[libc++][ranges] Applied [[nodiscard]] to drop_while_view

[H-G-Hristov]

[[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.drop.while

Towards #172124

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: Hristo Hristov (H-G-Hristov)

Changes

[[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.drop.while

Towards #172124

---

Full diff: https://github.com/llvm/llvm-project/pull/173211.diff

2 Files Affected:

• (modified) libcxx/include/__ranges/drop_while_view.h (+5-5)
• (added) libcxx/test/libcxx/ranges/range.adaptors/range.drop.while/nodiscard.verify.cpp (+46)

diff --git a/libcxx/include/__ranges/drop_while_view.h b/libcxx/include/__ranges/drop_while_view.h index bc7f019393a80..1fe4e17f8048b 100644 --- a/libcxx/include/__ranges/drop_while_view.h +++ b/libcxx/include/__ranges/drop_while_view.h @@ -57,17 +57,17 @@ class _LIBCPP_ABI_LLVM18_NO_UNIQUE_ADDRESS drop_while_view : public view_interfa _LIBCPP_HIDE_FROM_ABI constexpr _LIBCPP_EXPLICIT_SINCE_CXX23 drop_while_view(_View __base, _Pred __pred) : __base_(std::move(__base)), __pred_(std::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 auto begin() { + [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto begin() { // Note: this duplicates a check in `optional` but provides a better error message. _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS( __pred_.__has_value(), @@ -83,7 +83,7 @@ class _LIBCPP_ABI_LLVM18_NO_UNIQUE_ADDRESS drop_while_view : public view_interfa } } - _LIBCPP_HIDE_FROM_ABI constexpr auto end() { return ranges::end(__base_); } + [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto end() { return ranges::end(__base_); } private: _LIBCPP_NO_UNIQUE_ADDRESS _View __base_ = _View(); diff --git a/libcxx/test/libcxx/ranges/range.adaptors/range.drop.while/nodiscard.verify.cpp b/libcxx/test/libcxx/ranges/range.adaptors/range.drop.while/nodiscard.verify.cpp new file mode 100644 index 0000000000000..039e3ddffa312 --- /dev/null +++ b/libcxx/test/libcxx/ranges/range.adaptors/range.drop.while/nodiscard.verify.cpp @@ -0,0 +1,46 @@ +//===----------------------------------------------------------------------===// +// +// 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 <vector> +#include <utility> + +void test() { + std::vector<int> range; + auto pred = [](int) { return true; }; + + auto v = std::views::drop_while(range, pred); + + // [range.drop.while.view] + + // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}} + v.base(); + // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}} + std::move(v).base(); + + // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}} + v.pred(); + + // 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}} + v.end(); + + // [range.drop.while.overview] + + // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}} + std::views::drop_while(range, pred); + + // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}} + std::views::drop_while(range); +}