A conceivably pretty bad performance bug in bui...

A conceivably pretty bad performance bug in builders.

SI-4821 pointed out that ArrayBuffer's ++ checks for a cheap size method
by matching on IndexedSeq, but mutable.IndexedSeq, so all immutable
collections are thrown in the same group as linear seqs. I went looking
for other examples of this and found them, in key classes like Builder.

The "type shadowing trap" is a serious issue in the collections. Closes
SI-4821, no review.

Author: paulp

src/library/scala/collection/mutable/ArrayBuffer.scala

@@ -89,7 +89,7 @@ class ArrayBuffer[A](override protected val initialSize: Int)
    *  @return      the updated buffer.
    */
   override def ++=(xs: TraversableOnce[A]): this.type = xs match {
-    case v: IndexedSeq[_] =>
+    case v: collection.IndexedSeqLike[_, _] =>
       val n = v.length
       ensureSize(size0 + n)
       v.copyToArray(array.asInstanceOf[scala.Array[Any]], size0, n)

src/library/scala/collection/mutable/ArrayOps.scala

@@ -65,7 +65,7 @@ abstract class ArrayOps[T] extends ArrayLike[T, Array[T]] with CustomParalleliza
    */
   def flatten[U, To](implicit asTrav: T => collection.Traversable[U], m: ClassManifest[U]): Array[U] = {
     val b = Array.newBuilder[U]
-    b.sizeHint(map{case is: IndexedSeq[_] => is.size case _ => 0} sum)
+    b.sizeHint(map{case is: collection.IndexedSeq[_] => is.size case _ => 0} sum)
     for (xs <- this)
       b ++= asTrav(xs)
     b.result

src/library/scala/collection/mutable/Builder.scala

@@ -65,7 +65,7 @@ trait Builder[-Elem, +To] extends Growable[Elem] {
    *  @param delta a correction to add to the `coll.size` to produce the size hint.
    */
   def sizeHint(coll: TraversableLike[_, _], delta: Int = 0) {
-    if (coll.isInstanceOf[IndexedSeqLike[_,_]]) {
+    if (coll.isInstanceOf[collection.IndexedSeqLike[_,_]]) {
       sizeHint(coll.size + delta)
     }
   }
@@ -83,7 +83,7 @@ trait Builder[-Elem, +To] extends Growable[Elem] {
    *                       than collection's size are reduced.
    */
   def sizeHintBounded(size: Int, boundingColl: TraversableLike[_, _]) {
-    if (boundingColl.isInstanceOf[IndexedSeqLike[_,_]])
+    if (boundingColl.isInstanceOf[collection.IndexedSeqLike[_,_]])
       sizeHint(size min boundingColl.size)
   }