id	summary	reporter	owner	description	type	status	milestone	component	version	severity	resolution	keywords	cc
10676	[fusion] Compile errors with boost::fusion::fold due to boost::result_of picking wrong overload	Jared Grubb <jaredgrubb@…>	Joel de Guzman	"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.
}}}"	Bugs	closed	To Be Determined	fusion	Boost 1.56.0	Regression	fixed