src/HOL/Tools/inductive_codegen.ML

     val (in_ts', _) = get_args is 1 ts;
     val in_ts = filter (is_constrt thy) in_ts';
     val in_vs = terms_vs in_ts;
     val concl_vs = terms_vs ts
   in
-    forall is_eqT (map snd (duplicates (flat (map term_vTs in_ts')))) andalso
+    forall is_eqT (map snd (duplicates (flat (map term_vTs in_ts)))) andalso
     (case check_mode_prems (arg_vs union in_vs) ps of
        None => false
      | Some vs => concl_vs subset vs)
   end;