boost::gil::position_iterator::operator‐>

template<typename D = Derived>
constexpr
decltype(detail::make_pointer<pointer, reference>(*std::declval<D &>()))
operator‐>() noexcept(/* see-below */);

template<typename D = Derived>
constexpr
decltype(detail::make_pointer<pointer, reference>(
                    *std::declval<D const &>()))
operator‐>() const noexcept(/* see-below */);