Dereferencing `boost::fusion::vector<>`'s iterator is not SFINAE-friendly

[yaito3014]

Consider following code:

#include <boost/fusion/container/vector.hpp>

#include <type_traits>

template <class Iterator, class = void>
struct is_dereferenceable : std::false_type {};

template <class Iterator>
struct is_dereferenceable<Iterator, decltype(*boost::declval<Iterator>())> : std::true_type {};

using empty_vector = boost::fusion::vector<>;
using empty_vector_iterator = boost::fusion::result_of::begin<empty_vector>::type;
static_assert(!is_dereferenceable<empty_vector_iterator>::value);

int main() {}

This code should compile successfully, guaranteeing empty_vector_iterator is never dereferenceable.
However, this emits following hard errors:

In file included from /usr/include/boost/fusion/container/vector.hpp:10,
                 from source.cpp:1:
/usr/include/boost/fusion/container/vector/detail/value_at_impl.hpp: In instantiation of ‘struct boost::fusion::extension::value_at_impl<boost::fusion::vector_tag>::apply<boost::fusion::vector<>, mpl_::int_<0> >’:
/usr/include/boost/fusion/container/vector/detail/deref_impl.hpp:33:97:   required from ‘struct boost::fusion::extension::deref_impl<boost::fusion::vector_iterator_tag>::apply<boost::fusion::vector_iterator<boost::fusion::vector<>, 0> >’
/usr/include/boost/fusion/iterator/deref.hpp:51:16:   required from ‘struct boost::fusion::result_of::deref<boost::fusion::vector_iterator<boost::fusion::vector<>, 0> >’
/usr/include/boost/fusion/iterator/deref.hpp:69:5:   required by substitution of ‘template<class Iterator> constexpr typename boost::fusion::result_of::deref<Iterator>::type boost::fusion::operator*(const iterator_base<Iterator>&) [with Iterator = boost::fusion::vector_iterator<boost::fusion::vector<>, 0>]’
source.cpp:9:46:   required by substitution of ‘template<class Iterator> struct is_dereferenceable<Iterator, decltype (* declval<Iterator>())> [with Iterator = boost::fusion::vector_iterator<boost::fusion::vector<>, 0>]’
source.cpp:13:57:   required from here
/usr/include/boost/fusion/container/vector/detail/value_at_impl.hpp:50:28: error: no matching function for call to ‘value_at_impl<mpl_::int_<0>::value>(boost::fusion::vector<>*)’
   50 |             struct apply : BOOST_FUSION_DECLTYPE_N3031((
      |                            ^~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/boost/fusion/container/vector/detail/at_impl.hpp:12,
                 from /usr/include/boost/fusion/container/vector/vector.hpp:29,
                 from /usr/include/boost/fusion/container/vector.hpp:12:
/usr/include/boost/fusion/container/vector/detail/value_at_impl.hpp:38:26: note: candidate: ‘template<long unsigned int N, class U> boost::mpl::identity<U> boost::fusion::vector_detail::value_at_impl(const volatile store<N, U>*)’
   38 |         mpl::identity<U> value_at_impl(store<N, U> const volatile*);
      |                          ^~~~~~~~~~~~~
/usr/include/boost/fusion/container/vector/detail/value_at_impl.hpp:38:26: note:   template argument deduction/substitution failed:
/usr/include/boost/fusion/container/vector/detail/value_at_impl.hpp:50:28: note:   ‘boost::fusion::vector<>’ is not derived from ‘const volatile boost::fusion::vector_detail::store<0, U>’
   50 |             struct apply : BOOST_FUSION_DECLTYPE_N3031((
      |                            ^~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/boost/fusion/container/vector/vector_iterator.hpp:12,
                 from /usr/include/boost/fusion/container/vector/detail/begin_impl.hpp:11,
                 from /usr/include/boost/fusion/container/vector/vector.hpp:31:
/usr/include/boost/fusion/container/vector/detail/deref_impl.hpp: In instantiation of ‘struct boost::fusion::extension::deref_impl<boost::fusion::vector_iterator_tag>::apply<boost::fusion::vector_iterator<boost::fusion::vector<>, 0> >’:
/usr/include/boost/fusion/iterator/deref.hpp:51:16:   required from ‘struct boost::fusion::result_of::deref<boost::fusion::vector_iterator<boost::fusion::vector<>, 0> >’
/usr/include/boost/fusion/iterator/deref.hpp:69:5:   required by substitution of ‘template<class Iterator> constexpr typename boost::fusion::result_of::deref<Iterator>::type boost::fusion::operator*(const iterator_base<Iterator>&) [with Iterator = boost::fusion::vector_iterator<boost::fusion::vector<>, 0>]’
source.cpp:9:46:   required by substitution of ‘template<class Iterator> struct is_dereferenceable<Iterator, decltype (* declval<Iterator>())> [with Iterator = boost::fusion::vector_iterator<boost::fusion::vector<>, 0>]’
source.cpp:13:57:   required from here
/usr/include/boost/fusion/container/vector/detail/deref_impl.hpp:33:97: error: no type named ‘type’ in ‘struct boost::fusion::extension::value_at_impl<boost::fusion::vector_tag>::apply<boost::fusion::vector<>, mpl_::int_<0> >’
   33 |                 typedef typename value_at_impl<vector_tag>::template apply<vector, index>::type element;
      |                                                                                                 ^~~~~~~
/usr/include/boost/fusion/container/vector/detail/deref_impl.hpp:41:17: error: no type named ‘type’ in ‘struct boost::fusion::extension::value_at_impl<boost::fusion::vector_tag>::apply<boost::fusion::vector<>, mpl_::int_<0> >’
   41 |                 type;
      |                 ^~~~

This breaks some existing constrained codes.

[saki7]

Confirmed during pair programming. This raises false-positive hard error on Spirit.X3's constraints, which ultimately leads to hard error on Spirit.X3's test case:

https://github.com/boostorg/spirit/blob/e1973fcb314dfd051e6c30e7f1f8278adb1caf9f/test/x3/omit.cpp#L66-L67

The constraint is currently worked around so the error is not showing up on X3's CI.

https://github.com/boostorg/spirit/blob/e1973fcb314dfd051e6c30e7f1f8278adb1caf9f/include/boost/spirit/home/x3/support/traits/container_traits.hpp#L31