Although enc_filebuf now compiles (PR 13189), it's not possible to do any
actual IO with it.

The problem seems to be that basic_filebuf assumes that state_type is
assignable, but __enc_traits isn't. Although operator= is defined for
__enc_traits, t = u doesn't result in t being equivalent to u, since
it doesn't copy the iconv_t members (_M_*_desc):

    // Need assignment operator as well.
    __enc_traits&
    operator=(const __enc_traits& __obj)
    {
      strncpy(_M_int_enc, __obj._M_int_enc, _S_max_size);
      strncpy(_M_ext_enc, __obj._M_ext_enc, _S_max_size);
      _M_in_desc = 0;
      _M_out_desc = 0;
      _M_ext_bom = __obj._M_ext_bom;
      _M_int_bom = __obj._M_int_bom;
      return *this;
    }

(Note that there is a memory leak here if _M_in_desc or _M_out_desc are
non-null.)