Potential regression: incomplete type in tag_of is a hard error

[ldionne]

This issue was initially reported as boostorg/hana#386. The following is a hard error in Boost trunk:

#include <boost/fusion/include/tag_of.hpp>
struct incomplete;
using X = ::boost::fusion::traits::tag_of<incomplete>::type;

int main() { }

I believe this started happening with boostorg/type_traits@ac35139. fusion::tag_of is using boost::is_convertible, which requires its input to be a complete type, but I don't think it is documented anywhere that tags must be complete types. Possible workarounds are

1. Only call is_convertible if boost::is_complete is true
2. Request that tags be complete types (but that's a breaking change)
3. Reimplement the is_convertible without the assertion

I think (1) is the best solution, even though I am skeptical that a is_complete trait can even be implemented without compiler support. But since Boost.TypeTraits provides it, it seems like it's possible.

The full error message (with Clang) is:

In file included from [snip].cpp:5:
In file included from [snip]/boost/fusion/include/tag_of.hpp:11:
In file included from [snip]/boost/fusion/support/tag_of.hpp:14:
In file included from [snip]/boost/fusion/support/detail/is_mpl_sequence.hpp:16:
[snip]/boost/type_traits/is_convertible.hpp:498:4: error: static_assert failed "From argument type to is_convertible must be a complete type"
   BOOST_STATIC_ASSERT_MSG(boost::is_complete<From>::value || boost::is_void<From>::value || boost::is_array<From>::value, "From argument type to is_convertible must be a complete type");
   ^                       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
[snip]/boost/static_assert.hpp:31:45: note: expanded from macro 'BOOST_STATIC_ASSERT_MSG'
#     define BOOST_STATIC_ASSERT_MSG( ... ) static_assert(__VA_ARGS__)
                                            ^             ~~~~~~~~~~~
[snip]/boost/mpl/aux_/nested_type_wknd.hpp:27:7: note: in instantiation of template class 'boost::is_convertible<incomplete, boost::fusion::detail::from_sequence_convertible_type>' requested here
    : T::type
      ^
[snip]/boost/mpl/not.hpp:41:11: note: in instantiation of template class 'boost::mpl::aux::nested_type_wknd<boost::is_convertible<incomplete, boost::fusion::detail::from_sequence_convertible_type> >' requested here
          BOOST_MPL_AUX_NESTED_TYPE_WKND(T)::value
          ^
[snip]/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> \
                       ^
[snip]/boost/mpl/aux_/nested_type_wknd.hpp:27:7: note: in instantiation of template class 'boost::mpl::not_<boost::is_convertible<incomplete, boost::fusion::detail::from_sequence_convertible_type> >' requested here
    : T::type
      ^
[snip]/boost/mpl/aux_/preprocessed/gcc/and.hpp:51:11: note: in instantiation of template class 'boost::mpl::aux::nested_type_wknd<boost::mpl::not_<boost::is_convertible<incomplete, boost::fusion::detail::from_sequence_convertible_type> > >' requested here
          BOOST_MPL_AUX_NESTED_TYPE_WKND(T1)::value
          ^
[snip]/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> \
                       ^
[snip]/boost/fusion/support/detail/is_mpl_sequence.hpp:22:11: note: in instantiation of template class 'boost::mpl::and_<boost::mpl::not_<boost::is_convertible<incomplete, boost::fusion::detail::from_sequence_convertible_type> >, boost::mpl::is_sequence<incomplete>, mpl_::bool_<true>, mpl_::bool_<true>, mpl_::bool_<true> >' requested here
        : mpl::and_<
          ^
[snip]/boost/mpl/if.hpp:63:68: note: in instantiation of template class 'boost::fusion::detail::is_mpl_sequence<incomplete>' requested here
          BOOST_MPL_AUX_STATIC_CAST(bool, BOOST_MPL_AUX_VALUE_WKND(T1)::value)
                                                                   ^
[snip]/boost/fusion/support/tag_of.hpp:53:18: note: in instantiation of template class 'boost::mpl::if_<boost::fusion::detail::is_mpl_sequence<incomplete>, boost::mpl::identity<boost::fusion::mpl_sequence_tag>, boost::mpl::identity<boost::fusion::non_fusion_tag> >' requested here
          : mpl::if_<fusion::detail::is_mpl_sequence<Sequence>,
                 ^
[snip]/boost/fusion/support/tag_of.hpp:71:15: note: in instantiation of template class 'boost::fusion::detail::tag_of_impl<incomplete, void>' requested here
            : boost::fusion::detail::tag_of_impl<Sequence, Active>
              ^
[snip].cpp:7:36: note: in instantiation of template class 'boost::fusion::traits::tag_of<incomplete, void>' requested here
using X = ::boost::fusion::traits::tag_of<incomplete>::type;
                                   ^

[Flast]

Is hana tag a sequence? It seems hana passes tag type to tag_of.

../../../boost/fusion/support/tag_of.hpp:70:16: required from ‘struct boost::fusion::traits::tag_of<boost::hana::ext::boost::fusion::deque_tag, void>’ (from boostorg/hana#386)

[ldionne]

This is expected. hana::tag_of is idempotent, so hana::tag_of<boost::hana::ext::boost::fusion::deque_tag>::type should be boost::hana::ext::boost::fusion::deque_tag. More precisely, here is what happens:

hana::any_of(fusion::make_deque(...), pred)
using Tag = hana::tag_of<decltype(fusion::make_deque(...))>::type;
if constexpr (hana::Searchable<Tag>::value) {
  // do something
}

But then, hana::Searchable<Tag> is really hana::Searchable<boost::hana::ext::boost::fusion::deque_tag>, which does (with Tag = boost::hana::ext::boost::fusion::deque_tag:

hana::integral_constant<bool,
  !is_default<any_of_impl<hana::tag_of<Tag>::type>>::value &&
  !is_default<find_if_impl<tag_of<Tag>::type>>::value
>

Here, tag_of<Tag>::type is expected to return just Tag if it's already a tag, but it hard-errors when the fusion adapters are included and the tag is incomplete, because we probe for whether the Tag is a fusion tag in that case.

Regardless of Hana's adapters, other clients of Fusion may have been using incomplete types for tags since this wasn't documented anywhere.

[djowel]

By design, we should be able to use incomplete types.

[Flast]

OK. Additionally, the change also should be applied to fusion::is_sequence.