SI-8346 Re-established soundness of toSet (element type widening)

toSet needs to rebuild some child classes, but not others, as toSet is
allowed to widen element types (which the invariant Set normally cannot do),
and some sets rely upon their invariance.  Thus, sets that rely upon their
invariance now rebuild themselves into a generic set upon toSet, while those
that do not just sit there.

Note: there was a similar patch previously that fixed the same problem, but
this is a reimplementation to circumvent license issues.

Note: the newBuilder method was benchmarked as (surprisingly!) the most
efficient way to create small sets, so it is used where sets may need to
be rebuild.

1 files changed, 2 insertions, 0 deletions

diff --git a/src/library/scala/collection/immutable/ListSet.scala b/src/library/scala/collection/immutable/ListSet.scala
index 2e17677359..adc975479a 100644
--- a/src/library/scala/collection/immutable/ListSet.scala
+++ b/src/library/scala/collection/immutable/ListSet.scala
@@ -179,4 +179,6 @@ class ListSet[A] extends AbstractSet[A]
 
     override def tail: ListSet[A] = self
   }
+  
+  override def toSet[B >: A]: Set[B] = this.asInstanceOf[ListSet[B]]
 }