Hours later, I still can understand how reversed_binary_value() function contributes towards getting the desired string output!!!!

I wonder if the author also understands what he wrote as a challenge.

Once I understood what it was all about I came up with this:

template <bool...digits>
int reversed_binary_value() {
    vector<bool> values{digits...};
    int sum = 0;
    for(int i{0};i < values.size(); ++i)
        sum += ((1&values[i])<<i);
    return sum;
}

By this practice and the following answers, now I remeber again some topics that for a while, I forgot them. Thanks everybody for the solutions.

clean I think

template<bool F, bool ...I>
int reversed_binary_value()
{
    return F + 2 * reversed_binary_value<I...>();
}

template<>
int reversed_binary_value<0>()
{
    return 0;
}

template<>
int reversed_binary_value<1>()
{
    return 1;
}

That the task was mentioning the input format was not necessary and thus confusing! First problem I solved.