SI-8335 List.++ avoidably burns memory

Changed to check the identity of the CanBuildFrom instead of the identity of the generated builder to shortcut building.  Should reduce memory churn on ++ a little.

1 files changed, 2 insertions, 4 deletions

diff --git a/src/library/scala/collection/immutable/List.scala b/src/library/scala/collection/immutable/List.scala
index 930e13a9d3..aa9dec2761 100644
--- a/src/library/scala/collection/immutable/List.scala
+++ b/src/library/scala/collection/immutable/List.scala
@@ -190,11 +190,9 @@ sealed abstract class List[+A] extends AbstractSeq[A]
 
   // Overridden methods from IterableLike and SeqLike or overloaded variants of such methods
 
-  override def ++[B >: A, That](that: GenTraversableOnce[B])(implicit bf: CanBuildFrom[List[A], B, That]): That = {
-    val b = bf(this)
-    if (b.isInstanceOf[ListBuffer[_]]) (this ::: that.seq.toList).asInstanceOf[That]
+  override def ++[B >: A, That](that: GenTraversableOnce[B])(implicit bf: CanBuildFrom[List[A], B, That]): That =
+    if (bf eq List.ReusableCBF) (this ::: that.seq.toList).asInstanceOf[That]
     else super.++(that)
-  }
 
   override def +:[B >: A, That](elem: B)(implicit bf: CanBuildFrom[List[A], B, That]): That = bf match {
     case _: List.GenericCanBuildFrom[_] => (elem :: this).asInstanceOf[That]