[libcxx-commits] [libcxx] [libc++][ranges] Applied `[[nodiscard]]` to `single_view` (PR #173709)

[Hristo Hristov via libcxx-commits]

https://github.com/H-G-Hristov created https://github.com/llvm/llvm-project/pull/173709

`[[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/range.single

Towards #172124

>From 5835c3438e18160f213181840cc791186743c575 Mon Sep 17 00:00:00 2001
From: Hristo Hristov <hghristov.rmm at gmail.com>
Date: Sat, 27 Dec 2025 08:43:34 +0200
Subject: [PATCH] [libc++][ranges]  Applied `[[nodiscard]]` to `single_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/range.single

Towards #172124
---
 libcxx/include/__ranges/single_view.h         | 16 +++----
 .../range.single.view/nodiscard.verify.cpp    | 48 +++++++++++++++++++
 2 files changed, 56 insertions(+), 8 deletions(-)
 create mode 100644 libcxx/test/libcxx/ranges/range.factories/range.single.view/nodiscard.verify.cpp

diff --git a/libcxx/include/__ranges/single_view.h b/libcxx/include/__ranges/single_view.h
index 955578b99cf58..213c507138e68 100644
--- a/libcxx/include/__ranges/single_view.h
+++ b/libcxx/include/__ranges/single_view.h
@@ -63,21 +63,21 @@ class _LIBCPP_ABI_LLVM18_NO_UNIQUE_ADDRESS single_view : public view_interface<s
   _LIBCPP_HIDE_FROM_ABI constexpr explicit single_view(in_place_t, _Args&&... __args)
       : __value_{in_place, std::forward<_Args>(__args)...} {}
 
-  _LIBCPP_HIDE_FROM_ABI constexpr _Tp* begin() noexcept { return data(); }
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Tp* begin() noexcept { return data(); }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr const _Tp* begin() const noexcept { return data(); }
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr const _Tp* begin() const noexcept { return data(); }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr _Tp* end() noexcept { return data() + 1; }
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Tp* end() noexcept { return data() + 1; }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr const _Tp* end() const noexcept { return data() + 1; }
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr const _Tp* end() const noexcept { return data() + 1; }
 
-  _LIBCPP_HIDE_FROM_ABI static constexpr bool empty() noexcept { return false; }
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI static constexpr bool empty() noexcept { return false; }
 
-  _LIBCPP_HIDE_FROM_ABI static constexpr size_t size() noexcept { return 1; }
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI static constexpr size_t size() noexcept { return 1; }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr _Tp* data() noexcept { return __value_.operator->(); }
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Tp* data() noexcept { return __value_.operator->(); }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr const _Tp* data() const noexcept { return __value_.operator->(); }
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr const _Tp* data() const noexcept { return __value_.operator->(); }
 };
 
 template <class _Tp>
diff --git a/libcxx/test/libcxx/ranges/range.factories/range.single.view/nodiscard.verify.cpp b/libcxx/test/libcxx/ranges/range.factories/range.single.view/nodiscard.verify.cpp
new file mode 100644
index 0000000000000..6f4be85ee27a6
--- /dev/null
+++ b/libcxx/test/libcxx/ranges/range.factories/range.single.view/nodiscard.verify.cpp
@@ -0,0 +1,48 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+
+// <ranges>
+
+// Check that functions are marked [[nodiscard]]
+
+#include <ranges>
+#include <utility>
+
+void test() {
+  auto v = std::views::single(94);
+
+  // [range.single.view]
+
+  // 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}}
+  v.empty();
+
+  // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  v.data();
+  // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::as_const(v).data();
+
+  // [range.single.overview]
+
+  // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::views::single(82);
+}