src/Pure/Concurrent/future.ML

 fun cancel x = cancel_group (Task_Queue.group_of_task (task_of x));
 
 
 (* future jobs *)
 
-fun assign_result group result res =
-  let
+fun assign_result group result raw_res =
+  let
+    val res =
+      (case raw_res of
+        Exn.Exn exn => Exn.Exn (#2 (Par_Exn.serial exn))
+      | _ => raw_res);
     val _ = Single_Assignment.assign result res
       handle exn as Fail _ =>
         (case Single_Assignment.peek result of
           SOME (Exn.Exn e) => reraise (if Exn.is_interrupt e then e else exn)
         | _ => reraise exn);