[libc++][array] Applied `[[nodiscard]]` #168829

H-G-Hristov · 2025-11-20T06:14:51Z

[[nodiscard]] should be applied to functions where discarding the return value is most likely a correctness issue.

https://libcxx.llvm.org/CodingGuidelines.html#apply-nodiscard-where-relevant

`[[nodiscard]]` should be applied to functions where discarding the return value is most likely a correctness issue. - https://libcxx.llvm.org/CodingGuidelines.html#apply-nodiscard-where-relevant

llvmbot · 2025-11-20T06:15:25Z

@llvm/pr-subscribers-libcxx

Author: Hristo Hristov (H-G-Hristov)

Changes

[[nodiscard]] should be applied to functions where discarding the return value is most likely a correctness issue.

- https://libcxx.llvm.org/CodingGuidelines.html#apply-nodiscard-where-relevant

Patch is 27.96 KiB, truncated to 20.00 KiB below, full version: https://github.com/llvm/llvm-project/pull/168829.diff

3 Files Affected:

(modified) libcxx/include/array (+89-56)
(modified) libcxx/test/libcxx/diagnostics/array.nodiscard.verify.cpp (+87-4)
(modified) libcxx/test/std/containers/sequences/array/array.creation/to_array.verify.cpp (+12-8)

diff --git a/libcxx/include/array b/libcxx/include/array
index ff46838e2e8e2..d346b50cfcd7a 100644
--- a/libcxx/include/array
+++ b/libcxx/include/array
@@ -210,28 +210,28 @@ struct array {
   }
 
   // iterators:
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 iterator begin() _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 iterator begin() _NOEXCEPT {
 #  if defined(_LIBCPP_ABI_BOUNDED_ITERATORS_IN_STD_ARRAY)
     return std::__make_static_bounded_iter<_Size>(data(), data());
 #  else
     return iterator(data());
 #  endif
   }
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 const_iterator begin() const _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 const_iterator begin() const _NOEXCEPT {
 #  if defined(_LIBCPP_ABI_BOUNDED_ITERATORS_IN_STD_ARRAY)
     return std::__make_static_bounded_iter<_Size>(data(), data());
 #  else
     return const_iterator(data());
 #  endif
   }
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 iterator end() _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 iterator end() _NOEXCEPT {
 #  if defined(_LIBCPP_ABI_BOUNDED_ITERATORS_IN_STD_ARRAY)
     return std::__make_static_bounded_iter<_Size>(data() + _Size, data());
 #  else
     return iterator(data() + _Size);
 #  endif
   }
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 const_iterator end() const _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 const_iterator end() const _NOEXCEPT {
 #  if defined(_LIBCPP_ABI_BOUNDED_ITERATORS_IN_STD_ARRAY)
     return std::__make_static_bounded_iter<_Size>(data() + _Size, data());
 #  else
@@ -239,62 +239,81 @@ struct array {
 #  endif
   }
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 reverse_iterator rbegin() _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 reverse_iterator rbegin() _NOEXCEPT {
     return reverse_iterator(end());
   }
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 const_reverse_iterator rbegin() const _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 const_reverse_iterator
+  rbegin() const _NOEXCEPT {
     return const_reverse_iterator(end());
   }
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 reverse_iterator rend() _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 reverse_iterator rend() _NOEXCEPT {
     return reverse_iterator(begin());
   }
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 const_reverse_iterator rend() const _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 const_reverse_iterator rend() const _NOEXCEPT {
     return const_reverse_iterator(begin());
   }
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 const_iterator cbegin() const _NOEXCEPT { return begin(); }
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 const_iterator cend() const _NOEXCEPT { return end(); }
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 const_reverse_iterator crbegin() const _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 const_iterator cbegin() const _NOEXCEPT {
+    return begin();
+  }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 const_iterator cend() const _NOEXCEPT {
+    return end();
+  }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 const_reverse_iterator
+  crbegin() const _NOEXCEPT {
     return rbegin();
   }
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 const_reverse_iterator crend() const _NOEXCEPT { return rend(); }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 const_reverse_iterator crend() const _NOEXCEPT {
+    return rend();
+  }
 
   // capacity:
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR size_type size() const _NOEXCEPT { return _Size; }
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR size_type max_size() const _NOEXCEPT { return _Size; }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR size_type size() const _NOEXCEPT { return _Size; }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR size_type max_size() const _NOEXCEPT { return _Size; }
   [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR bool empty() const _NOEXCEPT { return _Size == 0; }
 
