Function isinfinity

ext/include/cppformat/format.h:392–392

// Portable version of isinf.
# ifdef isinf
inline int isinfinity(double x) { return isinf(x); }
inline int isinfinity(long double x) { return isinf(x); }
# else
inline int isinfinity(double x) { return std::isinf(x); }