[Bug c++/84768] [7/8 Regression] ICE with failed class template argument deduction because of invalid template parameter

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84768

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
oldelt is
 <tree_list 0x7fffefd9f280 purpose <error_mark 0x7fffefc45e58> value
<error_mark 0x7fffefc45e58>>
It has been created in:
      /* Add it to the list.  */
      if (parameter != error_mark_node)
        parameter_list = process_template_parm (parameter_list,
                                                parm_loc,
                                                parameter,
                                                is_non_type,
                                                is_parameter_pack);
      else
       {
         tree err_parm = build_tree_list (parameter, parameter);
         parameter_list = chainon (parameter_list, err_parm);
       }
which looks like it is intentional that way (r116661), so probably need to
handle error_mark_node somewhere else, maybe in build_deduction_guide?