boost::gil::bit_aligned_pixel_iterator::operator+=

constexpr
bit_aligned_pixel_iterator&
operator+=(difference_type i) noexcept;

template<typename D = Derived>
constexpr
decltype(access::base(std::declval<D &>()) += n, std::declval<D &>())
operator+=(difference_type n) noexcept(/* see-below */);