Phase 2 construction

// Phase 2 static member constructor function, which cannot throw
inline outcome::result<file_handle> file_handle::file(file_handle::path_type path, file_handle::mode mode) noexcept
{
  // Perform phase 1 of object construction
  file_handle ret;

  // Perform phase 2 of object construction
  int flags = 0;
  switch(mode)
  {
  case mode::attr_read:
  case mode::read:
    flags = O_RDONLY;
    break;
  case mode::attr_write:
  case mode::write:
    flags = O_RDWR;
    break;
  case mode::append:
    flags = O_APPEND;
    break;
  default:
    return std::errc::invalid_argument;
  }
  ret._fd = ::open(path.u8string().c_str(), flags);
  if(-1 == ret._fd)
  {
    // Note that if we bail out here, ~file_handle() will correctly not call ::close()
    return {errno, std::system_category()};
  }
  if(-1 == ::fstat(ret._fd, &ret._stat))
  {
    // Note that if we bail out here, ~file_handle() will correctly call ::close()
    return {errno, std::system_category()};
  }

  // Returning ret directly is an area full of compiler specific behaviour quirks,
  // so be explicit by wrapping into an initialiser list with embedded move.
  return {std::move(ret)};
}