src/Pure/Tools/build_cluster.scala

     if (attempts.forall(Exn.the_res.isDefinedAt)) {
       _sessions = attempts.map(Exn.the_res)
       _sessions
     }
     else {
-      for (Exn.Res(session) <- attempts) session.close()
+      for (case Exn.Res(session) <- attempts) session.close()
       error("Failed to connect build cluster")
     }
   }