[libcxx-commits] [libcxx] [libc++][vector] Apply `[[nodiscard]]` to `vector<bool>::iterator` (PR #202265)

[Hristo Hristov via libcxx-commits]

https://github.com/H-G-Hristov updated https://github.com/llvm/llvm-project/pull/202265

>From 45451be5428878f12a2781fb7ccc5410b36c5724 Mon Sep 17 00:00:00 2001
From: Hristo Hristov <hghristov.rmm at gmail.com>
Date: Mon, 8 Jun 2026 09:21:13 +0300
Subject: [PATCH] [libc++][vector] Apply `[[nodiscard]]` to
 `vector<bool>::iterator`

Towards #172124
---
 libcxx/include/__bit_reference                | 15 +++---
 .../vector.bool/nodiscard.iterator.verify.cpp | 52 +++++++++++++++++++
 2 files changed, 61 insertions(+), 6 deletions(-)
 create mode 100644 libcxx/test/libcxx/containers/sequences/vector.bool/nodiscard.iterator.verify.cpp

diff --git a/libcxx/include/__bit_reference b/libcxx/include/__bit_reference
index a4fd6602770ef..692686f1584d0 100644
--- a/libcxx/include/__bit_reference
+++ b/libcxx/include/__bit_reference
@@ -336,7 +336,7 @@ public:
   }
 #endif
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 reference operator*() const _NOEXCEPT {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 reference operator*() const _NOEXCEPT {
     _LIBCPP_ASSERT_INTERNAL(__ctz_ < __bits_per_word, "Dereferencing an invalid __bit_iterator.");
     return __conditional_t<_IsConst, __bit_const_reference<_Cp>, __bit_reference<_Cp> >(
         __seg_, __storage_type(1) << __ctz_);
@@ -389,29 +389,32 @@ public:
     return *this += -__n;
   }
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 __bit_iterator operator+(difference_type __n) const {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 __bit_iterator
+  operator+(difference_type __n) const {
     __bit_iterator __t(*this);
     __t += __n;
     return __t;
   }
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 __bit_iterator operator-(difference_type __n) const {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 __bit_iterator
+  operator-(difference_type __n) const {
     __bit_iterator __t(*this);
     __t -= __n;
     return __t;
   }
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 friend __bit_iterator
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 friend __bit_iterator
   operator+(difference_type __n, const __bit_iterator& __it) {
     return __it + __n;
   }
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 friend difference_type
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 friend difference_type
   operator-(const __bit_iterator& __x, const __bit_iterator& __y) {
     return (__x.__seg_ - __y.__seg_) * __bits_per_word + __x.__ctz_ - __y.__ctz_;
   }
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 reference operator[](difference_type __n) const {
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 reference
+  operator[](difference_type __n) const {
     return *(*this + __n);
   }
 
diff --git a/libcxx/test/libcxx/containers/sequences/vector.bool/nodiscard.iterator.verify.cpp b/libcxx/test/libcxx/containers/sequences/vector.bool/nodiscard.iterator.verify.cpp
new file mode 100644
index 0000000000000..6d8e652d0d8a2
--- /dev/null
+++ b/libcxx/test/libcxx/containers/sequences/vector.bool/nodiscard.iterator.verify.cpp
@@ -0,0 +1,52 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// <vector>
+
+// Check that functions are marked [[nodiscard]]
+
+#include <vector>
+
+#include "test_macros.h"
+
+void test() {
+  typedef std::vector<bool> Container;
+  Container c;
+  Container::iterator it        = c.begin();
+  Container::const_iterator cit = c.cbegin();
+
+  // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  *it;
+  // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  *cit;
+
+  // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  it + 1;
+  // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  cit + 1;
+
+  // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  it - 1;
+  // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  cit - 1;
+
+  // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  1 + it;
+  // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  1 + cit;
+
+  // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  it - it;
+  // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  cit - cit;
+
+  // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  it[0];
+  // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  cit[0];
+}