To elaborate on @ruakh answer.

you want a specialization for has_member<A>, which is has_member<A, void>

The reason is because has_member is typically going to be used like:

template<typename T, typename=std::enable_if_t<has_member<T>::value>> 
void resolve(const T& s) {}

template<typename T, typename=std::enable_if_t<!has_member<T>::value>, typename=void>
void resolve(const T& s) {}