const type_info& target_type() const noexcept;
Returns: If *this has a target of type T, typeid(T); otherwise, typeid(void).
template<class Executor> Executor* target() noexcept;
template<class Executor> const Executor* target() const noexcept;
Returns: If target_type() == typeid(Executor) a pointer to the stored executor target; otherwise a null pointer value.