libstdc++/9580: basic_filebuf<> with custom traits_type fails to link

[Paolo Carlini]

Gabriel Dos Reis wrote:

>This is something we should fix.  However I don't have necessary time
>next week to work on it. I'm sorry.
>
Well, so *I* will fix it ;)

Seriously, the PR is regression from 3.0.x, "due" to this checkin of 
yours and
Benjamin, apparently
(the problem is with std::basic_filebuf<char, 
MyTraits>::_M_underflow_common(bool):

2002-07-30  Benjamin Kosnik  <bkoz@redhat.com>
            Gabriel Dos Reis  <gdr@nerim.net>

...

    * include/std/std_fstream.h: Declare _M_underflow_common
    specializations.
    * src/fstream.cc: New. Add _M_underflow_common specializations.
    * include/bits/fstream.tcc (filebuf::close): Use traits_type.
    (filebuf::_M_underflow_common(bool)): Remove generic version, as
    sys_ungetc and custom int_types don't get along.

...

Can you please help me a bit understanding these Changelog entries?

Thanks,
Paolo.