Allow incomplete types in fusion::tag_of and fusion::is_native_fusion_sequence

[ldionne]

Fixes #170

Please merge this before the 1.67.0 release, as this is a regression.

[Flast]

Please confirm usage on hana, #170 (comment)

[Flast]

@ldionne can you add a patch for fusion::is_sequence?

[ldionne]

is_sequence does not suffer from the same problem:

template <typename T>
struct is_sequence
  : mpl::bool_<
        (bool)extension::is_sequence_impl<
            typename fusion::detail::tag_of<T>::type
        >::template apply<T>::type::value
    >
{};

If T is incomplete, tag_of will do the right thing and return something like non_fusion_tag. However, is_native_fusion_sequence appears to suffer from the same problem, so I'll patch this one too.

[Flast]

is_sequence does not suffer from the same problem:

You're right.

[Flast]

Thanks!

[djowel]

Thanks @ldionne