Conditionally remove method

#include <type_traits>
#include <memory>

struct dummy {};

template<class T> class Vector {
    T arr[42];

    template<class X = void> inline void push_back_impl(const dummy &x) {}
    template<class X = void> inline void push_back_impl(const T &x) {
        //copy assignment is used here
        arr[0] = x;
    }
public:
    void push_back(typename std::conditional<std::is_copy_assignable<T>::value, const T&, dummy>::type x) {
        push_back_impl(x);
    }
};

//explicit instantiations work properly
template class Vector<int>;
template class Vector<std::unique_ptr<int>>;

int main() {
    //this is OK:
    Vector<int> ww;
    ww.push_back(5);

    Vector<std::unique_ptr<int>> vv;
    //uncomment to get compile error:
//    vv.push_back(std::unique_ptr<int>());
}