[libcxx-commits] [libcxx] [libc++][ranges] Applied `[[nodiscard]]` to `chunk_by_view ` (PR #173178)

[Hristo Hristov via libcxx-commits]

https://github.com/H-G-Hristov created https://github.com/llvm/llvm-project/pull/173178

`[[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.chunk.by

Towards #172124

>From 94a592da37da36ddae527564bf19c136d2277f2e Mon Sep 17 00:00:00 2001
From: Hristo Hristov <hghristov.rmm at gmail.com>
Date: Sun, 21 Dec 2025 09:01:19 +0200
Subject: [PATCH] [libc++][ranges] Applied `[[nodiscard]]` to `chunk_by_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.chunk.by

Towards #172124
---
 libcxx/include/__ranges/chunk_by_view.h       | 12 ++---
 .../diagnostics/ranges.nodiscard.verify.cpp   |  3 --
 .../range.chunk.by/nodiscard.verify.cpp       | 53 +++++++++++++++++++
 3 files changed, 59 insertions(+), 9 deletions(-)
 create mode 100644 libcxx/test/libcxx/ranges/range.adaptors/range.chunk.by/nodiscard.verify.cpp

diff --git a/libcxx/include/__ranges/chunk_by_view.h b/libcxx/include/__ranges/chunk_by_view.h
index 71fee3a4f2d1e..8007f76f0c1e6 100644
--- a/libcxx/include/__ranges/chunk_by_view.h
+++ b/libcxx/include/__ranges/chunk_by_view.h
@@ -100,17 +100,17 @@ class _LIBCPP_ABI_LLVM18_NO_UNIQUE_ADDRESS chunk_by_view : public view_interface
   _LIBCPP_HIDE_FROM_ABI constexpr explicit chunk_by_view(_View __base, _Pred __pred)
       : __base_(std::move(__base)), __pred_(in_place, std::move(__pred)) {}
 
-  _LIBCPP_HIDE_FROM_ABI constexpr _View base() const&
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _View base() const&
     requires copy_constructible<_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 const _Pred& pred() const { return *__pred_; }
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr const _Pred& pred() const { return *__pred_; }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr __iterator begin() {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr __iterator begin() {
     // Note: this duplicates a check in `optional` but provides a better error message.
     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
         __pred_.__has_value(), "Trying to call begin() on a chunk_by_view that does not have a valid predicate.");
@@ -122,7 +122,7 @@ class _LIBCPP_ABI_LLVM18_NO_UNIQUE_ADDRESS chunk_by_view : public view_interface
     return {*this, std::move(__first), *__cached_begin_};
   }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr auto end() {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto end() {
     if constexpr (common_range<_View>) {
       return __iterator{*this, ranges::end(__base_), ranges::end(__base_)};
     } else {
@@ -155,7 +155,7 @@ class chunk_by_view<_View, _Pred>::__iterator {
 
   _LIBCPP_HIDE_FROM_ABI __iterator() = default;
 
-  _LIBCPP_HIDE_FROM_ABI constexpr value_type operator*() const {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr value_type operator*() const {
     // If the iterator is at end, this would return an empty range which can be checked by the calling code and doesn't
     // necessarily lead to a bad access.
     _LIBCPP_ASSERT_PEDANTIC(__current_ != __next_, "Trying to dereference past-the-end chunk_by_view iterator.");
diff --git a/libcxx/test/libcxx/diagnostics/ranges.nodiscard.verify.cpp b/libcxx/test/libcxx/diagnostics/ranges.nodiscard.verify.cpp
index 03b4df735edb5..ecdad4ae9ec64 100644
--- a/libcxx/test/libcxx/diagnostics/ranges.nodiscard.verify.cpp
+++ b/libcxx/test/libcxx/diagnostics/ranges.nodiscard.verify.cpp
@@ -40,9 +40,6 @@ void test() {
   std::views::as_rvalue(range);       // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
   std::views::as_rvalue(rvalue_view); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
 
-  std::views::chunk_by(pred);        // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
-  std::views::chunk_by(range, pred); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
-
   std::views::take(std::views::repeat(3), 3);                            // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
   std::views::take(std::views::repeat(3, std::unreachable_sentinel), 3); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
 
diff --git a/libcxx/test/libcxx/ranges/range.adaptors/range.chunk.by/nodiscard.verify.cpp b/libcxx/test/libcxx/ranges/range.adaptors/range.chunk.by/nodiscard.verify.cpp
new file mode 100644
index 0000000000000..bdd25b2c7a793
--- /dev/null
+++ b/libcxx/test/libcxx/ranges/range.adaptors/range.chunk.by/nodiscard.verify.cpp
@@ -0,0 +1,53 @@
+//===----------------------------------------------------------------------===//
+//
+// 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++23
+
+// Check that functions are marked [[nodiscard]]
+
+#include <ranges>
+#include <vector>
+#include <utility>
+
+void test() {
+  std::vector<int> range;
+  std::ranges::less_equal pred;
+
+  auto v = std::views::chunk_by(range, pred);
+
+  // [range.chunk.by.view]
+
+  // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  v.base();
+  // 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.pred();
+
+  // 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}}
+  v.end();
+
+  // [range.chunk.by.iter]
+
+  auto it = v.begin();
+
+  // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  *it;
+
+  // [range.chunk.by.overview]
+
+  // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::views::chunk_by(range, pred);
+
+  // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::views::chunk_by(range);
+}