Binary MPL transform on empty fusion sequences does not work

[zeratul976@…]

The following code does not compile with Boost 1.60 in C++11 mode. Tested with recent gcc and clang.

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

struct predicate {
    template <typename T, typename U>
    struct apply {
        typedef T type;
    };
};

typedef
    boost::mpl::transform<
        boost::fusion::vector<>,
        boost::fusion::vector<>,
        predicate>
    ::type
Result;

The error is:

In file included from test.cpp:2:
In file included from boost/include/boost/fusion/container/vector.hpp:12:
In file included from boost/include/boost/fusion/container/vector/vector.hpp:28:
In file included from boost/include/boost/fusion/container/vector/detail/at_impl.hpp:12:
boost/include/boost/fusion/container/vector/detail/value_at_impl.hpp:50:57: error: no matching function for call to 'value_at_impl'
                typedef typename mpl::identity<decltype(seq::template value_at_impl<N::value>(boost::declval<seq*>()))>::type::type type;
                                                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
boost/include/boost/fusion/container/vector/detail/value_of_impl.hpp:30:61: note: in instantiation of template class
      'boost::fusion::extension::value_at_impl<boost::fusion::vector_tag>::apply<boost::fusion::vector<>, mpl_::int_<0>>' requested here
                typedef typename value_at_impl<vector_tag>::template apply<vector, index>::type type;
                                                            ^
boost/include/boost/fusion/iterator/value_of.hpp:52:15: note: in instantiation of template class
      'boost::fusion::extension::value_of_impl<boost::fusion::vector_iterator_tag>::apply<boost::fusion::vector_iterator<boost::fusion::vector<>, 0>>' requested here
            : extension::value_of_impl<typename detail::tag_of<Iterator>::type>::
              ^
boost/include/boost/fusion/iterator/mpl/fusion_iterator.hpp:47:45: note: in instantiation of template class
      'boost::fusion::result_of::value_of<boost::fusion::vector_iterator<boost::fusion::vector<>, 0>>' requested here
        typedef typename fusion::result_of::value_of<Iterator>::type type;
                                            ^
boost/include/boost/mpl/iterator_category.hpp:27:22: note: in instantiation of template class
      'boost::mpl::fusion_iterator<boost::fusion::vector_iterator<boost::fusion::vector<>, 0>>' requested here
    typedef typename Iterator::category type;
                     ^
boost/include/boost/mpl/pair_view.hpp:152:20: note: in instantiation of template class
      'boost::mpl::iterator_category<boost::mpl::fusion_iterator<boost::fusion::vector_iterator<boost::fusion::vector<>, 0>>>' requested here
          typename iterator_category<iter1_>::type
                   ^
boost/include/boost/mpl/aux_/has_tag.hpp:20:1: note: in instantiation of template class 'boost::mpl::pair_view<boost::fusion::vector<>, boost::fusion::vector<>>'
      requested here
BOOST_MPL_HAS_XXX_TRAIT_NAMED_DEF(has_tag, tag, false)
^
boost/include/boost/mpl/has_xxx.hpp:245:65: note: expanded from macro 'BOOST_MPL_HAS_XXX_TRAIT_NAMED_DEF'
            , boost::mpl::aux::type_wrapper<BOOST_MSVC_TYPENAME U::name>* = 0 \
                                                                ^
boost/include/boost/mpl/aux_/has_tag.hpp:20:1: note: while substituting deduced template arguments into function template 'test' [with U =
      boost::mpl::pair_view<boost::fusion::vector<>, boost::fusion::vector<>>]
BOOST_MPL_HAS_XXX_TRAIT_NAMED_DEF(has_tag, tag, false)
^
boost/include/boost/mpl/has_xxx.hpp:253:18: note: expanded from macro 'BOOST_MPL_HAS_XXX_TRAIT_NAMED_DEF'
          sizeof(gcc_3_2_wknd::test(static_cast<t_*>(0))) \
                 ^
boost/include/boost/config/suffix.hpp:394:72: note: expanded from macro 'BOOST_STATIC_CONSTANT'
