I just hit this using VS2012/boost 1.54 beta. #include #include <future> boost::optional< std::future<void> > top() { return std::async( []{ int i=0; ++i; } ); } int main() { auto opt = top(); } Error: 1>E:\Projects\SDK\boost\boost\include\boost-1_54\boost/optional/optional.hpp(347): error C2248: 'std::future<void>::future' : cannot access private member declared in class 'std::future<void>' 1> C:\Program Files (x86)\Microsoft Visual Studio 11.0\VC\include\future(1238) : see declaration of 'std::future<void>::future' 1> C:\Program Files (x86)\Microsoft Visual Studio 11.0\VC\include\future(1200) : see declaration of 'std::future<void>' 1> E:\Projects\SDK\boost\boost\include\boost-1_54\boost/optional/optional.hpp(346) : while compiling class template member function 'void boost::optional_detail::optional_base<T>::construct(const std::future<void> &)' 1> with 1> [ 1> T=std::future<void> 1> ] 1> E:\Projects\SDK\boost\boost\include\boost-1_54\boost/optional/optional.hpp(231) : see reference to function template instantiation 'void boost::optional_detail::optional_base<T>::construct(const std::future<void> &)' being compiled 1> with 1> [ 1> T=std::future<void> 1> ] 1> E:\Projects\SDK\boost\boost\include\boost-1_54\boost/optional/optional.hpp(501) : see reference to class template instantiation 'boost::optional_detail::optional_base<T>' being compiled 1> with 1> [ 1> T=std::future<void> 1> ] 1> E:\Projects\Art_Of_Sequence\repos\aos-all\tool\aosdesigner\backend\test\workspace.cpp(30) : see reference to class template instantiation 'boost::optional<T>' being compiled 1> with 1> [ 1> T=std::future<void> 1> ] I also tried with http://coliru.stacked-crooked.com/ gcc4.8 (but I don't know the boost version) and got a similar error. I first hit this with boost::future but it don't seem to be the problem as it generate the same error. My question is: does std/boost::future don't match the requirements to be an optional value? Or is this an error I should log? Joel Lamotte