functional/forward broken with decltype-based boost::result_of

[Eric Niebler]

The clang c++11 test runner is showing failures with decltype-based result_of that do not happen with TR1-style result_of. See the results here (clang-cxx11 test runner):

http://www.boost.org/development/tests/trunk/developer/functional-forward.html

In particular:

In file included from ../libs/functional/forward/test/forward_adapter.cpp:16:
In file included from ../boost/functional/forward_adapter.hpp:21:
In file included from ../boost/utility/result_of.hpp:113:
In file included from ../boost/preprocessor/iteration/detail/iter/forward1.hpp:47:
../boost/utility/detail/result_of_iterate.hpp:66:5: error: no matching function for call to object of type 'const test_func<boost::blank>'
    boost::declval<F>()(
    ^~~~~~~~~~~~~~~~~~~
../boost/utility/detail/result_of_iterate.hpp:47:7: note: in instantiation of template class 'boost::detail::cpp0x_result_of_impl<const test_func<boost::blank> ()>' requested here
    : mpl::if_<
      ^
../boost/functional/forward_adapter.hpp:150:36: note: in instantiation of template class 'boost::result_of<const test_func<boost::blank> ()>' requested here
            inline typename boost::result_of< FC() >::type
                                   ^

[Eric Niebler]

the following patch fixes the problem

[Michel Morin]

Patch is attached by Eric Niebler.