   // element access:
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 reference operator[](size_type __n) _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 reference operator[](size_type __n) _NOEXCEPT {
     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(__n < _Size, "out-of-bounds access in std::array<T, N>");
     return __elems_[__n];
   }
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 const_reference operator[](size_type __n) const _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 const_reference
+  operator[](size_type __n) const _NOEXCEPT {
     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(__n < _Size, "out-of-bounds access in std::array<T, N>");
     return __elems_[__n];
   }
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 reference at(size_type __n) {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 reference at(size_type __n) {
     if (__n >= _Size)
       std::__throw_out_of_range("array::at");
     return __elems_[__n];
   }
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 const_reference at(size_type __n) const {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 const_reference at(size_type __n) const {
     if (__n >= _Size)
       std::__throw_out_of_range("array::at");
     return __elems_[__n];
   }
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 reference front() _NOEXCEPT { return (*this)[0]; }
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 const_reference front() const _NOEXCEPT { return (*this)[0]; }
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 reference back() _NOEXCEPT { return (*this)[_Size - 1]; }
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 const_reference back() const _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 reference front() _NOEXCEPT {
+    return (*this)[0];
+  }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 const_reference front() const _NOEXCEPT {
+    return (*this)[0];
+  }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 reference back() _NOEXCEPT {
+    return (*this)[_Size - 1];
+  }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 const_reference back() const _NOEXCEPT {
     return (*this)[_Size - 1];
   }
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 value_type* data() _NOEXCEPT { return __elems_; }
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 const value_type* data() const _NOEXCEPT { return __elems_; }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 value_type* data() _NOEXCEPT {
+    return __elems_;
+  }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 const value_type* data() const _NOEXCEPT {
+    return __elems_;
+  }
 };
 
 template <class _Tp>
@@ -328,8 +347,10 @@ struct array<_Tp, 0> {
   };
   _ALIGNAS_TYPE(_ArrayInStructT) _EmptyType __elems_[sizeof(_ArrayInStructT)];
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 value_type* data() _NOEXCEPT { return nullptr; }
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 const value_type* data() const _NOEXCEPT { return nullptr; }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 value_type* data() _NOEXCEPT { return nullptr; }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 const value_type* data() const _NOEXCEPT {
+    return nullptr;
+  }
 
   // No explicit construct/copy/destroy for aggregate type
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void fill(const value_type&) {
@@ -341,28 +362,28 @@ struct array<_Tp, 0> {
   }
 
   // iterators:
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 iterator begin() _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 iterator begin() _NOEXCEPT {
 #  if defined(_LIBCPP_ABI_BOUNDED_ITERATORS_IN_STD_ARRAY)
     return std::__make_static_bounded_iter<0>(data(), data());
 #  else
     return iterator(data());
 #  endif
   }
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 const_iterator begin() const _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 const_iterator begin() const _NOEXCEPT {
 #  if defined(_LIBCPP_ABI_BOUNDED_ITERATORS_IN_STD_ARRAY)
     return std::__make_static_bounded_iter<0>(data(), data());
 #  else
     return const_iterator(data());
 #  endif
   }
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 iterator end() _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 iterator end() _NOEXCEPT {
 #  if defined(_LIBCPP_ABI_BOUNDED_ITERATORS_IN_STD_ARRAY)
     return std::__make_static_bounded_iter<0>(data(), data());
 #  else
     return iterator(data());
 #  endif
   }
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 const_iterator end() const _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 const_iterator end() const _NOEXCEPT {
 #  if defined(_LIBCPP_ABI_BOUNDED_ITERATORS_IN_STD_ARRAY)
     return std::__make_static_bounded_iter<0>(data(), data());
 #  else
@@ -370,68 +391,77 @@ struct array<_Tp, 0> {
 #  endif
   }
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 reverse_iterator rbegin() _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 reverse_iterator rbegin() _NOEXCEPT {
     return reverse_iterator(end());
   }
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 const_reverse_iterator rbegin() const _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 const_reverse_iterator
+  rbegin() const _NOEXCEPT {
     return const_reverse_iterator(end());
   }
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 reverse_iterator rend() _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 reverse_iterator rend() _NOEXCEPT {
     return reverse_iterator(begin());
   }
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 const_reverse_iterator rend() const _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 const_reverse_iterator rend() const _NOEXCEPT {
     return const_reverse_iterator(begin());
   }
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 const_iterator cbegin() const _NOEXCEPT { return begin(); }
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 const_iterator cend() const _NOEXCEPT { return end(); }
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 const_reverse_iterator crbegin() const _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 const_iterator cbegin() const _NOEXCEPT {
+    return begin();
+  }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 const_iterator cend() const _NOEXCEPT {
+    return end();
+  }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 const_reverse_iterator
+  crbegin() const _NOEXCEPT {
     return rbegin();
   }
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 const_reverse_iterator crend() const _NOEXCEPT { return rend(); }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 const_reverse_iterator crend() const _NOEXCEPT {
+    return rend();
+  }
 
