[Boost-bugs] [Boost C++ Libraries] #10676: [fusion] Compile errors with boost::fusion::fold due to boost::result_of picking wrong overload

Subject: [Boost-bugs] [Boost C++ Libraries] #10676: [fusion] Compile errors with boost::fusion::fold due to boost::result_of picking wrong overload
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2014-10-19 17:47:18

#10676: [fusion] Compile errors with boost::fusion::fold due to boost::result_of
picking wrong overload
----------------------------------------+------------------------
 Reporter:  Jared Grubb <jaredgrubb@…>  |      Owner:  djowel
     Type:  Bugs                        |     Status:  new
Milestone:  To Be Determined            |  Component:  fusion
  Version:  Boost 1.56.0                |   Severity:  Regression
 Keywords:                              |
----------------------------------------+------------------------
 I was updating from boost-1.51 to boost-1.56 and some of a parsing library
 I have now fails to compile.

 Notice how the errors occur in overloads requiring "const long&" but the
 Visitor takes mutable values (long&). Somehow, it picks const and then
 digs down.

 I have two workarounds:
  * use BOOST_RESULT_OF_USE_TR1_WITH_DECLTYPE_FALLBACK
  * add an explicit instantiation of boost::fusion::result_of to disable
 the selection that is causing the problem.


 {{{
 #include <boost/fusion/container/vector.hpp>
 #include <boost/fusion/algorithm/iteration/fold.hpp>

 #if WORKAROUND
 struct Visitor;
 namespace boost { namespace fusion { namespace result_of {
      template<typename Seq>
      struct fold<Seq const, int const, Visitor> {
         /* Missing 'type' to disable this selection earlier */
     };
 } } }
 #endif

 struct Visitor
 {
     using result_type = int;

     int operator()(int sum, long&)
     {
         return sum;
     }
 };

 int main()
 {
     boost::fusion::vector<long> vec;

     Visitor visitor{};
     boost::fusion::fold(vec, 0, visitor);
 }
 }}}

 Error:

 {{{
 $ clang++ --version
 Apple LLVM version 6.0 (clang-600.0.39) (based on LLVM 3.5svn)
 Target: x86_64-apple-darwin14.0.0
 Thread model: posix

 [ Using boost-1.51 ]
 $ clang++ x.cpp --std=c++11 -DWORKAROUND
 $ clang++ x.cpp --std=c++11

 [ Using boost-1.56 ]
 $ clang++ x.cpp --std=c++11 -DWORKAROUND

 $ clang++ x.cpp --std=c++11
 -DBOOST_RESULT_OF_USE_TR1_WITH_DECLTYPE_FALLBACK

 $ clang++ x.cpp --std=c++11
 In file included from x.cpp:2:
 In file included from
 /Users/grubber/Downloads/boost_1_56_0/boost/fusion/algorithm/iteration/fold.hpp:33:
 /Users/grubber/Downloads/boost_1_56_0/boost/fusion/algorithm/iteration/detail/preprocessed/fold.hpp:265:24:
 error: no type named 'type' in
       'boost::result_of<Visitor (const int &, const long &)>'
                     >::type
                     ~~~^~~~
 /Users/grubber/Downloads/boost_1_56_0/boost/fusion/algorithm/iteration/detail/preprocessed/fold.hpp:278:17:
 note: in instantiation of template
       class 'boost::fusion::detail::result_of_first_unrolledfold<const int
 &, boost::fusion::vector_iterator<const boost::fusion::vector<long,
       boost::fusion::void_, boost::fusion::void_, boost::fusion::void_,
 boost::fusion::void_, boost::fusion::void_, boost::fusion::void_,
       boost::fusion::void_, boost::fusion::void_, boost::fusion::void_>,
 0>, Visitor, 1>' requested here
                 result_of_first_unrolledfold<
                 ^
 /Users/grubber/Downloads/boost_1_56_0/boost/fusion/algorithm/iteration/detail/preprocessed/fold.hpp:314:13:
 note: in instantiation of template
       class 'boost::fusion::detail::fold_impl<1, const int &, const
 boost::fusion::vector<long, boost::fusion::void_, boost::fusion::void_,
       boost::fusion::void_, boost::fusion::void_, boost::fusion::void_,
 boost::fusion::void_, boost::fusion::void_, boost::fusion::void_,
       boost::fusion::void_>, Visitor>' requested here
           : fold_impl<
             ^
 /Users/grubber/Downloads/boost_1_56_0/boost/fusion/algorithm/iteration/detail/preprocessed/fold.hpp:328:13:
 note: in instantiation of template
       class 'boost::fusion::detail::result_of_fold<const
 boost::fusion::vector<long, boost::fusion::void_, boost::fusion::void_,
       boost::fusion::void_, boost::fusion::void_, boost::fusion::void_,
 boost::fusion::void_, boost::fusion::void_, boost::fusion::void_,
       boost::fusion::void_>, const int, Visitor, false>' requested here
           : detail::result_of_fold<
             ^
 /Users/grubber/Downloads/boost_1_56_0/boost/fusion/algorithm/iteration/detail/preprocessed/fold.hpp:352:32:
 note: in instantiation of template
       class 'boost::fusion::result_of::fold<const
 boost::fusion::vector<long, boost::fusion::void_, boost::fusion::void_,
 boost::fusion::void_,
       boost::fusion::void_, boost::fusion::void_, boost::fusion::void_,
 boost::fusion::void_, boost::fusion::void_, boost::fusion::void_>, const
       int, Visitor>' requested here
     inline typename result_of::fold<
                                ^
 x.cpp:29:5: note: while substituting deduced template arguments into
 function template 'fold' [with Seq = boost::fusion::vector<long,
       boost::fusion::void_, boost::fusion::void_, boost::fusion::void_,
 boost::fusion::void_, boost::fusion::void_, boost::fusion::void_,
       boost::fusion::void_, boost::fusion::void_, boost::fusion::void_>,
 State = int, F = Visitor]
     boost::fusion::fold(vec, 0, visitor);
     ^
 1 error generated.
 }}}

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/10676>
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