//Purpose:
//  test possible replacement for mpl::at_c.
#include <boost/mpl/long.hpp>

  template
  < typename...
  >
  struct
pack
  //Purpose:
  //  Just "packages" a bunch of types.
  {
  };
  template
  < long Index
  , typename Head
  , typename... Tail
  >
  auto
at_c_fun
  ( boost::mpl::long_<Index>
  , pack<Head, Tail...>
  )//returns a value with type of the Index-th type in [Head,Tail...]
  ->decltype
    ( at_c_fun
      ( boost::mpl::long_<Index-1>()
      , pack<Tail...>()
      )
    )
  ;
  template
  < typename Head
  , typename... Tail
  >
  Head
at_c_fun
  ( boost::mpl::long_<0>
  , pack<Head, Tail...>
  )//returns a value with type of the 0-th type in [Head,Tail...] = Head
  ;
  template
  < long Index
  , typename... T
  >
  using 
at_c_type//the Index-th type in T...
  =decltype
    ( at_c_fun
      ( boost::mpl::long_<Index>()
      , pack<T...>()
      )
    )
  ;
  template
  < typename Sequence
  , long Index
  >
  struct
at_c
  //The possible replacement for mpl::at_c.
  ;
  template
  < typename... T
  , template<typename...> class Pack
  , long Index
  >
  struct
at_c
  < Pack<T...>
  , Index
  >
  {
        using 
      type=at_c_type<Index,T...>;
  };
  template
  < int I
  >
  struct
type_i
  {
      int operator()(){return I;}
  };
int main(void)
{
      at_c
      < pack<type_i<0>, type_i<1>, type_i<2> >
      , 2
      >::type 
    a_type;
    return a_type();
}