Boost/C++ eval_if fails

Question

I've been having a very difficult time with a problem illustrated by the following program: It looks like the eval_if in line 51 isn't working correctly. That is, the metafuction get_promotion_policy is being invoked when both types are not safe types. I've been working on this for two days and can't see what's going on. Any help from a TMP genious would be much apprecated.

#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;
}

Answer 1

typename std::is_same<
            typename get_promotion_policy<T>::type,
            typename get_promotion_policy<U>::type
        >::type

triggers instantiation of get_promotion_policy<T> and get_promotion_policy<U>, even if it in the false branch. You have to report the evaluation, for example by :

// 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>
        >,
        std::is_same<
            typename boost::mpl::eval_if<is_safe<T>, get_promotion_policy<T>, boost::mpl::identity<void>>::type,
            typename boost::mpl::eval_if<is_safe<U>, get_promotion_policy<U>, boost::mpl::identity<void>>::type
        >,
        boost::mpl::identity<boost::mpl::true_>
    >::type::value,
    "if both types are safe, the policies have to be the same!"
);

Demo