template <typename T, T... chars>
struct c_str
{
    constexpr c_str() noexcept {}
};

template <char... chars>
constexpr const c_str<char, chars...> operator "" _cs() noexcept
{
    return c_str<char, chars...>();
}

template <char... chars>
constexpr double operator "" _dbl() noexcept
{
    return 0.0;
}

int main()
{
    using d_t = decltype("2.7"_dbl);//error: no matching function for call to ‘operator""_dbl()’
    using s_t = decltype("abc"_cs);error: no matching function for call to ‘operator""_cs()’
    return 0;
}