src/Pure/Concurrent/lazy.ML

       Thread_Attributes.uninterruptible (fn restore_attributes => fn () =>
         let
           val (expr, x) =
             Synchronized.change_result var
               (fn Expr e =>
-                    let val x = Future.promise I
+                    let val x = Future.promise_name "lazy" I
                     in ((SOME e, x), Result x) end
                 | Result x => ((NONE, x), Result x));
         in
           (case expr of
             SOME e =>