#define assert(_)
namespace helper {
    template<char...> struct toInt;
    template<> struct toInt<> { enum {value = 0, log=1, valid = true}; };
    template<char h, char... t> struct toInt<h, t...> {
        typedef toInt<t...> tail;
        enum { i = h-'0',
               log = tail::log*10,
               value = i*tail::log + tail::value,
               valid = tail::valid && (unsigned(i) < 10)
        };
    };
}


class month_t {
    int m;
    month_t(int m) : m{m} { }
public:
    operator int() { return m; }
    static month_t fromInt(int m) { assert(m >= 1 && m <= 12); return {m}; }
};

class day_t {
    int d;
    day_t(int d) : d{d} { }
public:
    operator int() { return d; }
    static day_t fromInt(int d) { assert(d >= 1 && d <= 31); return {d}; }
};

template <char... a> month_t operator "" _month() {
        enum { m = helper::toInt<a...>::value  };
        static_assert(helper::toInt<a...>::valid && m >= 1 && m <= 12, "Invalid month");
        return month_t::fromInt(m);
};
template <char... a> day_t operator "" _day() {
        enum { m = helper::toInt<a...>::value  };
        static_assert(helper::toInt<a...>::valid && m >= 1 && m <= 31, "Invalid day");
        return day_t::fromInt(m);
};


#include <iostream>
void setBirthDay( day_t d, month_t m) {
    std::cout << d << "/" << m  << std::endl;
}

int main() {
    setBirthDay( 9_day , 7_month  );
 //   setBirthDay( 39_day , 7_month  ); //error: static_assert failed "Invalid day"
}