   // capacity:
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR size_type size() const _NOEXCEPT { return 0; }
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR size_type max_size() const _NOEXCEPT { return 0; }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR size_type size() const _NOEXCEPT { return 0; }
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR size_type max_size() const _NOEXCEPT { return 0; }
   [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR bool empty() const _NOEXCEPT { return true; }
 
   // element access:
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 reference operator[](size_type) _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 reference operator[](size_type) _NOEXCEPT {
     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(false, "cannot call array<T, 0>::operator[] on a zero-sized array");
     __libcpp_unreachable();
   }
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 const_reference operator[](size_type) const _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 const_reference
+  operator[](size_type) const _NOEXCEPT {
     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(false, "cannot call array<T, 0>::operator[] on a zero-sized array");
     __libcpp_unreachable();
   }
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 reference at(size_type) {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 reference at(size_type) {
     std::__throw_out_of_range("array<T, 0>::at");
     __libcpp_unreachable();
   }
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 const_reference at(size_type) const {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 const_reference at(size_type) const {
     std::__throw_out_of_range("array<T, 0>::at");
     __libcpp_unreachable();
   }
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 reference front() _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 reference front() _NOEXCEPT {
     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(false, "cannot call array<T, 0>::front() on a zero-sized array");
     __libcpp_unreachable();
   }
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 const_reference front() const _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 const_reference front() const _NOEXCEPT {
     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(false, "cannot call array<T, 0>::front() on a zero-sized array");
     __libcpp_unreachable();
   }
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 reference back() _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 reference back() _NOEXCEPT {
     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(false, "cannot call array<T, 0>::back() on a zero-sized array");
     __libcpp_unreachable();
   }
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 const_reference back() const _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 const_reference back() const _NOEXCEPT {
     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(false, "cannot call array<T, 0>::back() on a zero-sized array");
     __libcpp_unreachable();
   }
@@ -443,8 +473,8 @@ array(_Tp, _Args...) -> array<_Tp, 1 + sizeof...(_Args)>;
 #  endif
 
 template <class _Tp, size_t _Size>
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 bool
-operator==(const array<_Tp, _Size>& __x, const array<_Tp, _Size>& __y) {
+inline _LIBCPP_HIDE_FROM_ABI
+_LIBCPP_CONSTEXPR_SINCE_CXX20 bool operator==(const array<_Tp, _Size>& __x, const array<_Tp, _Size>& __y) {
   return std::equal(__x.begin(), __x.end(), __y.begin());
 }
 
@@ -501,25 +531,28 @@ struct tuple_element<_Ip, array<_Tp, _Size> > {
 };
 
 template <size_t _Ip, class _Tp, size_t _Size>
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _Tp& get(array<_Tp, _Size>& __a) _NOEXCEPT {
+[[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _Tp&
+get(array<_Tp, _Size>& __a) _NOEXCEPT {
   static_assert(_Ip < _Size, "Index out of bounds in std::get<> (std::array)");
   return __a.__elems_[_Ip];
 }
 
 template <size_t _Ip, class _Tp, size_t _Size>
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 const _Tp& get(const array<_Tp, _Size>& __a) _NOEXCEPT {
+[[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 const _Tp&
+get(const array<_Tp, _Size>& __a) _NOEXCEPT {
   static_assert(_Ip < _Size, "Index out of bounds in std::get<> (const std::array)");
   return __a.__elems_[_Ip];
 }
 
 template <size_t _Ip, class _Tp, size_t _Size>
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _Tp&& get(array<_Tp, _Size>&& __a) _NOEXCEPT {
+[[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _Tp&& get(array<_Tp, _Size>&& __a) _NOEXCEPT {
   static_assert(_Ip < _Size, "Index out of bounds in std::get<> (std::array &&)");
   return std::move(__a.__elems_[_Ip]);
 }
 
 template <size_t _Ip, class _Tp, size_t _Size>
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 const _Tp&& get(const array<_Tp, _Size>&& __a) _NOEXCEPT {
+[[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 const _Tp&&
+get(const array<_Tp, _Size>&& __a) _NOEXCEPT {
   static_assert(_Ip < _Size, "Index out of bounds in std::get<> (const std::array &&)");
   return std::move(__a.__elems_[_Ip]);
 }
@@ -539,7 +572,7 @@ __to_array_rvalue_impl(_Tp (&&__arr)[_Size], index_sequence<_Index...>) {
 }
 
