src/Pure/Concurrent/future.scala

--- a/src/Pure/Concurrent/future.scala	Tue Aug 12 15:31:24 2014 +0200
+++ b/src/Pure/Concurrent/future.scala	Tue Aug 12 15:46:20 2014 +0200
@@ -37,7 +37,7 @@
   def join: A
   def map[B](f: A => B): Future[B] = Future.fork { f(join) }
 
-  override def toString =
+  override def toString: String =
     peek match {
       case None => "<future>"
       case Some(Exn.Exn(_)) => "<failed>"