C++ puzzle 2

gistfile1.cpp

#include <iostream>

using namespace std;

template<class T>
struct B {
    struct X {
        T t;
    };
};

template<class T>
void foo(typename B<T>::X &x) {
    cout << "foo" << endl;
}

int main(void) {
    B<int>::X x;
    foo(x);
}