template <class F>
 future<typename result_of<typename decay<F>::type()>::type>
async(F f);
template <class F>
 future<typename result_of<typename decay<F>::type()>::type>
async(launch policy, F f);

In your example you are calling the second overload, which takes a launch policy in its first argument, boost::launch is an enum (se here), i guess the compiler is failing trying to create an enum from your class instance.

I tried  your example using boost::launch::async and capturing your tex object: didn't have any problems at all.

Best regards,

Juan