *  @param asTraversable an implicit conversion which asserts that the element
-   *          type of this $coll is a `Traversable`.
+   *          type of this $coll is a `GenTraversable`.
    *  @return a new $coll resulting from concatenating all element ${coll}s.
    *  @usecase def flatten[B]: $Coll[B]
    */
-  def flatten[B](implicit asTraversable: A => /*<:<!!!*/ TraversableOnce[B]): CC[B] = {
+  def flatten[B](implicit asTraversable: A => /*<:<!!!*/ GenTraversableOnce[B]): CC[B] = {
     val b = genericBuilder[B]
     for (xs <- sequential)
-      b ++= asTraversable(xs)
+      b ++= asTraversable(xs).seq
     b.result
   }
 
+  // cannot have a bridge, because it would have the same signature as the target method after erasure
+  // @bridge
+  // def flatten[B](implicit asTraversable: A => /*<:<!!!*/ TraversableOnce[B]): CC[B] =
+  //   flatten[B](asTraversable: A => GenTraversableOnce[B])
+
   /** Transposes this $coll of traversable collections into
    *  a $coll of ${coll}s.
    *
@@ -143,7 +149,7 @@ trait GenericTraversableTemplate[+A, +CC[X] <: GenTraversable[X]] extends HasNew
    *          are not of the same size.
    */
   @migration(2, 9, "As of 2.9, transpose throws an exception if collections are not uniformly sized.")
-  def transpose[B](implicit asTraversable: A => /*<:<!!!*/ TraversableOnce[B]): CC[CC[B] @uncheckedVariance] = {
+  def transpose[B](implicit asTraversable: A => /*<:<!!!*/ GenTraversableOnce[B]): CC[CC[B] @uncheckedVariance] = {
     if (isEmpty)
       return genericBuilder[CC[B]].result
 
diff --git a/src/library/scala/collection/immutable/Stream.scala b/src/library/scala/collection/immutable/Stream.scala
index 5f90202522..a0e5668349 100644
--- a/src/library/scala/collection/immutable/Stream.scala
+++ b/src/library/scala/collection/immutable/Stream.scala
@@ -484,7 +484,7 @@ self =>
     result
   }
 
-  override def flatten[B](implicit asTraversable: A => /*<:<!!!*/ TraversableOnce[B]): Stream[B] = {
+  override def flatten[B](implicit asTraversable: A => /*<:<!!!*/ GenTraversableOnce[B]): Stream[B] = {
     def flatten1(t: Traversable[B]): Stream[B] =
       if (!t.isEmpty)
         cons(t.head, flatten1(t.tail))
@@ -492,7 +492,7 @@ self =>
         tail.flatten
 
     if (isEmpty) Stream.empty
-    else flatten1(asTraversable(head).toTraversable)
+    else flatten1(asTraversable(head).seq.toTraversable)
   }
 
   override def view = new StreamView[A, Stream[A]] {
diff --git a/test/files/run/t4761.check b/test/files/run/t4761.check
new file mode 100644
index 0000000000..1698a57bfa
--- /dev/null
+++ b/test/files/run/t4761.check
@@ -0,0 +1,4 @@
+Vector(1, 1, 1, 1, 1)
+Vector(Vector(1, 1, 1, 1, 1))
+List(1, 2)
+List(List(1, 2))
diff --git a/test/files/run/t4761.scala b/test/files/run/t4761.scala
new file mode 100644
index 0000000000..205798b00e
--- /dev/null
+++ b/test/files/run/t4761.scala
@@ -0,0 +1,15 @@
+
+
+
+
+object Test {
+  def main(args: Array[String]) {
+    val gs = for (x <- (1 to 5)) yield { if (x % 2 == 0) List(1).seq else List(1).par }
+    println(gs.flatten)
+    println(gs.transpose)
+
+    val s = Stream(Vector(1).par, Vector(2).par)
+    println(s.flatten.toList)
+    println(s.transpose.map(_.toList).toList)
+  }
+}