这个builds在GCC中，但不是MSVC。GCC似乎可以从嵌套类中的友元函数访问外部类的私有成员，但MSVC不能。这是为什么呢？

template <typename T>
class Outer
{
public:
  struct Inner {
    friend bool operator==(const Inner& a, const Inner& b) {
      return a.outer_.private_member_ == b.outer_.private_member_;
    }
    // Both compilers are happy this if uncommented
//    bool operator==(const Inner& other) {
//      return outer_.private_member_ == other.outer_.private_member_;
//    }
   private:
    Outer<T>& outer_;
  };
  friend struct Iterator;

  private:
  int private_member_;
};

bool test(Outer<int>::Inner A, Outer<int>::Inner B)
{
    return A == B;
}

Demo