Re: [Boost-users] [TypeErasure] Reference of forward declared type requires complete type

Subject: Re: [Boost-users] [TypeErasure] Reference of forward declared type requires complete type
From: Yuan Yao (yaoyuan0123_at_[hidden])
Date: 2015-09-04 00:26:39

On 2015-09-03 23:36:03, Steven Watanabe wrote:
 > > <snip>
 > >
 > > struct Type;
 > >
 > > BOOST_TYPE_ERASURE_MEMBER((HasTest), Test, 1)
 > >
 > > <snip>
 > >     HasTest<void(Type&)>,
 > > <snip>
 > > ================================================
 > >
 > > However, the compiler (e.g. clang) complains about incomplete type:
 > >
 > > ================================================
 > > /usr/local/include/boost/type_traits/is_base_and_derived.hpp:228:42:
 > > error: incomplete type 'Type' used in type trait expression
 > >     BOOST_STATIC_CONSTANT(bool, value = (BOOST_IS_BASE_OF(B,D) && !
 > > ::boost::is_same<ncvB,ncvD>::value));
 > > ================================================
 > >
 > > Replacing Type& with Type* can fix the problem. Is this a bug?
 > >
 >
 > Probably. Is boost::type_erasure::is_placeholder in
 > the instantiation backtrace?

Yes. And the following is the full error message:

================================================
In file included from main.cpp:1:

In file included from /usr/local/include/boost/type_erasure/any.hpp:36:

In file included from 
/usr/local/include/boost/type_erasure/detail/normalize.hpp:30:

In file included from 
/usr/local/include/boost/type_erasure/detail/get_placeholders.hpp:25:

In file included from 
/usr/local/include/boost/type_erasure/is_placeholder.hpp:15:

/usr/local/include/boost/type_traits/is_base_and_derived.hpp:228:42: 
error: incomplete type 'Type' used in type trait expression

     BOOST_STATIC_CONSTANT(bool, value = (BOOST_IS_BASE_OF(B,D) && ! 
::boost::is_same<ncvB,ncvD>::value));

                                          ^

/usr/local/include/boost/type_traits/intrinsics.hpp:189:53: note: 
expanded from macro 'BOOST_IS_BASE_OF'

#     define BOOST_IS_BASE_OF(T,U) (__is_base_of(T,U) && 
!is_same<T,U>::value)

                                                     ^

/usr/local/include/boost/config/suffix.hpp:394:72: note: expanded from 
macro 'BOOST_STATIC_CONSTANT'

#     define BOOST_STATIC_CONSTANT(type, assignment) static const type 
assignment

^

/usr/local/include/boost/type_traits/is_base_and_derived.hpp:237:25: 
note: in instantiation of template class 
'boost::detail::is_base_and_derived_impl<boost::type_erasure::placeholder, 
Type>' requested here

     , (::boost::detail::is_base_and_derived_impl<Base,Derived>::value)

                         ^

/usr/local/include/boost/type_traits/detail/bool_trait_def.hpp:74:30: 
note: expanded from macro 'BOOST_TT_AUX_BOOL_TRAIT_DEF2'

     BOOST_TT_AUX_BOOL_C_BASE(C) \

                              ^

/usr/local/include/boost/type_traits/detail/bool_trait_def.hpp:55:81: 
note: expanded from macro 'BOOST_TT_AUX_BOOL_C_BASE'

#   define BOOST_TT_AUX_BOOL_C_BASE(C) : public 
::boost::integral_constant<bool,C>

^

/usr/local/include/boost/type_erasure/is_placeholder.hpp:27:25: note: in 
instantiation of template class 
'boost::is_base_and_derived<boost::type_erasure::placeholder, Type>' 
requested here

struct is_placeholder : ::boost::is_base_and_derived<placeholder, T> {};

                         ^

/usr/local/include/boost/mpl/aux_/nested_type_wknd.hpp:27:7: note: in 
instantiation of template class 
'boost::type_erasure::is_placeholder<Type>' requested here

     : T::type

       ^

/usr/local/include/boost/mpl/not.hpp:41:11: note: in instantiation of 
template class 
'boost::mpl::aux::nested_type_wknd<boost::type_erasure::is_placeholder<Type> 
 >' requested here

           BOOST_MPL_AUX_NESTED_TYPE_WKND(T)::value

           ^

/usr/local/include/boost/mpl/aux_/nested_type_wknd.hpp:38:24: note: 
expanded from macro 'BOOST_MPL_AUX_NESTED_TYPE_WKND'

     ::boost::mpl::aux::nested_type_wknd<T> \

                        ^

/usr/local/include/boost/mpl/aux_/nested_type_wknd.hpp:27:7: note: in 
instantiation of template class 
'boost::mpl::not_<boost::type_erasure::is_placeholder<Type> >' requested 
here

     : T::type

       ^

/usr/local/include/boost/mpl/aux_/preprocessed/gcc/or.hpp:51:11: note: 
(skipping 17 contexts in backtrace; use -ftemplate-backtrace-limit=0 to 
see all)

           BOOST_MPL_AUX_NESTED_TYPE_WKND(T1)::value

           ^

/usr/local/include/boost/mpl/aux_/nested_type_wknd.hpp:38:24: note: 
expanded from macro 'BOOST_MPL_AUX_NESTED_TYPE_WKND'

