C++11 user defined literals for std::integral_constant<int64_t, N>

literals.hpp

namespace literals { // inspired by http://codereview.stackexchange.com/a/51576

constexpr int64_t combine(int64_t accumulator) {
	return accumulator;
}

template <typename... Characters>
constexpr int64_t combine(int64_t accumulator, char character, Characters... Rest) {
	int digit = (character >= '0' && character <= '9') ? character - '0' 
		: throw std::range_error("Only decimal literals are supported.");
	return combine(accumulator * 10 + digit, Rest...);
}

} // end namespace


template<char... Characters>
constexpr auto operator"" _lit() {
	return std::integral_constant<int64_t, literals::combine(0, Characters...)>{};
}


template <int64_t N>
constexpr auto operator- (std::integral_constant<int64_t, N>) -> std::integral_constant<int64_t, -N> {
	static_assert(N > 0); return {};
}