Some tweaks to ListSet to make it less patholog...

Some tweaks to ListSet to make it less pathological in its outlook. We
can see some modest improvements in run time and answer quality via the
enclosed test case:

// with this patch: 2.250s elapsed, assertions pass.
// without this patch: 51.441s elapsed, and it's
a mercy killing: java.lang.StackOverflowError at
scala.collection.immutable.ListSet$Node.contains(ListSet.scala:117) at
scala.collection.immutable.ListSet$Node.contains(ListSet.scala:117)

Closes #3822, review by community.

1 files changed, 13 insertions, 0 deletions

diff --git a/test/files/run/bug3822.scala b/test/files/run/bug3822.scala
new file mode 100644
index 0000000000..7401ecbde2
--- /dev/null
+++ b/test/files/run/bug3822.scala
@@ -0,0 +1,13 @@
+import scala.collection.{ mutable, immutable, generic }
+import immutable.ListSet
+
+object Test {
+  def main(args: Array[String]): Unit = {
+    val xs = ListSet(-100000 to 100001: _*)
+
+    assert(xs.size == 200002)
+    assert(xs.sum == 100001)
+  }
+}
+
+