Требует ли конструктор копирования std::function, чтобы типы аргументов типа шаблона были полными типами?

Дано:

#include 
class world_building_gun;
class tile_bounding_box;
typedef std::function worldgen_function_t;
void foo() {
    worldgen_function_t v;
    worldgen_function_t w(v);
}

Следует ли это компилировать? Мои компиляторы говорят:

Да: GCC/stdlibc++ (также boost::function имеет значение yes как в GCC, так и в Clang)

Нет: Clang/libc++ ( http://libcxx.llvm.org/, Clang 3.0, libc++ SVN на сегодняшний день)

(Если правильный ответ «нет», я исправлю свой реальный код, чтобы поместить полные типы в большее количество заголовков или использовать boost::function.)

РЕДАКТИРОВАТЬ: Вот сообщение об ошибке Clang:

In file included from foo.cpp:2:
In file included from /usr/include/c++/v1/functional:462:
/usr/include/c++/v1/type_traits:2766:19: error: invalid appli
    static_assert(sizeof(_Tp) > 0, "Type must be complete.");
                  ^~~~~~~~~~~
/usr/include/c++/v1/type_traits:2752:15: note: in instantiation of template class 'std::__1::__check_complete,
              ^
/usr/include/c++/v1/type_traits:2753:15: note: in instantiation of template class 'std::__1::__check_complete
              ^
/usr/include/c++/v1/type_traits:2919:15: note: in instantiation of template class 'std::__1::__check_complete' requested here
    : private __check_complete<_Fp, _Args...>
              ^
/usr/include/c++/v1/type_traits:2930:11: note: in instantiation of template class 'std::__1::__invokable_imp' requested here
          __invokable_imp<_Fp, _Args...>::value>
          ^
/usr/include/c++/v1/functional:1115:33: note: in instantiation of template class 'std::__1::__invokable' requested here
    template ::value>
                                ^
/usr/include/c++/v1/functional:1141:35: note: in instantiation of default argument for '__callable >' required here
              typename enable_if<__callable<_Fp>::value>::type* = 0);
                                  ^~~~~~~~~~~~~~~
/usr/include/c++/v1/functional:1140:7: note: while substituting deduced template arguments into function template 'function' [with _Fp = std::__1::function]
      function(_Fp,
      ^
foo.cpp:4:7: note: forward declaration of 'world_building_gun'
class world_building_gun;
      ^
In file included from foo.cpp:2:
In file included from /usr/include/c++/v1/functional:462:
/usr/include/c++/v1/type_traits:2766:19: error: invalid application of 'sizeof' to an incomplete type 'tile_bounding_box'
    static_assert(sizeof(_Tp) > 0, "Type must be complete.");
                  ^~~~~~~~~~~
/usr/include/c++/v1/type_traits:2753:15: note: in instantiation of template class 'std::__1::__check_complete' requested here
      private __check_complete<_T0, _Tp...>
              ^
/usr/include/c++/v1/type_traits:2753:15: note: in instantiation of template class 'std::__1::__check_complete' requested here
      private __check_complete<_T0, _Tp...>
              ^
/usr/include/c++/v1/type_traits:2919:15: note: in instantiation of template class 'std::__1::__check_complete &,
      world_building_gun, tile_bounding_box>' requested here
    : private __check_complete<_Fp, _Args...>
              ^
/usr/include/c++/v1/type_traits:2930:11: note: in instantiation of template class 'std::__1::__invokable_imp &,
      world_building_gun, tile_bounding_box>' requested here
          __invokable_imp<_Fp, _Args...>::value>
          ^
/usr/include/c++/v1/functional:1115:33: note: in instantiation of template class 'std::__1::__invokable &,
      world_building_gun, tile_bounding_box>' requested here
    template ::value>
                                ^
/usr/include/c++/v1/functional:1141:35: note: in instantiation of default argument for '__callable >' required here
              typename enable_if<__callable<_Fp>::value>::type* = 0);
                                  ^~~~~~~~~~~~~~~
/usr/include/c++/v1/functional:1140:7: note: while substituting deduced template arguments into function template 'function' [with _Fp = std::__1::function]
      function(_Fp,
      ^
foo.cpp:5:7: note: forward declaration of 'tile_bounding_box'
class tile_bounding_box;
      ^
2 errors generated.

Clang+libc++ успешно компилируется, если я удалю строку «worldgen_function_t w(v);» или если я сделаю классы полными типами.