     ::boost::mpl::aux::nested_type_wknd<T> \

                        ^

/usr/local/include/boost/mpl/eval_if.hpp:38:22: note: in instantiation 
of template class 'boost::mpl::fold<boost::mpl::vector<HasTest<void 
(Type &), boost::type_erasure::_self>, 
boost::type_erasure::destructible<boost::type_erasure::_self>, 
boost::type_erasure::relaxed, mpl_::na, mpl_::na, mpl_::na, mpl_::na, 
mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, 
mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na>, 
boost::mpl::s_item<boost::mpl::vector<HasTest<void (Type &), 
boost::type_erasure::_self>, 
boost::type_erasure::destructible<boost::type_erasure::_self>, 
boost::type_erasure::relaxed, mpl_::na, mpl_::na, mpl_::na, mpl_::na, 
mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, 
mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na>, 
boost::mpl::set0<mpl_::na> >, 
boost::type_erasure::detail::collect_concepts<mpl_::arg<2>, 
boost::mpl::map0<mpl_::na>, mpl_::arg<1> > >' requested here

     typedef typename f_::type type;

                      ^

/usr/local/include/boost/type_erasure/detail/normalize.hpp:388:36: note: 
in instantiation of template class 
'boost::mpl::eval_if<boost::mpl::is_sequence<boost::mpl::vector<HasTest<void 
(Type &), boost::type_erasure::_self>, 
boost::type_erasure::destructible<boost::type_erasure::_self>, 
boost::type_erasure::relaxed, mpl_::na, mpl_::na, mpl_::na, mpl_::na, 
mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, 
mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na> >, 
boost::mpl::fold<boost::mpl::vector<HasTest<void (Type &), 
boost::type_erasure::_self>, 
boost::type_erasure::destructible<boost::type_erasure::_self>, 
boost::type_erasure::relaxed, mpl_::na, mpl_::na, mpl_::na, mpl_::na, 
mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, 
mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na>, 
boost::mpl::s_item<boost::mpl::vector<HasTest<void (Type &), 
boost::type_erasure::_self>, 
boost::type_erasure::destructible<boost::type_erasure::_self>, 
boost::type_erasure::relaxed, mpl_::na, mpl_::na, mpl_::na, mpl_::na, 
mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, 
mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na>, 
boost::mpl::set0<mpl_::na> >, 
boost::type_erasure::detail::collect_concepts<mpl_::arg<2>, 
boost::mpl::map0<mpl_::na>, mpl_::arg<1> > >, 
boost::mpl::identity<boost::mpl::s_item<boost::mpl::vector<HasTest<void 
(Type &), boost::type_erasure::_self>, 
boost::type_erasure::destructible<boost::type_erasure::_self>, 
boost::type_erasure::relaxed, mpl_::na, mpl_::na, mpl_::na, mpl_::na, 
mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, 
mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na>, 
boost::mpl::set0<mpl_::na> > > >' requested here

     typedef typename ::boost::mpl::eval_if< 
::boost::mpl::is_sequence<Concept>,

                                    ^

/usr/local/include/boost/type_erasure/any.hpp:65:49: note: in 
instantiation of template class 
'boost::type_erasure::detail::collect_concepts<boost::mpl::vector<HasTest<void 
(Type &), boost::type_erasure::_self>, 
boost::type_erasure::destructible<boost::type_erasure::_self>, 
boost::type_erasure::relaxed, mpl_::na, mpl_::na, mpl_::na, mpl_::na, 
mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, 
mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na>, 
boost::mpl::map0<mpl_::na>, boost::mpl::set0<mpl_::na> >' requested here

         typename ::boost::type_erasure::detail::collect_concepts<

                                                 ^

/usr/local/include/boost/type_erasure/any.hpp:132:43: note: in 
instantiation of template class 
'boost::type_erasure::detail::compute_bases<boost::type_erasure::any<boost::mpl::vector<HasTest<void 
(Type &), boost::type_erasure::_self>, 
boost::type_erasure::destructible<boost::type_erasure::_self>, 
boost::type_erasure::relaxed, mpl_::na, mpl_::na, mpl_::na, mpl_::na, 
mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, 
mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na>, 
boost::type_erasure::_self>, boost::mpl::vector<HasTest<void (Type &), 
boost::type_erasure::_self>, 
boost::type_erasure::destructible<boost::type_erasure::_self>, 
boost::type_erasure::relaxed, mpl_::na, mpl_::na, mpl_::na, mpl_::na, 
mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, 
mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na>, 
boost::type_erasure::_self>' requested here

     public ::boost::type_erasure::detail::compute_bases<

                                           ^

main.cpp:17:10: note: in instantiation of template class 
'boost::type_erasure::any<boost::mpl::vector<HasTest<void (Type &), 
boost::type_erasure::_self>, 
boost::type_erasure::destructible<boost::type_erasure::_self>, 
boost::type_erasure::relaxed, mpl_::na, mpl_::na, mpl_::na, mpl_::na, 
mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, 
mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na>, 
boost::type_erasure::_self>' requested here

      Any obj;

          ^

main.cpp:4:8: note: forward declaration of 'Type'

struct Type;

        ^

1 error generated.
================================================