Potential regression: incomplete type in tag_of is a hard error about fusion

[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.