Function is_code_point_valid

third-party/utf8cpp/source/utf8/core.h:110–113

    template <typename u32>
    inline bool is_code_point_valid(u32 cp)
    {
        return (cp <= CODE_POINT_MAX && !utf8::internal::is_surrogate(cp));
    }

    template <typename octet_iterator>
    inline typename std::iterator_traits<octet_iterator>::difference_type