#     define BOOST_STATIC_CONSTANT(type, assignment) static const type assignment
                                                                       ^
boost/include/boost/mpl/sequence_tag.hpp:112:30: note: in instantiation of template class 'boost::mpl::aux::has_tag<boost::mpl::pair_view<boost::fusion::vector<>,
      boost::fusion::vector<>>, mpl_::bool_<false>>' requested here
          ::boost::mpl::aux::has_tag<Sequence>::value
                             ^
boost/include/boost/mpl/O1_size.hpp:30:30: note: in instantiation of template class 'boost::mpl::sequence_tag<boost::mpl::pair_view<boost::fusion::vector<>,
      boost::fusion::vector<>>>' requested here
    : O1_size_impl< typename sequence_tag<Sequence>::type>
                             ^
boost/include/boost/mpl/fold.hpp:34:25: note: in instantiation of template class 'boost::mpl::O1_size<boost::mpl::pair_view<boost::fusion::vector<>,
      boost::fusion::vector<>>>' requested here
          ::boost::mpl::O1_size<Sequence>::value
                        ^
boost/include/boost/mpl/transform.hpp:75:7: note: in instantiation of template class 'boost::mpl::fold<boost::mpl::pair_view<boost::fusion::vector<>,
      boost::fusion::vector<>>, boost::fusion::vector<>, boost::mpl::bind2<boost::mpl::push_back<mpl_::na, mpl_::na>, mpl_::arg<1>, boost::mpl::bind2<predicate,
      boost::mpl::bind1<boost::mpl::first<mpl_::na>, mpl_::arg<2>>, boost::mpl::bind1<boost::mpl::second<mpl_::na>, mpl_::arg<2>>>>>' requested here
    : fold<
      ^
boost/include/boost/mpl/transform.hpp:114:1: note: in instantiation of template class 'boost::mpl::aux::transform2_impl<boost::fusion::vector<>,
      boost::fusion::vector<>, predicate, boost::mpl::back_inserter<boost::fusion::vector<>>>' requested here
BOOST_MPL_AUX_INSERTER_ALGORITHM_DEF(4, transform2)
^
boost/include/boost/mpl/aux_/inserter_algorithm.hpp:52:7: note: expanded from macro 'BOOST_MPL_AUX_INSERTER_ALGORITHM_DEF'
    : if_< has_push_back< typename clear<P1>::type> \
      ^
boost/include/boost/mpl/eval_if.hpp:38:22: note: in instantiation of template class 'boost::mpl::transform2<boost::fusion::vector<>, boost::fusion::vector<>, predicate,
      mpl_::na>' requested here
    typedef typename f_::type type;
                     ^
boost/include/boost/mpl/transform.hpp:138:1: note: in instantiation of template class 'boost::mpl::eval_if<boost::mpl::or_<boost::mpl::is_na<predicate>,
      boost::mpl::is_lambda_expression<boost::fusion::vector<>>, boost::mpl::not_<boost::mpl::is_sequence<boost::fusion::vector<>>>, mpl_::bool_<false>, mpl_::bool_<false>>,
      boost::mpl::transform1<boost::fusion::vector<>, boost::fusion::vector<>, predicate>, boost::mpl::transform2<boost::fusion::vector<>, boost::fusion::vector<>, predicate, mpl_::na>>'
      requested here
AUX778076_TRANSFORM_DEF(transform)
^
boost/include/boost/mpl/transform.hpp:125:22: note: expanded from macro 'AUX778076_TRANSFORM_DEF'
    typedef typename eval_if<                                           \
                     ^
test.cpp:13:17: note: in instantiation of template class 'boost::mpl::transform<boost::fusion::vector<>, boost::fusion::vector<>, predicate, mpl_::na>' requested here
    boost::mpl::transform<
                ^
boost/include/boost/fusion/container/vector/vector.hpp:275:30: note: candidate template ignored: could not match 'store' against 'vector'
            mpl::identity<U> value_at_impl(store<N, U>*);
                             ^ 

Kohei has indicated on the mailing list [1] that this affects C++11 mode only, and seems to be a regression in 1.60.

[1] ​http://boost.2283326.n4.nabble.com/fusion-mpl-binary-MPL-transform-on-empty-fusion-vectors-tp4682832p4682837.html

[Kohei Takahashi <flast@…>]

Thanks!

[Kohei Takahashi <flast@…>]

Here is workaround: ​http://melpon.org/wandbox/permlink/NgLFdyqAua16KStL.

[Kohei Takahashi <flast@…>]

It seems not a regression but major defect: all of fusion sequences except c++98 vector don't compile. see: ​http://melpon.org/wandbox/permlink/WXfmXnSH7Iqa1kat