SI-8434 Make generic Set operations build the same kind of Set

When building from a `Set` implementation that was statically seen as
a `collection.GenSet` or `collection.Set`, we used to build a default
`Set` implementation determined by `GenSetFactory.setCanBuildFrom`.
This change modifies `setCanBuildFrom` to determine the correct
implementation at runtime by asking the source `Set`’s companion object
for the `Builder`.

Tests are in `NewBuilderTest.mapPreservesCollectionType`, including lots
of disabled tests for which I believe there is no solution under the
current collection library design.

`Map` suffers from the same problem as `Set`. This *can* be fixed in the
same way as for `Set` with some non-trivial changes (see the note in
`NewBuilderTest`), so it is probably best left for Scala 2.13.

1 files changed, 5 insertions, 1 deletions

diff --git a/src/library/scala/collection/generic/GenSetFactory.scala b/src/library/scala/collection/generic/GenSetFactory.scala
index 800f66eb53..65404a4991 100644
--- a/src/library/scala/collection/generic/GenSetFactory.scala
+++ b/src/library/scala/collection/generic/GenSetFactory.scala
@@ -40,7 +40,11 @@ abstract class GenSetFactory[CC[X] <: GenSet[X] with GenSetLike[X, CC[X]]]
   /** $setCanBuildFromInfo
    */
   def setCanBuildFrom[A] = new CanBuildFrom[CC[_], A, CC[A]] {
-    def apply(from: CC[_]) = newBuilder[A]
+    def apply(from: CC[_]) = from match {
+      // When building from an existing Set, try to preserve its type:
+      case from: Set[_] => from.genericBuilder.asInstanceOf[Builder[A, CC[A]]]
+      case _ => newBuilder[A]
+    }
     def apply() = newBuilder[A]
   }
 }