#define USE_RESULT_OF
#define BOOST_RESULT_OF_USE_DECLTYPE

#include <boost/utility/enable_if.hpp>
#include <boost/utility/result_of.hpp>
#include <utility>

template<class>
struct is_callable;

template<class F, class T>
struct is_callable<F(T)> {
    typedef char (&pass)[1];
    typedef char (&fail)[2];

    template<class F1, class T1>
    struct sub {};

    template<class F1, class T1>
    static pass test(sub<F1, T1>,
                     boost::mpl::int_<
                         sizeof(boost::declval<F1>()(boost::declval<T1>()))
                     >* x = 0);
    static fail test(...);

    const static bool value = sizeof(test(sub<F,T>())) == sizeof(pass);
    typedef typename boost::mpl::bool_<value>::type type;
};

template <typename T>
T const& cref();

template <typename T>
T& ref();

template <typename F, typename Arg>
#ifndef USE_RESULT_OF
  decltype(F()(cref<Arg>()))
#else
  typename boost::lazy_enable_if<
    is_callable<F(Arg const&)>,
    boost::result_of<F(Arg const&)>
  >::type
#endif
invoke(F f, Arg const& arg)
{
    return f(arg);
}

template <typename F, typename Arg>
#ifndef USE_RESULT_OF
  decltype(F()(ref<Arg>()))
#else
  typename boost::lazy_enable_if<
    is_callable<F(Arg&)>,
    boost::result_of<F(Arg&)>
  >::type
#endif
invoke(F f, Arg& arg)
{
    return f(arg);
}

///////////////////////////////////////////////////////////////////////////////
// Client code
///////////////////////////////////////////////////////////////////////////////
#include <iostream>

int foo(int& i)
{
    std::cout << i << std::endl;
    return i;
}

int main()
{
    int i = 123;
    invoke(foo, i);
}