Function FASTFLOAT_IF_CONSTEXPR17

app/src/ThirdParty/fast_float.h:2026–2031

          p, parse_error::missing_integer_or_dot_after_sign);
    }
    FASTFLOAT_IF_CONSTEXPR17(basic_json_fmt) {
      if (!is_integer(*p)) { // a sign must be followed by an integer
        return report_parse_error<UC>(p,
                                      parse_error::missing_integer_after_sign);
      }
    }
    else {
      if (!is_integer(*p) &&
          (*p !=