Making Tick's IntegralConstant a Constant for Hana

main.cpp

#include <boost/hana.hpp>
#include <tick/integral_constant.h>
namespace hana = boost::hana;


template <typename T>
struct TickIntegralConstant { using value_type = T; };

namespace boost { namespace hana {
    template <typename T, T n>
    struct datatype<tick::integral_constant<T, n>> {
        using type = TickIntegralConstant<T>;
    };

    template <typename T>
    struct value_impl<TickIntegralConstant<T>> {
        template <typename C>
        static constexpr auto apply() { return C::value; }
    };

    template <typename T, typename C>
    struct to_impl<TickIntegralConstant<T>, C, when<
        models<Constant, C>() &&
        std::is_integral<typename C::value_type>{}()
    >>
        : embedding<is_embedded<typename C::value_type, T>{}>
    {
        static_assert(std::is_integral<T>{},
        "trying to convert a Constant to a tick::integral_constant of a "
        "non-integral type; tick::integral_constant may only hold "
        "integral types");

        template <typename X>
        static constexpr auto apply(X const&) {
            constexpr T v = hana::value<X>();
            return tick::integral_constant<T, v>{};
        }
    };
}}

static_assert(
    hana::plus(tick::integral_constant<int, 3>{},
               tick::integral_constant<int, 7>{})
        ==
    tick::integral_constant<int, 10>{}
, "");

int main() { }