Bonus : to use it without knowing i at compile-time :

template <size_t current_value = 0>
size_t& inc(size_t& i) {
	if (i == current_value) {
		i = increment<current_value>::value;
		return i;
	} else {
		if constexpr (current_value != std::numeric_limits<size_t>::max()) {
			return inc<increment<current_value>::value>(i);
		}
	}
}

int main() {
	int i;
	std::cin >> i;
	inc(i);  // this increments i
	std::cout << i << std::endl;
	return 0;
}

Hope you’re not in a hurry

C++ templates my beloved

#pragma pack(1)
template <size_t i> struct increment;
template <> struct increment<0> {
	char _plusone;
	constexpr static size_t value = 1;
};

template <size_t i> struct increment<i> {
	char _plusone;  // this increments i
    constexpr static size_t value = sizeof(increment<i - 1>);
};

template <unsigned i>
using increment_v<i> = increment<i>::value;