[libcxx-commits] [libcxx] [libc+][ranges] Applied `[[nodiscard]]` to `drop_view` (PR #173557)
via libcxx-commits
libcxx-commits at lists.llvm.org
Thu Dec 25 07:23:25 PST 2025
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-libcxx
Author: Hristo Hristov (H-G-Hristov)
<details>
<summary>Changes</summary>
`[[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.drop
Towards #<!-- -->172124
---
Full diff: https://github.com/llvm/llvm-project/pull/173557.diff
2 Files Affected:
- (modified) libcxx/include/__ranges/drop_view.h (+8-8)
- (added) libcxx/test/libcxx/ranges/range.adaptors/range.drop/nodiscard.verify.cpp (+144)
``````````diff
diff --git a/libcxx/include/__ranges/drop_view.h b/libcxx/include/__ranges/drop_view.h
index 42ada9299a779..feb3705d2df6c 100644
--- a/libcxx/include/__ranges/drop_view.h
+++ b/libcxx/include/__ranges/drop_view.h
@@ -80,14 +80,14 @@ class drop_view : public view_interface<drop_view<_View>> {
_LIBCPP_ASSERT_UNCATEGORIZED(__count_ >= 0, "count must be greater than or equal to zero.");
}
- _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 auto begin()
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto begin()
requires(!(__simple_view<_View> && random_access_range<const _View> && sized_range<const _View>))
{
if constexpr (random_access_range<_View> && sized_range<_View>) {
@@ -104,20 +104,20 @@ class drop_view : public view_interface<drop_view<_View>> {
return __tmp;
}
- _LIBCPP_HIDE_FROM_ABI constexpr auto begin() const
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto begin() const
requires random_access_range<const _View> && sized_range<const _View>
{
const auto __dist = std::min(ranges::distance(__base_), __count_);
return ranges::begin(__base_) + __dist;
}
- _LIBCPP_HIDE_FROM_ABI constexpr auto end()
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto end()
requires(!__simple_view<_View>)
{
return ranges::end(__base_);
}
- _LIBCPP_HIDE_FROM_ABI constexpr auto end() const
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto end() const
requires range<const _View>
{
return ranges::end(__base_);
@@ -129,13 +129,13 @@ class drop_view : public view_interface<drop_view<_View>> {
return __s < __c ? 0 : __s - __c;
}
- _LIBCPP_HIDE_FROM_ABI constexpr auto size()
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto size()
requires sized_range<_View>
{
return __size(*this);
}
- _LIBCPP_HIDE_FROM_ABI constexpr auto size() const
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto size() const
requires sized_range<const _View>
{
return __size(*this);
diff --git a/libcxx/test/libcxx/ranges/range.adaptors/range.drop/nodiscard.verify.cpp b/libcxx/test/libcxx/ranges/range.adaptors/range.drop/nodiscard.verify.cpp
new file mode 100644
index 0000000000000..2633c3b778c01
--- /dev/null
+++ b/libcxx/test/libcxx/ranges/range.adaptors/range.drop/nodiscard.verify.cpp
@@ -0,0 +1,144 @@
+//===----------------------------------------------------------------------===//
+//
+// 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 <span>
+#include <vector>
+#include <utility>
+
+#include "test_macros.h"
+#include "test_iterators.h"
+
+void test() {
+ // [range.drop.view]
+
+ {
+ std::vector<int> range;
+
+ auto v = std::views::drop(range, 0);
+
+ // 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.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.drop.overview]
+
+ { // The `empty_view` case.
+ auto emptyView = std::views::empty<int>;
+
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::views::drop(emptyView, 0);
+ }
+
+ { // The `span | basic_string_view | iota_view | subrange (StoreSize == false)` case.
+ int arr[]{94, 82, 49};
+ auto subrange = std::ranges::subrange(arr, arr + 3);
+ LIBCPP_STATIC_ASSERT(!decltype(subrange)::_StoreSize);
+
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::views::drop(subrange, 3);
+ }
+
+ { // The the `subrange (StoreSize == true)` case.
+ struct SizedViewWithUnsizedSentinel : std::ranges::view_base {
+ using iterator = random_access_iterator<int*>;
+ using sentinel = sentinel_wrapper<random_access_iterator<int*>>;
+
+ int* begin_ = nullptr;
+ int* end_ = nullptr;
+ constexpr SizedViewWithUnsizedSentinel(int* begin, int* end) : begin_(begin), end_(end) {}
+
+ constexpr auto begin() const { return iterator(begin_); }
+ constexpr auto end() const { return sentinel(iterator(end_)); }
+ constexpr std::size_t size() const { return end_ - begin_; }
+ };
+ static_assert(std::ranges::random_access_range<SizedViewWithUnsizedSentinel>);
+ static_assert(std::ranges::sized_range<SizedViewWithUnsizedSentinel>);
+ static_assert(
+ !std::sized_sentinel_for<SizedViewWithUnsizedSentinel::sentinel, SizedViewWithUnsizedSentinel::iterator>);
+ static_assert(std::ranges::view<SizedViewWithUnsizedSentinel>);
+
+ int arr[]{94, 82, 49};
+
+ using View = SizedViewWithUnsizedSentinel;
+ View view{arr, arr + 3};
+
+ using Subrange = std::ranges::subrange<View::iterator, View::sentinel, std::ranges::subrange_kind::sized>;
+ auto subrange = Subrange(view.begin(), view.end(), std::ranges::distance(view.begin(), view.end()));
+ LIBCPP_STATIC_ASSERT(decltype(subrange)::_StoreSize);
+
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::views::drop(subrange, 3);
+ }
+
+#if TEST_STD_VER >= 23
+ { // The `repeat_view` "_RawRange models sized_range" case.
+ auto repeatView = std::ranges::repeat_view<int, int>(1, 8);
+
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::views::drop(repeatView, 3);
+ }
+
+ { // The `repeat_view` "otherwise" case.
+ auto repeatView = std::ranges::repeat_view<int>(1);
+
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::views::drop(repeatView, 3);
+ }
+#endif
+
+ { // The "otherwise" case.
+ struct SizedView : std::ranges::view_base {
+ int* begin_ = nullptr;
+ int* end_ = nullptr;
+ constexpr SizedView(int* begin, int* end) : begin_(begin), end_(end) {}
+
+ constexpr auto begin() const { return forward_iterator<int*>(begin_); }
+ constexpr auto end() const { return sized_sentinel<forward_iterator<int*>>(forward_iterator<int*>(end_)); }
+ };
+ static_assert(std::ranges::forward_range<SizedView>);
+ static_assert(std::ranges::sized_range<SizedView>);
+ static_assert(std::ranges::view<SizedView>);
+
+ int arr[]{94, 82, 49};
+
+ SizedView view{arr, arr + 3};
+ auto subrange = std::ranges::subrange(view.begin(), view.end());
+
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::views::drop(subrange, 3);
+ }
+
+ {
+ struct X {};
+
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::views::drop(X{});
+ }
+}
``````````
</details>
https://github.com/llvm/llvm-project/pull/173557
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