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, 34 insertions, 0 deletions

diff --git a/test/files/run/t8346.scala b/test/files/run/t8346.scala
new file mode 100644
index 0000000000..5f3df84174
--- /dev/null
+++ b/test/files/run/t8346.scala
@@ -0,0 +1,34 @@
+object Test extends App {
+  import reflect.ClassTag
+
+  object SomeEnum extends Enumeration {
+    val one, two, three, four = Value
+  }
+
+  def sctor[A <: Set[Int]](f: Int => A)(implicit A: ClassTag[A])
+      : (String, Int => Set[Int]) =
+    (A.runtimeClass.getSimpleName, f)
+
+  val inits: Seq[(String, Int => Set[Int])] = {
+    import collection.immutable.{Seq => _, _}
+    Seq(sctor(BitSet(_)),
+        sctor(HashSet(_)),
+        sctor(ListSet(_)),
+        sctor(SortedSet(_)),
+        sctor(TreeSet(_)))
+  }
+
+  def sVarInfo[A](sa: Set[A]): String = {
+    val saa = sa.toSet[Any]
+    if (sa eq saa) s"""covariant (${(saa + "hi") contains "hi"})"""
+    else "invariant"
+  }
+
+  inits foreach {case (name, singleton) =>
+    print(s"${name}: ")
+    val one = singleton(1)
+    println(Seq(2,3,4).scanLeft(one)(_ + _) map sVarInfo toList)
+  }
+
+  println(s"ValueSet: ${sVarInfo(SomeEnum.values)}")
+}