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10.516 isnan

POSIX specification:
https://pubs.opengroup.org/onlinepubs/9699919799/functions/isnan.html

Gnulib module: isnan

Portability problems fixed by Gnulib:

• This macro is missing on some platforms: MSVC 9.
• isnan was introduced with C99 and is thus commonly not present on pre-C99 systems.
• isnan is not a macro on some platforms: Solaris 11.4.
• This macro does not work on some platforms: MSVC 14.

Portability problems not fixed by Gnulib:

• This macro returns an unspecified result when given noncanonical values such as unnormalized numbers, pseudo-denormals, pseudo-NaNs, pseudo-Infinities, and pseudo-zeroes.