Potentially Tail Call

fib-tailrec.cpp

template <int n, int prev, int next, bool isPositive>
struct fib_impl {
    static constexpr int val = fib_impl<n - 1, next, prev + next, isPositive>::val;
};

template <int prev, int next>
struct fib_impl<0, prev, next, true> {
    static constexpr int val = prev;
};

template <int n, int prev, int next>
struct fib_impl<n, prev, next, false> {
    static constexpr int val = -1;
};

template <int n>
struct fib {
    static_assert(n >= 0, "Error: fib can't be called with negative numbers!");
    static constexpr int val = fib_impl<n, 0, 1, (n >= 0)>::val;
};

int main() {
    static_assert(fib<10>::val == 55);
    static_assert(fib<0>::val == 0);
    static_assert(fib<1>::val == 1);
//    static_assert(fib<-1>::val); // this will fail :)
    return 0;
}