C++: A type trait detecting operator ->

#include 
#include 
#include 
#include hpp>


/* type trait : has_arrow_operator */
template <typename T>
struct has_arrow_operator {
    template <typename> struct test;

    template<typename U>
    static boost::type_traits::yes_type check_sig(testU::operator ->)>* = NULL);

    template<typename U>
    static boost::type_traits::no_type check_sig(...);

    static const bool value = sizeof(check_sig<T>(0)) == sizeof(boost::type_traits::yes_type);
};

/* operator -> available */
class A {
public:
    A* operator -> ();
};

/* operator -> hidden */
class B {
protected:
    B* operator -> ();
};

/* operator -> not available */
class C {
};

int main() {
    std::cout << has_arrow_operator<int>::value << std::endl;
    std::cout << has_arrow_operator<boost::shared_ptr<int> >::value << std::endl;
    std::cout << has_arrow_operator<A>::value << std::endl;
    // ????? how to make this false
    //std::cout << has_arrow_operator::value << std::endl;
    std::cout << has_arrow_operator<C>::value << std::endl;
}

Executing result:

$ dync++ has_arrow_operator.cpp 
0
1
1
0

Question is how to make the private operator version class B works also?