Closes #3493. Review by extempore.

git-svn-id: http://lampsvn.epfl.ch/svn-repos/scala/scala/trunk@22527 5e8d7ff9-d8ef-0310-90f0-a4852d11357a

src/library/scala/runtime/ScalaRunTime.scala

@@ -235,6 +235,12 @@ object ScalaRunTime {
    *
    */  
   def stringOf(arg: Any): String = {
+    import collection.{SortedSet, SortedMap}
+    def mapTraversable(x: Traversable[_], f: Any => String) = x match {
+      case ss: SortedSet[_] => ss.map(f)
+      case ss: SortedMap[_, _] => ss.map(f)
+      case _ => x.map(f)
+    }
     def inner(arg: Any): String = arg match {
       case null                     => "null"
       // Node extends NodeSeq extends Seq[Node] strikes again
@@ -252,7 +258,7 @@ object ScalaRunTime {
         // exception if you call iterator.  What a world.
         // And they can't be infinite either.
         if (x.getClass.getName startsWith "scala.tools.nsc.io") x.toString
-        else (x map inner) mkString (x.stringPrefix + "(", ", ", ")")
+        else (mapTraversable(x, inner)) mkString (x.stringPrefix + "(", ", ", ")")
       case x                        => x toString
     }
     val s = inner(arg)

test/files/run/t3493.scala

@@ -0,0 +1,15 @@
+
+
+
+
+object Test {
+
+  def main(args: Array[String]) {
+    import scala.collection.immutable._
+    val x = TreeSet("a", "b", "c", "d")
+    val x2 = x + "e"
+    assert(x2.toString == "TreeSet(a, b, c, d, e)")
+    assert(x2.toString == runtime.ScalaRunTime.stringOf(x2).trim)
+  }
+
+}