Boost/C++ eval_if fails

我遇到了一个非常困难的问题，下面的程序说明了这个问题:看起来第51行中的eval_if不能正常工作。也就是说，当这两种类型都不是安全类型时，调用元函数get_promotion_policy。我已经研究了两天了，还是看不出发生了什么。任何来自TMP天才的帮助将是非常感激的。

#include <type_traits>
#include <boost/mpl/and.hpp>
#include <boost/mpl/or.hpp>
#include <boost/mpl/if.hpp>
#include <boost/mpl/eval_if.hpp>
#include <boost/mpl/identity.hpp>
struct safe_tag {};
template<class T, class P = void>
struct safe : public safe_tag {
    T m_t;
    typedef P PromotionPolicy;
};
template<class T>
struct is_safe : public
    //std::is_arithmetic<T>
    std::is_base_of<safe_tag, T>
{};
template<class T, class U>
struct common_policies {
    static_assert(
        boost::mpl::or_<
            is_safe<T>,
            is_safe<U>
        >::value,
        "at least one type must be a safe type"
    );
    template<typename Z>
    struct get_promotion_policy {
        static_assert(
            is_safe<Z>::value,
            "only safe types have promotion policies"
        );
        typedef typename Z::PromotionPolicy type;
    };

    // if both types are safe, the policies have to be the same!
    static_assert(
        boost::mpl::eval_if<
            boost::mpl::and_<
                is_safe<T>,
                is_safe<U>
            >,
            typename std::is_same<
                typename get_promotion_policy<T>::type,
                typename get_promotion_policy<U>::type
            >::type,
            boost::mpl::identity<boost::mpl::true_>
        >::type::value,
        "if both types are safe, the policies have to be the same!"
    );
    // now we've verified that there is no conflict between policies
    // return the one from the first safe type
    typedef typename boost::mpl::if_<
        is_safe<T>,
        T,
    typename boost::mpl::if_<
        is_safe<U>,
        U,
    //
        boost::mpl::void_
    >::type
    >::type safe_type;
    typedef typename get_promotion_policy<safe_type>::type promotion_policy;
};
common_policies<int, safe<int> > t;
int main(){
    return 0;
}