https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61761
Bug ID: 61761
Summary: [C++11] std::proj returns incorrect values
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: kariya_mitsuru at hotmail dot com
std::proj returns incorrect values if _GLIBCXX_USE_C99_COMPLEX is not defined.
For example, std::proj(std::complex<double>(1, 2)) should be
std::complex<double>(1, 2) but libstdc++ returns
std::complex<double>(0.333333,0.666667).
If _GLIBCXX_USE_C99_COMPLEX is defined, the sample code below can show the
difference between a glibc version and a libstdc++ version.
============ sample source ========================================
#include <iostream>
#include <complex>
#include <climits>
void check(const std::complex<double>& c)
{
// libstdc++ version
std::complex<double> r1 = std::__complex_proj(c);
std::cout << "libstdc++ version : proj( " << c << " ) = " << r1 << std::endl;
// glibc version
std::complex<double> r2 = std::__complex_proj(c.__rep());
std::cout << "glibc version : proj( " << c << " ) = " << r2 << std::endl;
}
int main()
{
check(std::complex<double>(1, 2));
check(std::complex<double>(1, NAN));
}
============ sample output ========================================
libstdc++ version : proj( (1,2) ) = (0.333333,0.666667)
glibc version : proj( (1,2) ) = (1,2)
libstdc++ version : proj( (1,nan) ) = (nan,nan)
glibc version : proj( (1,nan) ) = (1,nan)
===================================================================
According to C99 standard 7.3.9.4 (referenced from C++11 standard 26.4.7
[complex.value.ops] paragraph 8), "z projects to z except that all complex
infinities (even those with one infinite part and one NaN part) project to
positive infinity on the real axis."
Note that the glibc version 2.11 or below had the same bug.
(For std::complex<double>(1, NAN), the glibc version 2.18 or below had a
different bug.)
