src/HOL/Tools/primrec.ML

 fun get_fns fns ((i : int, (tname, _, constrs)), rec_name) (fs, defs) =
   case AList.lookup (op =) fns i of
      NONE =>
        let
          val dummy_fns = map (fn (_, cargs) => Const ("HOL.undefined",
-           replicate ((length cargs) + (length (List.filter is_rec_type cargs)))
+           replicate (length cargs + length (filter is_rec_type cargs))
              dummyT ---> HOLogic.unitT)) constrs;
          val _ = warning ("No function definition for datatype " ^ quote tname)
        in
          (dummy_fns @ fs, defs)
        end