Compiler chokes on dataflow overload with launch policy

[mariomulansky]

i tried using the new launch policy feature of local dataflows, but gcc doesnt like the additional overload. If I write:

f1 = hpx::lcos::local::dataflow( hpx::launch::sync , unwrap(mul()) , f1 , f2 );

the compiler tries to instantiate the dataflow with 3 arguments using the launch policy as functor, which obviously fails. Specifically providing the template parameters works:

f1 = hpx::lcos::local::dataflow<hpx::util::detail::unwrap_impl<mul>, future_type& , future_type& >( 
    hpx::launch::sync , unwrap(mul()) , f1 , f2 );

[hkaiser]

Is this still a problem after the latest changes?

[mariomulansky]

yes, compilation still fails on gcc 4.7.1

[hkaiser]

I'm almost certain that this is a compiler problem. Have you tried other compilers?

[mariomulansky]

clang 3.1 reports the same problem, though with a much more readable error message:

/home/mario/hpx/include/hpx/lcos/local/preprocessed/dataflow_10.hpp:532:20: error: no type named 'type' in
      'boost::result_of<hpx::launch (hpx::util::detail::unwrap_impl<mul>, hpx::lcos::future<double>, hpx::lcos::future<double>)>'
                >::type
                ~~~^~~~
/home/mario/hpx/include/hpx/lcos/local/preprocessed/dataflow_10.hpp:727:22: note: in instantiation of template class
      'hpx::lcos::local::detail::dataflow_frame_3<hpx::launch, hpx::util::detail::unwrap_impl<mul>, hpx::lcos::future<double> &,
      hpx::lcos::future<double> &>' requested here
    typename detail::dataflow_frame_3<
                     ^
/home/mario/hpx/include/hpx/lcos/local/preprocessed/dataflow_10.hpp:731:5: note: while substituting deduced template arguments into
      function template 'dataflow' [with Func = hpx::launch, F0 = <no value>, F1 = , F2 = <no value>]
    dataflow(BOOST_FWD_REF(Func) func, BOOST_FWD_REF(F0) f0 , BOOST_FWD_REF(F1) f1 , BOOST_FWD_REF(F2) f2)

I think this indeed is a bug because template substitution works with hpx::launch interpreted as Func, but only then the error occurs when trying to obtain the result_of. Maybe there should be a disable_if< is_same< Func , hpx::launch> > added?

[sithhell]

You are right, the overloads without a launch policy need to be disabled when Func is not a callable.

[sithhell]

The reason why it works with MSVC is because it is very lazy with template instantiations.

[mariomulansky]

i don't quite understand why the internal forward is working, though (when dataflow is called without a specific policy).

[sithhell]

Someone removed the internal forwarding.