For operating systems that are based on POSIX,
implementations should define the std::system_category()
identical to the POSIX errno
values, with additional values as defined by the
operating system's documentation.
Implementations for operating systems that are not
based on POSIX should define values identical to the operating system's
For errors that do not originate from the operating system, the implementation
may provide enums for the associated values.
— end example