[Bug libstdc++/105580] [12/13 Regression] warning "potential null pointer dereference" raised when using istreambuf_iterator and any "-O" flag

[jason at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105580

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|c++                         |libstdc++
            Summary|[12/13 Regression] False    |[12/13 Regression] warning
                   |warning "potential null     |"potential null pointer
                   |pointer dereference" raised |dereference" raised when
                   |when using                  |using istreambuf_iterator
                   |istreambuf_iterator and any |and any "-O" flag
                   |"-O" flag                   |
                 CC|                            |jason at gcc dot gnu.org

--- Comment #1 from Jason Merrill <jason at gcc dot gnu.org> ---
The theory of the warning seems to be that if istreambuf_iterator::_M_get,
called from operator* for *__beg in _M_construct, hits EOF, it clears _M_sbuf,
and then ++__beg will try to refer to members of the now-null __beg._M_sbuf. 
At first glance, this seems like a plausible theory.  Why does _M_get clear
_M_sbuf?

      int_type
      _M_get() const
      {
        int_type __ret = _M_c;
        if (_M_sbuf && _S_is_eof(__ret) && _S_is_eof(__ret = _M_sbuf->sgetc()))
          _M_sbuf = 0;
        return __ret;
      }