std::isnan(nan) and nan!=nan return false

If I write the following code in c++:

#include <math.h>
float bad_number = sqrt(0)/0;
std::cout << "nan = " << bad_number << "\n";

The value of bad_number will be NaN and it will always print NaN. The output of the program will always be 'nan = nan'.

If I'm in a random c++ file on my hard drive, and I add these lines:

bool is_nan_homebrew = bad_number != bad_number;
bool is_nan_c11stdlib = std::isnan(bad_number);
bool c99_macro_nan = isnan(bad_number);

All these bools will end up true, because, well, they're nan.

Maybe I'm being dumb (I hope so): I tried the same code inside FFEA (inside rod_math_v9.cpp), and all three of those methods returned false, even though the program still printed 'nan = nan'.

No idea what caused it but this is a really malevolent bug. is_nan_homebrew should always be true, it's in the standard.

A workaround: use boost::math::isnan(x) from boost/math/special_functions/fpclassify.hpp. That seems to work as intended.

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