Ambiguous template instantiation for is_future_range and future_range_traits.

[ddemidov]

The following example results in a compile error (on the master branch of hpx, fc3d1bf):

#include <vector>
#include <hpx/hpx.hpp>

typedef boost::iterator_range<
    std::vector< hpx::shared_future<void> >::iterator
    > future_range;

typedef hpx::traits::is_future_range<future_range>::type error1;

typedef hpx::traits::future_range_traits<future_range>::future_type error2;

int main() { }

The errors are:

/usr/include/boost/core/enable_if.hpp:41:10: error: ambiguous template instantiation for ‘struct hpx::traits::is_future_range<boost::iterator_range<__gnu_cxx::__normal_iterator<hpx::lcos::shared_future<void>*, std::vector<hpx::lcos::shared_future<void> > > >, void>’
   struct enable_if : public enable_if_c<Cond::value, T> {};
          ^
In file included from /usr/include/hpx/traits/acquire_shared_state.hpp:14:0,
                 from /usr/include/hpx/lcos/future.hpp:13,
                 from /usr/include/hpx/lcos/detail/async_implementations_fwd.hpp:11,
                 from /usr/include/hpx/runtime/components/stubs/stub_base.hpp:16,
                 from /usr/include/hpx/runtime/components/client_base.hpp:16,
                 from /usr/include/hpx/runtime/basename_registration.hpp:12,
                 from /usr/include/hpx/hpx_fwd.hpp:746,
                 from /usr/include/hpx/include/components.hpp:10,
                 from /usr/include/hpx/include/runtime.hpp:10,
                 from /usr/include/hpx/hpx.hpp:10,
                 from /home/demidov/tmp/hello/range.cpp:2:
/usr/include/hpx/traits/is_future_range.hpp:28:12: note: candidates are: template<class Range> struct hpx::traits::is_future_range<Range, typename boost::enable_if<hpx::traits::is_range<Range> >::type> [with Range = boost::iterator_range<__gnu_cxx::__normal_iterator<hpx::lcos::shared_future<void>*, std::vector<hpx::lcos::shared_future<void> > > >]
     struct is_future_range<Range,
            ^
/usr/include/hpx/traits/is_future_range.hpp:34:12: note:                 template<class Iterator> struct hpx::traits::is_future_range<boost::iterator_range<IteratorT> > [with Iterator = __gnu_cxx::__normal_iterator<hpx::lcos::shared_future<void>*, std::vector<hpx::lcos::shared_future<void> > >]
     struct is_future_range<boost::iterator_range<Iterator> >
            ^

and (after the second definition of is_future_range is commented out)

/home/demidov/tmp/hello/range.cpp:10:55: error: ambiguous template instantiation for ‘struct hpx::traits::future_range_traits<boost::iterator_range<__gnu_cxx::__normal_iterator<hpx::lcos::shared_future<void>*, std::vector<hpx::lcos::shared_future<void> > > > >’
 typedef hpx::traits::future_range_traits<future_range>::future_type error2;
                                                       ^
In file included from /usr/include/hpx/traits/acquire_shared_state.hpp:14:0,
                 from /usr/include/hpx/lcos/future.hpp:13,
                 from /usr/include/hpx/lcos/detail/async_implementations_fwd.hpp:11,
                 from /usr/include/hpx/runtime/components/stubs/stub_base.hpp:16,
                 from /usr/include/hpx/runtime/components/client_base.hpp:16,
                 from /usr/include/hpx/runtime/basename_registration.hpp:12,
                 from /usr/include/hpx/hpx_fwd.hpp:746,
                 from /usr/include/hpx/include/components.hpp:10,
                 from /usr/include/hpx/include/runtime.hpp:10,
                 from /usr/include/hpx/hpx.hpp:10,
                 from /home/demidov/tmp/hello/range.cpp:2:
/usr/include/hpx/traits/is_future_range.hpp:45:12: note: candidates are: template<class Range> struct hpx::traits::future_range_traits<Range, typename boost::enable_if<hpx::traits::is_future_range<Range> >::type> [with Range = boost::iterator_range<__gnu_cxx::__normal_iterator<hpx::lcos::shared_future<void>*, std::vector<hpx::lcos::shared_future<void> > > >]
     struct future_range_traits<
            ^
/usr/include/hpx/traits/is_future_range.hpp:53:12: note:                 template<class Iterator> struct hpx::traits::future_range_traits<boost::iterator_range<IteratorT>, typename boost::enable_if<hpx::traits::is_future<typename std::iterator_traits<_Iterator>::value_type> >::type> [with Iterator = __gnu_cxx::__normal_iterator<hpx::lcos::shared_future<void>*, std::vector<hpx::lcos::shared_future<void> > >]
     struct future_range_traits<
            ^

The second error may be fixed by commenting out the second definition of the future_range_traits. The candidates in the both errors do seem ambiguous to me, as is_range<boost::iterator_range> is true_type.

[ddemidov]

If this is a correct way to fix this, I can make a PR.