boost::gil::iterator_from_2d::operator‐

constexpr
auto
operator‐(iterator_from_2d other) const noexcept;

template<typename D = Derived>
constexpr
decltype(access::base(std::declval<D const &>()) ‐ access::base(other))
operator‐(D other) const noexcept(/* see-below */);