[libc++] Add [[nodiscard]] to std::prev and std::next

Author: marcauberer

libcxx/include/__iterator/next.h

@@ -43,25 +43,25 @@ next(_InputIter __x, typename iterator_traits<_InputIter>::difference_type __n =
 namespace ranges {
 struct __next {
   template <input_or_output_iterator _Ip>
-  _LIBCPP_HIDE_FROM_ABI constexpr _Ip operator()(_Ip __x) const {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Ip operator()(_Ip __x) const {
     ++__x;
     return __x;
   }
 
   template <input_or_output_iterator _Ip>
-  _LIBCPP_HIDE_FROM_ABI constexpr _Ip operator()(_Ip __x, iter_difference_t<_Ip> __n) const {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Ip operator()(_Ip __x, iter_difference_t<_Ip> __n) const {
     ranges::advance(__x, __n);
     return __x;
   }
 
   template <input_or_output_iterator _Ip, sentinel_for<_Ip> _Sp>
-  _LIBCPP_HIDE_FROM_ABI constexpr _Ip operator()(_Ip __x, _Sp __bound_sentinel) const {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Ip operator()(_Ip __x, _Sp __bound_sentinel) const {
     ranges::advance(__x, __bound_sentinel);
     return __x;
   }
 
   template <input_or_output_iterator _Ip, sentinel_for<_Ip> _Sp>
-  _LIBCPP_HIDE_FROM_ABI constexpr _Ip operator()(_Ip __x, iter_difference_t<_Ip> __n, _Sp __bound_sentinel) const {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Ip operator()(_Ip __x, iter_difference_t<_Ip> __n, _Sp __bound_sentinel) const {
     ranges::advance(__x, __n, __bound_sentinel);
     return __x;
   }

libcxx/include/__iterator/prev.h

@@ -42,19 +42,19 @@ prev(_InputIter __x, typename iterator_traits<_InputIter>::difference_type __n =
 namespace ranges {
 struct __prev {
   template <bidirectional_iterator _Ip>
-  _LIBCPP_HIDE_FROM_ABI constexpr _Ip operator()(_Ip __x) const {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Ip operator()(_Ip __x) const {
     --__x;
     return __x;
   }
 
   template <bidirectional_iterator _Ip>
-  _LIBCPP_HIDE_FROM_ABI constexpr _Ip operator()(_Ip __x, iter_difference_t<_Ip> __n) const {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Ip operator()(_Ip __x, iter_difference_t<_Ip> __n) const {
     ranges::advance(__x, -__n);
     return __x;
   }
 
   template <bidirectional_iterator _Ip>
-  _LIBCPP_HIDE_FROM_ABI constexpr _Ip operator()(_Ip __x, iter_difference_t<_Ip> __n, _Ip __bound_iter) const {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Ip operator()(_Ip __x, iter_difference_t<_Ip> __n, _Ip __bound_iter) const {
     ranges::advance(__x, -__n, __bound_iter);
     return __x;
   }