求高手解决这个C++问题

 

#include <stdio.h>

template<class T>
class C
{
    public:
    struct A
    {
        T x;
    };

    struct B: A
    {
        int y;
        B() {
            x = 10;//error
        }
    };
};

int main(void)
{
    C<int>::B b;
    b.x = 5; //right
    b.y = 6;
    return 0;
}
 

 

 

编译以上代码会报如下错误：

test.cpp: In constructor `C<T>::B::B()':
test.cpp:16: error: `x' was not declared in this scope

哪位朋友可以给出一个有说服力的解释？