  bool agg_arg_charsets_for_comparison(DTCollation &c,
                                       Item **items, uint nitems,
                                       int item_sep= 1)
  {
    uint flags= MY_COLL_ALLOW_SUPERSET_CONV |
                MY_COLL_ALLOW_COERCIBLE_CONV |
                MY_COLL_DISALLOW_NONE;
    return agg_arg_charsets(c, items, nitems, flags, item_sep);
  }