old not allowed in programs

bench/programs/bad-typing/old3.mlw

+(* old not allowed in programs *)
+
+module Test
+
+  use import module ref.Refint
+
+  let test (x: ref int) =
+    if !x = old !x then 1 else 2
+
+end
+
+(*
+Local Variables:
+compile-command: "unset LANG; make -C ../../.. bench/programs/bad-typing/old3"
+End:
+*)
\ No newline at end of file

src/programs/pgm_typing.ml

@@ -410,6 +410,7 @@ and dexpr_desc ~ghost env loc = function
         with Not_found ->
           errorm ~loc "unbound symbol %a" print_qualid p
       in
+      if ls_equal ls fs_old then errorm ~loc "old not allowed in programs";
       let ps = get_psymbol ls in
       begin match ps.ps_kind with
         | PSvar v ->