如何将模板类型限制为基类，而不是基类的子类？

#include <type_traits>

template <typename Base, typename Derived>
struct is_base_of {
  private:
    template <typename T>
    static std::true_type test(const Base*);
  
    template <typename T>
    static std::false_type test(...);
  
  public:
    static constexpr bool value = decltype(test<Derived>(nullptr))::value;
};

template <typename Base, typename Derived>
constexpr bool is_base_of_v = is_base_of<Base, Derived>::value;

#include <iostream>

class Base {
  public:
    virtual void foo() {
        std::cout << "Base::foo()" << std::endl;
    }
};

class Derived : public Base {
  public:
    void foo() override {
        std::cout << "Derived::foo()" << std::endl;
    }
};

template <typename T, typename = std::enable_if_t<is_base_of_v<Base, T>>>
void bar(T& obj) {
    obj.foo();
}

int main() {
    Base base;
    Derived derived;

    bar(base);    // 输出：Base::foo()
    bar(derived); // 输出：Derived::foo()

    return 0;
}