Ensure return type of templated generic parameter

I'm trying to make a compile time generics example. I've made it enforce that the methods exists, but I can't figure out how to enforce the return type:

#include <type_traits>
template<class...> struct voider { using type = void; }
template<class... T> using void_t = typename voider<T...>::type;

template<class T, class = void> struct add_able : std::false_type {};
template<class T> struct add_able
< T, void_t < std::is_same < decltype(std::declval<T>().addOne()), void >,
              std::is_same < decltype(std::declval<T>().subOne()), void > > >
: std::true_type {};

class A { public: int addOne(); void subOne(); } // compiles and runs
class B { public: void addOne(int); void subOne(); } // fails
class C { public: void addOne(); void subOne(); } // compiles and runs as expected
class D { public: void subOne(); } // fails

This ensures that there is a addOne() and subOne() method but the return types are completely arbitrary. How do I enforce this?