Merge "C++17 compatibility: add missing `constexpr`."

     * Convert to a smaller counter type. This is always safe.

     */

    template<typename U, typename E=typename std::enable_if<(sizeof(U) < sizeof(T))>::type>

    inline operator c2_cntr_t<U>() {

    inline constexpr operator c2_cntr_t<U>() {

        return c2_cntr_t<U>(mValue);

    }