src/HOL/BNF/Tools/bnf_fp_rec_sugar.ML

         |> map_filter (try (apsnd (fn Nested_Rec p => p)));
 
       val args = replicate n_args ("", dummyT)
         |> Term.rename_wrt_term t
         |> map Free
-        |> fold (fn (ctr_arg_idx, (arg_idx, T)) =>
+        |> fold (fn (ctr_arg_idx, (arg_idx, _)) =>
             nth_map arg_idx (K (nth ctr_args ctr_arg_idx)))
           no_calls'
         |> fold (fn (ctr_arg_idx, (arg_idx, T)) =>
             nth_map arg_idx (K (retype_free T (nth ctr_args ctr_arg_idx))))
           mutual_calls'