From 441e44db3442043eb23c19a76acbc58fc3e99e9f Mon Sep 17 00:00:00 2001
From: Hristo Hristov <hghristov.rmm@gmail.com>
Date: Thu, 4 Dec 2025 07:41:50 +0200
Subject: [PATCH] [libc++][pair] Applied `[[nodiscard]]`

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

- https://libcxx.llvm.org/CodingGuidelines.html
- https://wg21.link/pairs
---
 libcxx/include/__utility/pair.h               | 32 +++++++-----
 .../utility/pairs/nodiscard.verify.cpp        | 52 +++++++++++++++++++
 2 files changed, 70 insertions(+), 14 deletions(-)
 create mode 100644 libcxx/test/libcxx/utilities/utility/pairs/nodiscard.verify.cpp

diff --git a/libcxx/include/__utility/pair.h b/libcxx/include/__utility/pair.h
index d3914f655f2a6..a8232bc9da9fa 100644
--- a/libcxx/include/__utility/pair.h
+++ b/libcxx/include/__utility/pair.h
@@ -539,8 +539,8 @@ swap(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y) noexcept(noexcept(__x
 #endif
 
 template <class _T1, class _T2>
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 pair<__unwrap_ref_decay_t<_T1>, __unwrap_ref_decay_t<_T2> >
-make_pair(_T1&& __t1, _T2&& __t2) {
+[[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14
+pair<__unwrap_ref_decay_t<_T1>, __unwrap_ref_decay_t<_T2> > make_pair(_T1&& __t1, _T2&& __t2) {
   return pair<__unwrap_ref_decay_t<_T1>, __unwrap_ref_decay_t<_T2> >(std::forward<_T1>(__t1), std::forward<_T2>(__t2));
 }
 
@@ -612,67 +612,71 @@ struct __get_pair<1> {
 };
 
 template <size_t _Ip, class _T1, class _T2>
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 typename tuple_element<_Ip, pair<_T1, _T2> >::type&
+[[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14
+typename tuple_element<_Ip, pair<_T1, _T2> >::type&
 get(pair<_T1, _T2>& __p) _NOEXCEPT {
   return __get_pair<_Ip>::get(__p);
 }
 
 template <size_t _Ip, class _T1, class _T2>
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 const typename tuple_element<_Ip, pair<_T1, _T2> >::type&
+[[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI
+_LIBCPP_CONSTEXPR_SINCE_CXX14 const typename tuple_element<_Ip, pair<_T1, _T2> >::type&
 get(const pair<_T1, _T2>& __p) _NOEXCEPT {
   return __get_pair<_Ip>::get(__p);
 }
 
 template <size_t _Ip, class _T1, class _T2>
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 typename tuple_element<_Ip, pair<_T1, _T2> >::type&&
+[[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14
+typename tuple_element<_Ip, pair<_T1, _T2> >::type&&
 get(pair<_T1, _T2>&& __p) _NOEXCEPT {
   return __get_pair<_Ip>::get(std::move(__p));
 }
 
 template <size_t _Ip, class _T1, class _T2>
-inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 const typename tuple_element<_Ip, pair<_T1, _T2> >::type&&
+[[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI
+_LIBCPP_CONSTEXPR_SINCE_CXX14 const typename tuple_element<_Ip, pair<_T1, _T2> >::type&&
 get(const pair<_T1, _T2>&& __p) _NOEXCEPT {
   return __get_pair<_Ip>::get(std::move(__p));
 }
 
 #if _LIBCPP_STD_VER >= 14
 template <class _T1, class _T2>
-inline _LIBCPP_HIDE_FROM_ABI constexpr _T1& get(pair<_T1, _T2>& __p) _NOEXCEPT {
+[[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI constexpr _T1& get(pair<_T1, _T2>& __p) _NOEXCEPT {
   return __p.first;
 }
 
 template <class _T1, class _T2>
-inline _LIBCPP_HIDE_FROM_ABI constexpr _T1 const& get(pair<_T1, _T2> const& __p) _NOEXCEPT {
+[[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI constexpr _T1 const& get(pair<_T1, _T2> const& __p) _NOEXCEPT {
   return __p.first;
 }
 
 template <class _T1, class _T2>
-inline _LIBCPP_HIDE_FROM_ABI constexpr _T1&& get(pair<_T1, _T2>&& __p) _NOEXCEPT {
+[[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI constexpr _T1&& get(pair<_T1, _T2>&& __p) _NOEXCEPT {
   return std::forward<_T1&&>(__p.first);
 }
 
 template <class _T1, class _T2>
-inline _LIBCPP_HIDE_FROM_ABI constexpr _T1 const&& get(pair<_T1, _T2> const&& __p) _NOEXCEPT {
+[[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI constexpr _T1 const&& get(pair<_T1, _T2> const&& __p) _NOEXCEPT {
   return std::forward<_T1 const&&>(__p.first);
 }
 
 template <class _T2, class _T1>
-inline _LIBCPP_HIDE_FROM_ABI constexpr _T2& get(pair<_T1, _T2>& __p) _NOEXCEPT {
+[[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI constexpr _T2& get(pair<_T1, _T2>& __p) _NOEXCEPT {
   return __p.second;
 }
 
 template <class _T2, class _T1>
-inline _LIBCPP_HIDE_FROM_ABI constexpr _T2 const& get(pair<_T1, _T2> const& __p) _NOEXCEPT {
+[[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI constexpr _T2 const& get(pair<_T1, _T2> const& __p) _NOEXCEPT {
   return __p.second;
 }
 
 template <class _T2, class _T1>
-inline _LIBCPP_HIDE_FROM_ABI constexpr _T2&& get(pair<_T1, _T2>&& __p) _NOEXCEPT {
+[[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI constexpr _T2&& get(pair<_T1, _T2>&& __p) _NOEXCEPT {
   return std::forward<_T2&&>(__p.second);
 }
 
 template <class _T2, class _T1>
-inline _LIBCPP_HIDE_FROM_ABI constexpr _T2 const&& get(pair<_T1, _T2> const&& __p) _NOEXCEPT {
+[[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI constexpr _T2 const&& get(pair<_T1, _T2> const&& __p) _NOEXCEPT {
   return std::forward<_T2 const&&>(__p.second);
 }
 
diff --git a/libcxx/test/libcxx/utilities/utility/pairs/nodiscard.verify.cpp b/libcxx/test/libcxx/utilities/utility/pairs/nodiscard.verify.cpp
new file mode 100644
index 0000000000000..87ecbcb01ac43
--- /dev/null
+++ b/libcxx/test/libcxx/utilities/utility/pairs/nodiscard.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
+//
+//===----------------------------------------------------------------------===//
+
+// Check that functions are marked [[nodiscard]]
+
+#include <utility>
+
+#include <test_macros.h>
+
+void test() {
+  struct First {};
+  struct Second {};
+
+  std::pair<First, Second> p;
+  const std::pair<First, Second> cp;
+
+  std::make_pair(94, 82); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+
+#if TEST_STD_VER >= 11
+  // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::get<0>(p);
+  // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::get<0>(cp);
+  // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::get<0>(std::move(p));
+  // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::get<0>(std::move(cp));
+#endif // TEST_STD_VER >= 11
+#if TEST_STD_VER >= 14
+  // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::get<First>(p);
+  // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::get<First>(cp);
+  // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::get<First>(std::move(p));
+  // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::get<First>(std::move(cp));
+  // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::get<Second>(p);
+  // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::get<Second>(cp);
+  // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::get<Second>(std::move(p));
+  // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::get<Second>(std::move(cp));
+#endif // TEST_STD_VER >= 14
+}