 template <typename _Tp, size_t _Size>
-_LIBCPP_HIDE_FROM_ABI constexpr array<remove_cv_t<_Tp>, _Size>
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr array<remove_cv_t<_Tp>, _Size>
 to_array(_Tp (&__arr)[_Size]) noexcept(is_nothrow_constructible_v<_Tp, _Tp&>) {
   static_assert(!is_array_v<_Tp>, "[array.creation]/1: to_array does not accept multidimensional arrays.");
   static_assert(is_constructible_v<_Tp, _Tp&>, "[array.creation]/1: to_array requires copy constructible elements.");
@@ -547,7 +580,7 @@ to_array(_Tp (&__arr)[_Size]) noexcept(is_nothrow_constructible_v<_Tp, _Tp&>) {
 }
 
 template <typename _Tp, size_t _Size>
-_LIBCPP_HIDE_FROM_ABI constexpr array<remove_cv_t<_Tp>, _Size>
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr array<remove_cv_t<_Tp>, _Size>
 to_array(_Tp (&&__arr)[_Size]) noexcept(is_nothrow_move_constructible_v<_Tp>) {
   static_assert(!is_array_v<_Tp>, "[array.creation]/4: to_array does not accept multidimensional arrays.");
   static_assert(is_move_constructible_v<_Tp>, "[array.creation]/4: to_array requires move constructible elements.");
diff --git a/libcxx/test/libcxx/diagnostics/array.nodiscard.verify.cpp b/libcxx/test/libcxx/diagnostics/array.nodiscard.verify.cpp
index 25a2f80b48f02..15bfdcc4f9e8c 100644
--- a/libcxx/test/libcxx/diagnostics/array.nodiscard.verify.cpp
+++ b/libcxx/test/libcxx/diagnostics/array.nodiscard.verify.cpp
@@ -11,13 +11,96 @@
 // check that <array> functions are marked [[nodiscard]]
 
 #include <array>
+#include <utility>
+
+#include <test_macros.h>
 
 void array_test() {
-  std::array<int, 1> array;
-  array.empty(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::array<int, 1> a;
+  const std::array<int, 1> ca{9482};
+
+  a.begin();    // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  ca.begin();   // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  a.end();      // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  ca.end();     // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  a.rbegin();   // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  ca.rbegin();  // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  a.rend();     // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  ca.rend();    // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  a.cbegin();   // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  ca.cbegin();  // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  a.cend();     // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  ca.cend();    // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  a.crbegin();  // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  ca.crbegin(); // expected-warning {{ignor...
[truncated]

libcxx/include/array

Thibault-Monnier · 2025-11-20T11:45:14Z

Perhaps we should add this to the changelog, together with #168826?

libcxx/include/array

libcxx/test/libcxx/diagnostics/array.nodiscard.verify.cpp

libcxx/test/std/containers/sequences/array/array.creation/to_array.verify.cpp

Zingam · 2025-11-20T12:11:35Z

Perhaps we should add this to the changelog, together with #168826?

I think we haven't upadated the release notes for any of the previous patches. @philnik777 Do we want to add a note about the [[nodiscard]] improvements?

Thibault-Monnier

LGTM

libcxx/test/libcxx/diagnostics/array.nodiscard.verify.cpp

H-G-Hristov · 2025-11-24T10:15:47Z

Thank you!

`[[nodiscard]]` should be applied to functions where discarding the return value is most likely a correctness issue. - https://libcxx.llvm.org/CodingGuidelines.html#apply-nodiscard-where-relevant --------- Co-authored-by: Hristo Hristov <[email protected]>

[libc++][array] Applied [[nodiscard]]`

5cbfae5

`[[nodiscard]]` should be applied to functions where discarding the return value is most likely a correctness issue. - https://libcxx.llvm.org/CodingGuidelines.html#apply-nodiscard-where-relevant

H-G-Hristov requested a review from a team as a code owner November 20, 2025 06:14

llvmbot added the libc++ libc++ C++ Standard Library. Not GNU libstdc++. Not libc++abi. label Nov 20, 2025

H-G-Hristov changed the title ~~[libc++][array] Applied [[nodiscard]]`~~ [libc++][array] Applied [[nodiscard]] Nov 20, 2025

frederick-vs-ja approved these changes Nov 20, 2025

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H-G-Hristov added 3 commits November 20, 2025 18:57

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Addressed review comments

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Formatting

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H-G-Hristov requested review from Thibault-Monnier, frederick-vs-ja and philnik777 November 21, 2025 05:29

Shorten test

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Thibault-Monnier approved these changes Nov 21, 2025

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H-G-Hristov and others added 3 commits November 21, 2025 18:18

Lengthen test

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Fix

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Merge branch 'main' into hgh/libcxx/nodiscard-to-array

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philnik777 approved these changes Nov 24, 2025

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Zingam merged commit 8c6ec12 into llvm:main Nov 24, 2025
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H-G-Hristov deleted the hgh/libcxx/nodiscard-to-array branch November 24, 2025 10:38

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