In backends/fs/windows/windows-fs.cpp, the macros F_OK/R_OK/W_OK are not defined by system includes of MSVC, so they have been added manually into the code.
While this solution works, in my opinion it would be much cleaner to use GetFileAttributes() for getting this information.
Actually, this is what the _access()/_waccess() functions do and, afterall, this is a piece of code expected to work on Windows only.
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