namespace detail {

    template<typename U>
    constexpr U FibOverflowIndicator = 4; //Not a Fibonacci number

    template<typename U, size_t N>
    struct Fib {
        public:
            static constexpr U val =
                (static_cast<U>(static_cast<unsigned long long>(Fib<U, N-1>::val) + Fib<U, N-2>::val) <= Fib<U, N-1>::val)
                ? FibOverflowIndicator<U>
                : Fib<U, N-1>::val + Fib<U, N-2>::val;
    };

    template<typename U>
    struct Fib<U, 0> {
        static constexpr U val = 1;
    };

    template<typename U>
    struct Fib<U, 1> {
        static constexpr U val = 1;
    };

    template<typename U, size_t N>
    constexpr bool isValidFibIndex() {
        return Fib<U, N>::val != FibOverflowIndicator<U>;
    }

    template<typename U, size_t N = 0>
    constexpr std::enable_if_t<!isValidFibIndex<U, N + 1>(), size_t>
    greatestStoreableFibIndex() {
        return N;
    }

    template<typename U, size_t N = 0>
    constexpr std::enable_if_t<isValidFibIndex<U, N + 1>(), size_t>
    greatestStoreableFibIndex() {
        return greatestStoreableFibIndex<U, N + 1>();
    }
} // namespace detail

template<typename U, size_t N>
constexpr U Fib() {
    return detail::Fib<U, N>::val;
}

template<typename U>
struct FibLimit {
    // The number of distinct Fibonacci numbers U can store.
    static constexpr size_t value = detail::greatestStoreableFibIndex<U>() + 1;
};

// Fibonacci number n is too large to fit in U, let's return the sequence.
template<typename U, typename Container, size_t n, U... us>
constexpr std::enable_if_t<Fib<U, n>() == detail::FibOverflowIndicator<U>, Container>
make_fibonacci_seq() {
    return {{us...}};
}

// Fibonacci number n can fit inside a U, continue.
template<typename U, typename Container, size_t n, U... us>
constexpr std::enable_if_t<Fib<U, n>() != detail::FibOverflowIndicator<U>, Container>
make_fibonacci_seq() {
    return make_fibonacci_seq<U, Container, n+1, us..., Fib<U, n>()>();
}



//In class A:

const std::array<U, FibLimit<U>::value> fibonacci = make_fibonacci_seq<U, std::array<U, FibLimit<U>::value>, 0>();