mpl::has_push_back_arg/mpl:::has_push_front_arg are incomplete types

[Christopher Schmidt]

boost::mpl::has_push_back_arg (​https://svn.boost.org/trac/boost/browser/trunk/boost/mpl/aux_/push_back_impl.hpp?rev=54948#L28) and boost::mpl::push_front_arg (​https://svn.boost.org/trac/boost/browser/trunk/boost/mpl/aux_/push_front_impl.hpp?rev=54948#L28) are both declared but not defined. This leads to a compiler error when using mpl::has_push_back (​https://svn.boost.org/trac/boost/browser/trunk/boost/mpl/push_back.hpp?rev=54948#L40) / mpl::has_push_front (​https://svn.boost.org/trac/boost/browser/trunk/boost/mpl/push_front.hpp?rev=54948#L40) on a mpl sequence (to be precise a fusion sequence) which needs complete 'argument' types when being instanced.

Minimal testcase:
#include <boost/mpl/transform.hpp>
#include <boost/mpl/placeholders.hpp>
#include <boost/fusion/container/vector.hpp>
#include <boost/fusion/mpl.hpp>
boost::mpl::transform<boost::fusion::vector<>,boost::mpl::_1>::type t;

[Steven Watanabe]

I don't think that it's safe in general to rely on the default implementation of has_push_back for sequences where push_back has any requirements on the arguments. I guess it doesn't hurt to make them complete types, though, since this is a pretty minimal requirement.

[Steven Watanabe]

(In [55672]) Provide definitions for the dummy structs used by has_push_back_impl and has_push_front_impl so that they work with Fusion containers. Fixes #3337.