#include <iostream>
#include <type_traits>
#include <tuple>

// This implementation of FDBB_RETURNS is c++11 compliant, but does not compile
// (compiler complains that template argument deduction failed)
// #define FDBB_RETURNS(expr) -> decltype(expr) { return expr; }

// This works, but is c++14 and above only:
#define FDBB_RETURNS(expr) { return expr; }

namespace detail {

template <int I, class... T>
auto tuple_sum(const std::tuple<T...>& t, std::false_type)
    FDBB_RETURNS(std::get<I>(t))

template <int I, class... T>
auto tuple_sum(const std::tuple<T...>& t, std::true_type)
    FDBB_RETURNS(std::get<I>(t) + tuple_sum<(I+1)>(t, std::integral_constant<bool, (I+2<sizeof...(T))>()))

} // namespace detail

template<class... T>
auto tuple_sum(const std::tuple<T...>& t)
    FDBB_RETURNS(detail::tuple_sum<0>(t, std::integral_constant<bool, (1<sizeof...(T))>()))

int main() {
    std::cout << tuple_sum(std::make_tuple(1,2,3)) << std::endl;
}