#include <boost/mpl/unique.hpp>
#include <boost/mpl/vector.hpp>
#include <boost/type_traits/is_same.hpp>
#include <boost/variant.hpp>

template<typename Variant>
void do_something(const Variant*, int);

template<typename T1, typename T2, typename T3, typename T4>
void foo(const T1& arg1, const T2& arg2, const T3& arg3, const T4& arg4)
{
    using namespace boost::mpl;

    typedef vector<const T1&, const T2&, const T3&, const T4&> template_types;
    // using this instead works, but of course isn't what I want:
    // typedef vector<const int&, const int&, const char&, const float&> template_types;
    typedef unique< template_types, boost::is_same<_1, _2> >::type
            unique_template_types;
    boost::make_variant_over< unique_template_types >::type args[4] = {
        arg1, arg2, arg3, arg4
    };

    do_something(args, 4);
}