I’m trying to write std::complex as an HLSL library. For this, I set out to implement the most basic functionality, starting with arithmetic operators. For finite numbers, all is well as expected. But things get weird at infinity. I won’t be touching any HLSL here, it’s all C++. All the following code was ran on Godbolt.

If I run the following code

std::complex<float> a(-3.0, 4.0);
std::complex<float> inf(1.0f / 0.0f, 0.0f);
a /= inf;
std::cout << a << std::endl;

Then the output will be (-0,0), which is expected. To implement this in HLSL I copied the definition for operator/= found here (same gcc version as Godbolt to make sure).

Now running this code:

std::complex<float> a(-3.0, 4.0);
std::complex<float> inf(1.0f / 0.0f, 0.0f);
float r = a.real() * inf.real() + a.imag() * inf.imag();
float n = std::norm(inf);
float imag = (a.imag() * inf.real() - a.real() * inf.imag()) / n;
float real = r / n;
std::cout << std::complex<float>(real, imag) << std::endl;

The output is now (-nan, -nan) which is different than what I got from just doing /=, but is what I expect from reading the code

Is the STL doing something I’m not seeing in the source code, like some sort of sanity check? What am I missing?