CRTP: How to infer type of member to be used as return type?

I would like to make the return type of a CRTP base method depend on the type of a member in the derived, as for example in:

template <typename C>
struct sum_a_b {
    ??? sum() { return static_cast<C*>(this)->a + static_cast<C*>(this)->b; }
}

template <typename T> struct a_b : sum_a_b<a_b<T>> { T a,b; };

What should I put in place of ???

I tried different ways to declare the return type :

template <typename T>
struct base {
    int get_ok() {  
        return static_cast<T*>(this)->value; 
    }
    auto get_invalid() -> decltype(static_cast<T*>(this)->value) {
        return static_cast<T*>(this)->value; 
    }
    typename T::value_type get_incomplete_type_foo() {  
        return static_cast<T*>(this)->value; 
    }
    auto get_incomplete_type_again() -> decltype(T().value) {  
        return static_cast<T*>(this)->value; 
    }
};

struct foo : base<foo> {
        typedef int value_type;
        value_type value;
};

The only methods that compiles is int get_ok, for the others I get either (for get_invalid_cast):

invalid static_cast from type 'base<foo>*' to type 'foo*'
     auto get_invalid() -> decltype(static_cast<T*>(this)->value) {  return static_cast<T*>(this)->value; }
                                    ^

or (the other two)

invalid use of incomplete type 'struct foo'
     typename T::value_type get_incomplete_type_foo() {  return static_cast<T*>(this)->value; }
                            ^