How std::bind(&T::memberFunc, this) can always bind to std::function

As far as I know, the member function pointer only can be assigned to the pointer to member function type, and converted to any other except this will violate the standard, right?

And when calling std::bind(&T::memberFunc, this), it should return a dependent type which depend on T.(in std of VC++ version, it's a class template called _Binder).

So the question becomes to why one std::funcion can cover all _Binder(VC++ version) types.

class A
{
public:
    void func(){}
};
class B
{
public:
    void func(){}
};

std::function<void(void)> f[2];

A a;
B b;
f[0] = std::bind(&A::func, &a);
f[1] = std::bind(&B::func, &b);

And I can't picture what type of the member of std::funcion which stored the function would be like, unless I am wrong from the first beginning.