[mp] P1450 copy_cv

[GuillaumeDua]

As currently in csl::ag:

namespace csl::mp {
    // P1450 Enriching type modification traits : https://github.com/cplusplus/papers/issues/216
    // Note that this is a good-enough implementation of P1450 to only fit this project's needs

    // P1450 copy_ref
    template <typename from, typename to>
    struct copy_ref : std::remove_reference<to>{};
    template <typename from, typename to>
    struct copy_ref<from&, to> : std::add_lvalue_reference<to>{};
    template <typename from, typename to>
    struct copy_ref<from&&, to> : std::add_rvalue_reference<std::remove_reference_t<to>>{};
    template <typename from, typename to>
    using copy_ref_t = typename copy_ref<from, to>::type;

    // P1450 - add cv - impl detail (also for ref-qualified types)
    template <typename T> struct add_const : std::type_identity<const T>{};
    template <typename T> struct add_const<T&> : std::type_identity<const T&>{};
    template <typename T> struct add_const<T&&> : std::type_identity<const T&&>{};
    template <typename T> using add_const_t = typename add_const<T>::type;

    template <typename T> struct add_volatile : std::type_identity<volatile T>{};
    template <typename T> struct add_volatile<T&> : std::type_identity<volatile T&>{};
    template <typename T> struct add_volatile<T&&> : std::type_identity<volatile T&&>{};
    template <typename T> using add_volatile_t  = typename add_volatile<T>::type;

    template <typename T> struct add_cv : add_const<typename add_volatile<T>::type>{};
    template <typename T> using add_cv_t = typename add_cv<T>::type;

    // P1450 copy_cv
    template <typename from, typename to>
    struct copy_cv : std::remove_cv<to>{};
    template <typename from, typename to> requires (std::is_reference_v<from>)
    struct copy_cv<from, to> : copy_cv<std::remove_reference_t<from>, to>{};
    template <typename from, typename to>
    struct copy_cv<const volatile from, to> : add_cv<to>{};
    template <typename from, typename to>
    struct copy_cv<const from, to> : add_const<to>{};
    template <typename from, typename to>
    struct copy_cv<volatile from, to> : add_volatile<to>{};
    template <typename from, typename to>
    using copy_cv_t = typename copy_cv<from, to>::type;

    // P1450 copy_cvref
    template <typename from, typename to>
    struct copy_cvref : copy_cv<from, copy_ref_t<from, to>>{};
    template <typename from, typename to>
    using copy_cvref_t = typename copy_cvref<from, to>::type;
}

---

C++17 version:

namespace csl::mp {

    template <typename T>
    struct type_identity{ using type = T; };

    template <typename T>
    struct remove_cvref {
        using type = std::remove_cv_t<std::remove_reference_t<T>>;
    };
    template <typename T>
    using remove_cvref_t = typename remove_cvref<T>::type;

    // P1450 Enriching type modification traits : https://github.com/cplusplus/papers/issues/216
    // Note that this is a good-enough implementation of P1450 to only fit this project's needs

    // P1450 copy_ref
    template <typename from, typename to>
    struct copy_ref : std::remove_reference<to>{};
    template <typename from, typename to>
    struct copy_ref<from&, to> : std::add_lvalue_reference<to>{};
    template <typename from, typename to>
    struct copy_ref<from&&, to> : std::add_rvalue_reference<std::remove_reference_t<to>>{};
    template <typename from, typename to>
    using copy_ref_t = typename copy_ref<from, to>::type;

    // P1450 - add cv - impl detail (also for ref-qualified types)
    template <typename T> struct add_const : type_identity<const T>{};
    template <typename T> struct add_const<T&> : type_identity<const T&>{};
    template <typename T> struct add_const<T&&> : type_identity<const T&&>{};
    template <typename T> using add_const_t = typename add_const<T>::type;

    template <typename T> struct add_volatile : type_identity<volatile T>{};
    template <typename T> struct add_volatile<T&> : type_identity<volatile T&>{};
    template <typename T> struct add_volatile<T&&> : type_identity<volatile T&&>{};
    template <typename T> using add_volatile_t  = typename add_volatile<T>::type;

    template <typename T> struct add_cv : add_const<typename add_volatile<T>::type>{};
    template <typename T> using add_cv_t = typename add_cv<T>::type;

    // P1450 copy_cv
    template <typename from, typename to, typename = void>
    struct copy_cv : std::remove_cv<to>{};
    template <typename from, typename to>
    struct copy_cv<from, to, std::enable_if_t<std::is_reference_v<from>>> : copy_cv<std::remove_reference_t<from>, to>{};
    template <typename from, typename to>
    struct copy_cv<const volatile from, to> : add_cv<to>{};
    template <typename from, typename to>
    struct copy_cv<const from, to> : add_const<to>{};
    template <typename from, typename to>
    struct copy_cv<volatile from, to> : add_volatile<to>{};
    template <typename from, typename to>
    using copy_cv_t = typename copy_cv<from, to>::type;

    // P1450 copy_cvref
    template <typename from, typename to>
    struct copy_cvref : copy_cv<from, copy_ref_t<from, to>>{};
    template <typename from, typename to>
    using copy_cvref_t = typename copy_cvref<from, to>::type;
}