In file included from scratch.cpp:1:
In file included from /usr/include/boost/signals2/signal.hpp:38:
In file included from /usr/include/boost/signals2/variadic_signal.hpp:21:
/usr/include/boost/signals2/detail/variadic_slot_invoker.hpp:81:18: error: call to member function 'm_invoke' is ambiguous
          return m_invoke(resolver, func, indices_type(), args);
                 ^~~~~~~~
/usr/include/boost/signals2/detail/variadic_slot_invoker.hpp:117:18: note: in instantiation of function template specialization 'boost::signals2::detail::call_with_tuple_args<boost::signals2::detail::void_type>::operator()<boost::function<void ()>, , 0>' requested here
          return call_with_tuple_args<result_type>()(connectionBody->slot.slot_function(), _args, mpl::size_t<sizeof...(Args)>());
                 ^
/usr/include/boost/signals2/detail/variadic_slot_invoker.hpp:109:18: note: in instantiation of function template specialization 'boost::signals2::detail::variadic_slot_invoker<boost::signals2::detail::void_type, >::m_invoke<boost::shared_ptr<boost::signals2::detail::connection_body<std::pair<boost::signals2::detail::slot_meta_group, boost::optional<int> >, boost::signals2::slot<void (), boost::function<void ()> >, boost::signals2::mutex> > >' requested here
          return m_invoke(connectionBody,
                 ^
/usr/include/boost/signals2/detail/slot_call_iterator.hpp:82:35: note: in instantiation of function template specialization 'boost::signals2::detail::variadic_slot_invoker<boost::signals2::detail::void_type, >::operator()<boost::shared_ptr<boost::signals2::detail::connection_body<std::pair<boost::signals2::detail::slot_meta_group, boost::optional<int> >, boost::signals2::slot<void (), boost::function<void ()> >, boost::signals2::mutex> > >' requested here
              cache->result.reset(cache->f(*iter));
                                  ^
/usr/include/boost/iterator/iterator_facade.hpp:514:20: note: in instantiation of member function 'boost::signals2::detail::slot_call_iterator_t<boost::signals2::detail::variadic_slot_invoker<boost::signals2::detail::void_type, >, std::_List_iterator<boost::shared_ptr<boost::signals2::detail::connection_body<std::pair<boost::signals2::detail::slot_meta_group, boost::optional<int> >, boost::signals2::slot<void (), boost::function<void ()> >, boost::signals2::mutex> > >, boost::signals2::detail::connection_body<std::pair<boost::signals2::detail::slot_meta_group, boost::optional<int> >, boost::signals2::slot<void (), boost::function<void ()> >, boost::signals2::mutex> >::dereference' requested here
          return f.dereference();
                   ^
/usr/include/boost/iterator/iterator_facade.hpp:639:18: note: in instantiation of function template specialization 'boost::iterator_core_access::dereference<boost::signals2::detail::slot_call_iterator_t<boost::signals2::detail::variadic_slot_invoker<boost::signals2::detail::void_type, >, std::_List_iterator<boost::shared_ptr<boost::signals2::detail::connection_body<std::pair<boost::signals2::detail::slot_meta_group, boost::optional<int> >, boost::signals2::slot<void (), boost::function<void ()> >, boost::signals2::mutex> > >, boost::signals2::detail::connection_body<std::pair<boost::signals2::detail::slot_meta_group, boost::optional<int> >, boost::signals2::slot<void (), boost::function<void ()> >, boost::signals2::mutex> > >' requested here
          return iterator_core_access::dereference(this->derived());
                 ^
