[libcxx-commits] [libcxx] [libc++][ranges] Applied `[[nodiscard]]` to `common_view` (PR #173176)

[Hristo Hristov via libcxx-commits]

https://github.com/H-G-Hristov created https://github.com/llvm/llvm-project/pull/173176

`[[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/ranges
- https://wg21.link/range.common

Towards #172124

>From 063f5c6773f4e6199104dc4f0ce73144c322ef15 Mon Sep 17 00:00:00 2001
From: Hristo Hristov <hghristov.rmm at gmail.com>
Date: Sun, 21 Dec 2025 07:43:19 +0200
Subject: [PATCH] [libc++][ranges] Applied `[[nodiscard]]` to `common_view`

`[[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/ranges
- https://wg21.link/range.common

Towards #172124
---
 libcxx/include/__ranges/common_view.h         | 14 ++---
 .../range.common.view/nodiscard.verify.cpp    | 61 +++++++++++++++++++
 2 files changed, 68 insertions(+), 7 deletions(-)
 create mode 100644 libcxx/test/libcxx/ranges/range.adaptors/range.common.view/nodiscard.verify.cpp

diff --git a/libcxx/include/__ranges/common_view.h b/libcxx/include/__ranges/common_view.h
index 133236dd1d78a..eec1045c8a758 100644
--- a/libcxx/include/__ranges/common_view.h
+++ b/libcxx/include/__ranges/common_view.h
@@ -56,16 +56,16 @@ class common_view : public view_interface<common_view<_View>> {
     return __base_;
   }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr _View base() && { return std::move(__base_); }
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _View base() && { return std::move(__base_); }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr auto begin() {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto begin() {
     if constexpr (random_access_range<_View> && sized_range<_View>)
       return ranges::begin(__base_);
     else
       return common_iterator<iterator_t<_View>, sentinel_t<_View>>(ranges::begin(__base_));
   }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr auto begin() const
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto begin() const
     requires range<const _View>
   {
     if constexpr (random_access_range<const _View> && sized_range<const _View>)
@@ -74,14 +74,14 @@ class common_view : public view_interface<common_view<_View>> {
       return common_iterator<iterator_t<const _View>, sentinel_t<const _View>>(ranges::begin(__base_));
   }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr auto end() {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto end() {
     if constexpr (random_access_range<_View> && sized_range<_View>)
       return ranges::begin(__base_) + ranges::size(__base_);
     else
       return common_iterator<iterator_t<_View>, sentinel_t<_View>>(ranges::end(__base_));
   }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr auto end() const
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto end() const
     requires range<const _View>
   {
     if constexpr (random_access_range<const _View> && sized_range<const _View>)
@@ -90,13 +90,13 @@ class common_view : public view_interface<common_view<_View>> {
       return common_iterator<iterator_t<const _View>, sentinel_t<const _View>>(ranges::end(__base_));
   }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr auto size()
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto size()
     requires sized_range<_View>
   {
     return ranges::size(__base_);
   }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr auto size() const
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto size() const
     requires sized_range<const _View>
   {
     return ranges::size(__base_);
diff --git a/libcxx/test/libcxx/ranges/range.adaptors/range.common.view/nodiscard.verify.cpp b/libcxx/test/libcxx/ranges/range.adaptors/range.common.view/nodiscard.verify.cpp
new file mode 100644
index 0000000000000..3542251b7d436
--- /dev/null
+++ b/libcxx/test/libcxx/ranges/range.adaptors/range.common.view/nodiscard.verify.cpp
@@ -0,0 +1,61 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: std-at-least-c++20
+
+// Check that functions are marked [[nodiscard]]
+
+#include <ranges>
+#include <utility>
+
+struct NonCommonView : std::ranges::view_base {
+  int* begin() const;
+  const int* end() const;
+
+  int* base();
+
+  int* begin();
+  const int* end();
+
+  constexpr std::size_t size() const { return 0; };
+};
+static_assert(!std::ranges::common_range<NonCommonView>);
+
+void test() {
+  NonCommonView range;
+
+  auto v = std::views::common(range);
+
+  // [range.common.view]
+
+  // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::move(v).base();
+
+  // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  v.begin();
+  // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::as_const(v).begin();
+
+  // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  v.end();
+  // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::as_const(v).end();
+
+  // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  v.size();
+  // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::as_const(v).size();
+
+  // [range.common.overview]
+
+  // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::views::common(v);
+
+  // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::views::common(range);
+}