#include <boost/utility/enable_if.hpp>
#include <boost/mpl/has_xxx.hpp>
#include <boost/preprocessor/cat.hpp>

#define TRAIT_HAS_NAMED(trait) \
  BOOST_MPL_HAS_XXX_TRAIT_NAMED_DEF(BOOST_PP_CAT(has_, trait), trait, false) 

namespace mpl = boost::mpl;

namespace tag {
    struct foo {
        struct foo_tag;
    };
}

TRAIT_HAS_NAMED(foo_tag)

template <class Tag, class T, class Enable = void> 
struct foo;

template <class Tag, class T>
struct foo<Tag, T, typename boost::enable_if<has_foo_tag<Tag> >::type
> {
    foo();
};

// definition
template <class Tag, class T>
foo<Tag, T, mpl::true_ //typename boost::enable_if<has_foo_tag<Tag> >::type
>::foo() { }


int main() {

    foo<tag::foo, int> f;
    
    return 0;
}