[Bug libstdc++/104883] New: <system_error> should define all std::errc enumerators

["redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104883

            Bug ID: 104883
           Summary: <system_error> should define all std::errc enumerators
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

Currently we only define std::errc enumerators when the OS defines the
corresponding errno macro:

#ifdef EOVERFLOW
      value_too_large =                         EOVERFLOW,
#endif


Which causes errors when we rely on that being present, e.g. for AVR:

/home/jwakely/src/gcc/build-avr/avr/libstdc++-v3/include/charconv: In function
‘std::to_chars_result std::__detail::__to_chars(char*, char*, _Tp, int)’:
/home/jwakely/src/gcc/build-avr/avr/libstdc++-v3/include/charconv:132:28:
error: ‘value_too_large’ is not a member of ‘std::errc’; did you mean
‘file_too_large’?
  132 |           __res.ec = errc::value_too_large;
      |                            ^~~~~~~~~~~~~~~
      |                            file_too_large


And src/filesystem/ops-common.h does this to workaround the fact that
std::errc::not_supported isn't always defined:

  inline error_code
  __unsupported() noexcept
  {
#if defined ENOTSUP
    return std::make_error_code(std::errc::not_supported);
#elif defined EOPNOTSUPP
    // This is supposed to be for socket operations
    return std::make_error_code(std::errc::operation_not_supported);
#else
    return std::make_error_code(std::errc::invalid_argument);
#endif
  }


We should consider defining all the enumerators unconditionally, picking values
outside the range used by the OS for <errno.h> constants, e.g.

#ifdef EOVERFLOW
      value_too_large =                         EOVERFLOW,
#else
      value_too_large =                         1001,
#endif

The tricky part is picking a value range that doesn't clash with the OS. For
the OS-specific headers such as config/os/mingw32-w64/error_constants.h we can
just inspect <errno.h> and make an educated choice. For
config/os/generic/error_constants.h maybe we want to do something in configure
or with the preprocessor to find the largest value among all the errno macros
that *are* defined, and add 100. Or just take a gamble and assume the OS uses
small numbers and we can start from 1000, or 32000, or something.