find_indices (enhancement)

[gabyx]

I suggest the following handy plural form of find_index:

namespace details {

        template<typename List, std::size_t I>
        struct find_ind_state {
            using L = List;
            static const std::size_t Idx = I;
        };

        template<typename Type>
        struct find_indices {
            template<typename State, typename T>
            using invoke = find_ind_state<
                                              meta::if_< std::is_same<Type,T> ,
                                                          meta::push_back< typename State::L, meta::size_t<State::Idx> >,
                                                          typename State::L
                                              >,
                                              State::Idx+1
                                          >;
        };
    };

    /** Returns a list of all indices of types equal to \p T in list \p */
    template<typename T, typename List>
    using find_indices =  typename  meta::accumulate< List,
                                                      details::find_ind_state< meta::list<> , 0 >,
                                                      /* accumulate the index list and the current index of the element*/
                                                      details::find_indices<T>
                                                      >::L;