为什么友元访问基类静态保护成员时，不能使用成员名限定方法访问？

#include <iostream>
using namespace std;
class A{
protected:
    static int a,b;
};
int A::a=1,A::b=2;
class C:public A{
    int m;
public:
    friend class F;
    C(int p){
        m=p;
    }
};
class F{
public:
    void func(){
        cout<<A::a <<endl;
    }
};

int main(){
    C c(100);
    F f;
    f.func();
    return 0;
}
error C2248: 'a' : cannot access protected member declared in class 'A'