constexpr const F& base_function(Ts&&... xs) const { return always_ref(*this)(xs...); }
constexpr const detail::callable_base<F>& base_function(Ts&&... xs) const { return always_ref(*this)(xs...); }
FIT_FIX_CONSTEXPR const Derived& derived_function(Ts&&... xs) const { return static_cast<const Derived&>(always_ref(*this)(xs...)); }
constexpr const Base& get_base(Xs&&... xs) const { return always_ref(*this)(xs...); }
FIT_FIX_CONSTEXPR const detail::callable_base<F>& base_function(Ts&&... xs) const { return always_ref(*this)(xs...); }