[Bug libstdc++/64064] New: basic_filebuf seekoff return value is unusable for files opened in text mode on Windows

["lukeallardyce at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64064

            Bug ID: 64064
           Summary: basic_filebuf seekoff return value is unusable for
                    files opened in text mode on Windows
           Product: gcc
           Version: 4.9.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: lukeallardyce at gmail dot com

Discussion here

https://gcc.gnu.org/ml/libstdc++/2014-11/msg00145.html

#include <fstream>
#include <iostream>

int main(int, char* argv[])
{
  using traits   = std::filebuf::traits_type;
  using int_type = std::filebuf::int_type;

  std::filebuf fb;
  fb.open(argv[1], std::ios::in);

  while (!traits::eq_int_type(fb.sbumpc(), traits::eof()))
    std::cout << fb.pubseekoff(0, std::ios::cur, std::ios::in) << ' ';

  std::cout << '\n';

  fb.close();
  fb.pubsetbuf(nullptr, 0);
  fb.open(argv[1], std::ios::in);

  while (!traits::eq_int_type(fb.sbumpc(), traits::eof()))
    std::cout << fb.pubseekoff(0, std::ios::cur, std::ios::in) << ' ';
}

With the following 3-line Windows-style text file:
hello
world

Produces the following:
4 5 6 7 8 9 10 11 12 13 14 15 16
1 2 3 4 5 7 8 9 10 11 12 14 16

On a buffered stream, the value returned by pubseekoff cannot be used to seek
back to that point due to the way Windows implements the POSIX read and lseek64
functions (i.e. read performs end of line conversion, lseek64 doesn't). The
value is off by one for each unconsumed end of line in the buffer.

It is still unclear whether this should be fixed, if possible, by libstdc++ or
mingw-w64.