Perfect forwarding container element

Question

Similar to this question, but instead of perfect forwarding member of an object, I would like to know how to perfect forwarding elements of an STL container, i.e. similar to

Answer 1

namespace detail {
    template
    using forwarded_type = std::conditional_t::value,
                                              std::remove_reference_t&, 
                                              std::remove_reference_t&&>;
}
template
detail::forwarded_type forward_like(U&& u) {
    return std::forward>(std::forward(u));
}

template 
void g(Vector&& v, size_t i) {
  f(forward_like(v[i]));
}

Demo. Using std::forward in the implementation automatically prevents you from doing a dangerous forward of rvalue as lvalue.

For your actual use case

I'd like to create vector from vector, vector, ...., where each element T is constructed from U1, U2, .... Each array of vector could be either & or &&, and I'd like the Ui to be perfectly forwarded.

this becomes something like

template
std::vector make_vector(Vectors&&...vectors){
    auto n = std::min({vectors.size()...});
    std::vector ret; 
    ret.reserve(n);
    for(decltype(n) i = 0; i < n; ++i)
        ret.emplace_back(forward_like(vectors[i])...);
    return ret;
}