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

Towards #172124

@H-G-Hristov H-G-Hristov marked this pull request as ready for review December 27, 2025 08:16
@H-G-Hristov H-G-Hristov requested a review from a team as a code owner December 27, 2025 08:16
@llvmbot llvmbot added the libc++ libc++ C++ Standard Library. Not GNU libstdc++. Not libc++abi. label Dec 27, 2025
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llvmbot commented Dec 27, 2025

@llvm/pr-subscribers-libcxx

Author: Hristo Hristov (H-G-Hristov)

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

Towards #172124


Full diff: https://github.com/llvm/llvm-project/pull/173709.diff

2 Files Affected:

  • (modified) libcxx/include/__ranges/single_view.h (+8-8)
  • (added) libcxx/test/libcxx/ranges/range.factories/range.single.view/nodiscard.verify.cpp (+48)
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@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  v.begin();
+  // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::as_const(v).begin();
+
+  // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  v.end();
+  // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::as_const(v).end();
+
+  // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  v.size();
+
+  // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  v.empty();
+
+  // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  v.data();
+  // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::as_const(v).data();
+
+  // [range.single.overview]
+
+  // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::views::single(82);
+}

@Zingam Zingam added the ranges Issues related to `<ranges>` label Dec 27, 2025
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