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

[marcauberer]

Add [[nodiscard]] attribute to std::prev and std::next. Those are potential pitfalls for users who might think they mutate the iterator.

Fixes #109452

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: Marc Auberer (marcauberer)

Changes

Add [[nodiscard]] attribute to std::prev and std::next. Those are potential pitfalls for users who might think they mutate the iterator.

---

Full diff: https://github.com/llvm/llvm-project/pull/109550.diff

2 Files Affected:

• (modified) libcxx/include/__iterator/next.h (+5-4)
• (modified) libcxx/include/__iterator/prev.h (+4-3)

diff --git a/libcxx/include/__iterator/next.h b/libcxx/include/__iterator/next.h
index fb6c8ea6d75508..27b21e6f52c272 100644
--- a/libcxx/include/__iterator/next.h
+++ b/libcxx/include/__iterator/next.h
@@ -43,25 +43,26 @@ 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;
   }
diff --git a/libcxx/include/__iterator/prev.h b/libcxx/include/__iterator/prev.h
index e950d8dc414717..6ee9f08308660e 100644
--- a/libcxx/include/__iterator/prev.h
+++ b/libcxx/include/__iterator/prev.h
@@ -42,19 +42,20 @@ 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;
   }

[ldionne]

Thanks for the patch! A few comments that should be easy to fix.

[ldionne]

LGTM once the CI is green, thanks!