/usr/include/boost/signals2/optional_last_value.hpp:55:13: note: in instantiation of member function 'boost::iterator_facade<boost::signals2::detail::slot_call_iterator_t<boost::signals2::detail::variadic_slot_invoker<boost::signals2::detail::void_type, >, std::_List_iterator<boost::shared_ptr<boost::signals2::detail::connection_body<std::pair<boost::signals2::detail::slot_meta_group, boost::optional<int> >, boost::signals2::slot<void (), boost::function<void ()> >, boost::signals2::mutex> > >, boost::signals2::detail::connection_body<std::pair<boost::signals2::detail::slot_meta_group, boost::optional<int> >, boost::signals2::slot<void (), boost::function<void ()> >, boost::signals2::mutex> >, boost::signals2::detail::void_type, boost::single_pass_traversal_tag, const boost::signals2::detail::void_type &, int>::operator*' requested here
            *first;
            ^
/usr/include/boost/signals2/detail/result_type_wrapper.hpp:64:11: note: in instantiation of function template specialization 'boost::signals2::optional_last_value<void>::operator()<boost::signals2::detail::slot_call_iterator_t<boost::signals2::detail::variadic_slot_invoker<boost::signals2::detail::void_type, >, std::_List_iterator<boost::shared_ptr<boost::signals2::detail::connection_body<std::pair<boost::signals2::detail::slot_meta_group, boost::optional<int> >, boost::signals2::slot<void (), boost::function<void ()> >, boost::signals2::mutex> > >, boost::signals2::detail::connection_body<std::pair<boost::signals2::detail::slot_meta_group, boost::optional<int> >, boost::signals2::slot<void (), boost::function<void ()> >, boost::signals2::mutex> > >' requested here
          combiner(first, last);
          ^
/usr/include/boost/signals2/detail/signal_template.hpp:241:18: note: in instantiation of function template specialization 'boost::signals2::detail::combiner_invoker<void>::operator()<boost::signals2::optional_last_value<void>, boost::signals2::detail::slot_call_iterator_t<boost::signals2::detail::variadic_slot_invoker<boost::signals2::detail::void_type, >, std::_List_iterator<boost::shared_ptr<boost::signals2::detail::connection_body<std::pair<boost::signals2::detail::slot_meta_group, boost::optional<int> >, boost::signals2::slot<void (), boost::function<void ()> >, boost::signals2::mutex> > >, boost::signals2::detail::connection_body<std::pair<boost::signals2::detail::slot_meta_group, boost::optional<int> >, boost::signals2::slot<void (), boost::function<void ()> >, boost::signals2::mutex> > >' requested here
          return detail::combiner_invoker<typename combiner_type::result_type>()
                 ^
/usr/include/boost/signals2/detail/signal_template.hpp:695:16: note: in instantiation of member function 'boost::signals2::detail::signal_impl<void (), boost::signals2::optional_last_value<void>, int, std::less<int>, boost::function<void ()>, boost::function<void (const boost::signals2::connection &)>, boost::signals2::mutex>::operator()' requested here
        return (*_pimpl)(BOOST_SIGNALS2_SIGNATURE_ARG_NAMES(BOOST_SIGNALS2_NUM_ARGS));
               ^
scratch.cpp:6:6: note: in instantiation of member function 'boost::signals2::signal<void (), boost::signals2::optional_last_value<void>, int, std::less<int>, boost::function<void ()>, boost::function<void (const boost::signals2::connection &)>, boost::signals2::mutex>::operator()' requested here
    s();
     ^
/usr/include/boost/signals2/detail/variadic_slot_invoker.hpp:85:13: note: candidate function [with T = void, Func = boost::function<void ()>, indices = <>, Args = <>]
          R m_invoke(T *, Func &func, unsigned_meta_array<indices...>, BOOST_SIGNALS2_TUPLE<Args...> args) const
            ^
/usr/include/boost/signals2/detail/variadic_slot_invoker.hpp:90:13: note: candidate function [with Func = boost::function<void ()>, indices = <>, Args = <>]
          R m_invoke(void *, Func &func, unsigned_meta_array<indices...>, BOOST_SIGNALS2_TUPLE<Args...> args) const
            ^
1 error generated.