[Bug libstdc++/50957] New: complex<T> ctor drops sign of zero (sometimes)

[kreckel at ginac dot de]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50957

             Bug #: 50957
           Summary: complex<T> ctor drops sign of zero (sometimes)
    Classification: Unclassified
           Product: gcc
           Version: 4.6.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: kreckel@ginac.de


When compiling with -std=c++0x (and only then), the ctor of g++ drops the sign
of zero. (This sometimes devastates the correct choice of branch cuts in the
complex plain.) Interestingly, it doesn't happen with my mock complex class.

#include <iostream>
#include <complex>
using namespace std;

template<typename _Tp>
struct mock_complex
{
    typedef _Tp value_type;
    _GLIBCXX_CONSTEXPR mock_complex(const _Tp& __r = _Tp(), const _Tp& __i =
_Tp())
    : _M_real(__r), _M_imag(__i) { }
    const _Tp
    real() const { return _M_real; }
    const _Tp
    imag() const { return _M_imag; }
private:
    _Tp _M_real;
    _Tp _M_imag;
};

int main()
{
    mock_complex<double> z1(-0.0, 1.0);
    cout << signbit(z1.real()) << signbit(z1.imag()) << endl;  // correct: 10

    complex<double> z2(-0.0, 1.0);
    cout << signbit(z2.real()) << signbit(z2.imag()) << endl;  // wrong: 00
}