::copy_cv

#include <type_traits>

template<typename From, typename To>
struct copy_const {

  using type = std::conditional_t<std::is_const<From>::value,
    std::add_const_t<To>, To>;

};

template<typename From, typename To>
struct copy_volatile {

  using type = std::conditional_t<std::is_volatile<From>::value,
    std::add_volatile_t<To>, To>;

};

template<typename From, typename To>
struct copy_cv {

  using type = typename copy_volatile<
    From, typename copy_const<From, To>::type>::type;

};

template<typename From, typename To>
using copy_const_t = typename copy_const<From, To>::type;

template<typename From, typename To>
using copy_volatile_t = typename copy_volatile<From, To>::type;

template<typename From, typename To>
using copy_cv_t = typename copy_cv<From, To>::type;