SI-8346: Rebuild invariant sets in #toSet, avoiding CCE.

SI-3953 caused several types of sets' operations to trivially throw
`ClassCastException` after using inherited covariant #toSet method, by
doing an unchecked cast that is only safe for intrinsically covariant
set data structures like `HashSet`, but totally unsafe for others like
`TreeSet` or `Enumeration.ValueSet`.

This change moves the cast to the leaves of the class hierarchy where
that is safe, and incidentally undeprecates overriding Set#toSet.

1 files changed, 7 insertions, 0 deletions

diff --git a/src/library/scala/collection/immutable/ListSet.scala b/src/library/scala/collection/immutable/ListSet.scala
index 1bb07eb02d..89d1a9640e 100644
--- a/src/library/scala/collection/immutable/ListSet.scala
+++ b/src/library/scala/collection/immutable/ListSet.scala
@@ -138,6 +138,13 @@ class ListSet[A] extends AbstractSet[A]
 
   override def stringPrefix = "ListSet"
 
+  /** Returns this $coll as an immutable set.
+   *  
+   *  A new set will not be built; lazy collections will stay lazy.
+   */
+  @deprecatedOverriding("Immutable sets should do nothing on toSet but return themselves cast as a Set.", "2.11.0")
+  override def toSet[B >: A]: Set[B] = this.asInstanceOf[Set[B]]
+
   /** Represents an entry in the `ListSet`.
    */
   protected class Node(override val head: A) extends ListSet[A] with Serializable {