[Bug libstdc++/67600] New: [5/6] Segfault when assigning only one char to ostreambuf_iterator<char> compiled with -O2 or -O3

[cctsai57 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67600

            Bug ID: 67600
           Summary: [5/6] Segfault when assigning only one char to
                    ostreambuf_iterator<char> compiled with -O2 or -O3
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: cctsai57 at gmail dot com
  Target Milestone: ---

#include <iostream>
#include <sstream>

int main()
{
  std::basic_ostringstream<char> ostr;
  std::ostreambuf_iterator<char> iter(ostr.rdbuf());
  *iter++ = 'X';  // Segmentation fault if -O2 or -O3.
// *iter++ = 'Y'; // But pass if adding this statement.

  std::cout << ostr.str() << '\n';
  return 0;
}


Description:
If the above simplified code is compiled with -O2 or -O3, segmentation fault
happens in "*iter++ = 'x'" statement, but -O0, -O1 or -Os are O.K.
If I add the "*iter++ = 'Y'" statement and compile it with -O2 or -O3, it
can print "XY" successfully.

gcc versions:
Fails on gcc-5 (x86_64-redhat-linux on Fedora-22)
and gcc-6.0.0 (20150916 (experimental)).

Pass on gcc-4.8 and gcc-4.9