Here is an update to set _M_sbuf to null if the user advance too much.

Note that in this case the streambuf remains un-modified which is 
different from the current implementation. I think it is another 
enhancement.

I also change the Debug assertion message for something more dedicated 
to std::advance algo.

François

On 10/14/19 10:12 PM, François Dumont wrote:
> The same way I proposed to review std::copy overload for 
> istreambuf_iterator we can implement std::advance using pubseekoff.
>
> It is both a cleaner implementation and avoids yet another friend 
> declaration.
>
>
>     * include/std/streambuf
>     (advance(istreambuf_iterator<>&, _Distance)): Remove friend 
> declaration.
>     * include/bits/streambuf_iterator.h (__copy_move_a2): Re-implement 
> using
>     streambuf pubseekoff.
>
> Tested under Linux x86_64.
>
> Ok to commit ?
>
> François
>