Recursive fibonacci at compile time (protected from negative numbers blowing up compiler)

fib.cpp

namespace impl {
    template <int n, bool isPositive>
    struct fib_impl {
        static constexpr int val = fib_impl<n - 1, isPositive>::val + fib_impl<n - 2, isPositive>::val;
    };

    template <>
    struct fib_impl<1, true> {
        static constexpr int val = 1;
    };

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

    // If calling fib<-1>::val it will try to do the recursion up to -ftemplate-depth=N
    // -> On GCC 7.2.1, N=900
    // -> Template specialization for that case
    template <int n>
    struct fib_impl<n, false> {
        static constexpr int val = -1;
    };
}

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

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