C++ Metaprogramming to check type existence

Question

The source code below is excerpted from MongoDB.

I understand that detect_clone_factory_type_member_impl serves the purpose of checking whether the type T possesses a clone_factory_type or not. However, what puzzles me is that since Derived inherits from both T and Fallback, regardless of whether T has a clone_factory_type, Derived should always possess a clone_factory_type. Wouldn't that mean the first test function, No& test(typename U::clone_factory_type*), should always be chosen?

When I actually run experiments, this routine works as expected, which makes me very curious as to how this mechanism functions. I would appreciate the expertise of various professionals.

template <typename T>
struct detect_clone_factory_type_member_impl {
    struct Fallback {
        struct clone_factory_type {};
    };

    struct Derived : T, Fallback {};

    using Yes = char[2];
    using No = char[1];

    template <typename U>
    static No& test(typename U::clone_factory_type*);

    template <typename U>
    static Yes& test(U*);

    static constexpr bool value = sizeof(test<Derived>(nullptr)) == sizeof(Yes);

    using type = typename std::integral_constant<bool, value>::type;
};