[libcxx-commits] [libcxx] WIP [libc++][ranges] Applied [[nodiscard]] to `elements_view` (PR #206589)
Lucas Mellone via libcxx-commits
libcxx-commits at lists.llvm.org
Tue Jun 30 10:48:26 PDT 2026
https://github.com/lknknm updated https://github.com/llvm/llvm-project/pull/206589
>From fde2218d5a8d799e9c2806a51ddd9758c2efd13e Mon Sep 17 00:00:00 2001
From: Lucas Mellone <github.snugness349 at passinbox.com>
Date: Mon, 29 Jun 2026 23:30:56 +0200
Subject: [PATCH 1/3] add: nodiscard to elements_view
---
libcxx/include/__ranges/elements_view.h | 40 ++++++++++++-------------
1 file changed, 20 insertions(+), 20 deletions(-)
diff --git a/libcxx/include/__ranges/elements_view.h b/libcxx/include/__ranges/elements_view.h
index b1419f2a1dd91..fc8c46b29b57f 100644
--- a/libcxx/include/__ranges/elements_view.h
+++ b/libcxx/include/__ranges/elements_view.h
@@ -77,57 +77,57 @@ class elements_view : public view_interface<elements_view<_View, _Np>> {
_LIBCPP_HIDE_FROM_ABI constexpr explicit elements_view(_View __base) : __base_(std::move(__base)) {}
- _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>)
{
return __iterator</*_Const=*/false>(ranges::begin(__base_));
}
- _LIBCPP_HIDE_FROM_ABI constexpr auto begin() const
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto begin() const
requires range<const _View>
{
return __iterator</*_Const=*/true>(ranges::begin(__base_));
}
- _LIBCPP_HIDE_FROM_ABI constexpr auto end()
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto end()
requires(!__simple_view<_View> && !common_range<_View>)
{
return __sentinel</*_Const=*/false>{ranges::end(__base_)};
}
- _LIBCPP_HIDE_FROM_ABI constexpr auto end()
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto end()
requires(!__simple_view<_View> && common_range<_View>)
{
return __iterator</*_Const=*/false>{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 __sentinel</*_Const=*/true>{ranges::end(__base_)};
}
- _LIBCPP_HIDE_FROM_ABI constexpr auto end() const
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto end() const
requires common_range<const _View>
{
return __iterator</*_Const=*/true>{ranges::end(__base_)};
}
- _LIBCPP_HIDE_FROM_ABI constexpr auto size()
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto size()
requires sized_range<_View>
{
return ranges::size(__base_);
}
- _LIBCPP_HIDE_FROM_ABI constexpr auto size() const
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto size() const
requires sized_range<const _View>
{
return ranges::size(__base_);
@@ -211,11 +211,11 @@ class elements_view<_View, _Np>::__iterator
requires _Const && convertible_to<iterator_t<_View>, iterator_t<_Base>>
: __current_(std::move(__i.__current_)) {}
- _LIBCPP_HIDE_FROM_ABI constexpr const iterator_t<_Base>& base() const& noexcept { return __current_; }
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr const iterator_t<_Base>& base() const& noexcept { return __current_; }
- _LIBCPP_HIDE_FROM_ABI constexpr iterator_t<_Base> base() && { return std::move(__current_); }
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr iterator_t<_Base> base() && { return std::move(__current_); }
- _LIBCPP_HIDE_FROM_ABI constexpr decltype(auto) operator*() const { return __get_element(__current_); }
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr decltype(auto) operator*() const { return __get_element(__current_); }
_LIBCPP_HIDE_FROM_ABI constexpr __iterator& operator++() {
++__current_;
@@ -261,7 +261,7 @@ class elements_view<_View, _Np>::__iterator
return *this;
}
- _LIBCPP_HIDE_FROM_ABI constexpr decltype(auto) operator[](difference_type __n) const
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr decltype(auto) operator[](difference_type __n) const
requires random_access_range<_Base>
{
return __get_element(__current_ + __n);
@@ -303,25 +303,25 @@ class elements_view<_View, _Np>::__iterator
return __x.__current_ <=> __y.__current_;
}
- _LIBCPP_HIDE_FROM_ABI friend constexpr __iterator operator+(const __iterator& __x, difference_type __y)
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI friend constexpr __iterator operator+(const __iterator& __x, difference_type __y)
requires random_access_range<_Base>
{
return __iterator{__x} += __y;
}
- _LIBCPP_HIDE_FROM_ABI friend constexpr __iterator operator+(difference_type __x, const __iterator& __y)
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI friend constexpr __iterator operator+(difference_type __x, const __iterator& __y)
requires random_access_range<_Base>
{
return __y + __x;
}
- _LIBCPP_HIDE_FROM_ABI friend constexpr __iterator operator-(const __iterator& __x, difference_type __y)
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI friend constexpr __iterator operator-(const __iterator& __x, difference_type __y)
requires random_access_range<_Base>
{
return __iterator{__x} -= __y;
}
- _LIBCPP_HIDE_FROM_ABI friend constexpr difference_type operator-(const __iterator& __x, const __iterator& __y)
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI friend constexpr difference_type operator-(const __iterator& __x, const __iterator& __y)
requires sized_sentinel_for<iterator_t<_Base>, iterator_t<_Base>>
{
return __x.__current_ - __y.__current_;
@@ -365,14 +365,14 @@ class elements_view<_View, _Np>::__sentinel {
template <bool _OtherConst>
requires sized_sentinel_for<sentinel_t<_Base>, iterator_t<__maybe_const<_OtherConst, _View>>>
- _LIBCPP_HIDE_FROM_ABI friend constexpr range_difference_t<__maybe_const<_OtherConst, _View>>
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI friend constexpr range_difference_t<__maybe_const<_OtherConst, _View>>
operator-(const __iterator<_OtherConst>& __x, const __sentinel& __y) {
return __get_current(__x) - __y.__end_;
}
template <bool _OtherConst>
requires sized_sentinel_for<sentinel_t<_Base>, iterator_t<__maybe_const<_OtherConst, _View>>>
- _LIBCPP_HIDE_FROM_ABI friend constexpr range_difference_t<__maybe_const<_OtherConst, _View>>
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI friend constexpr range_difference_t<__maybe_const<_OtherConst, _View>>
operator-(const __sentinel& __x, const __iterator<_OtherConst>& __y) {
return __x.__end_ - __get_current(__y);
}
>From 4c961f7efb3efb1985259cd16b13fae3c23ef871 Mon Sep 17 00:00:00 2001
From: Lucas Mellone <github.snugness349 at passinbox.com>
Date: Mon, 29 Jun 2026 23:36:42 +0200
Subject: [PATCH 2/3] fix: clang format
---
libcxx/include/__ranges/elements_view.h | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/libcxx/include/__ranges/elements_view.h b/libcxx/include/__ranges/elements_view.h
index fc8c46b29b57f..33c152bbd03b4 100644
--- a/libcxx/include/__ranges/elements_view.h
+++ b/libcxx/include/__ranges/elements_view.h
@@ -321,7 +321,8 @@ class elements_view<_View, _Np>::__iterator
return __iterator{__x} -= __y;
}
- [[nodiscard]] _LIBCPP_HIDE_FROM_ABI friend constexpr difference_type operator-(const __iterator& __x, const __iterator& __y)
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI friend constexpr difference_type
+ operator-(const __iterator& __x, const __iterator& __y)
requires sized_sentinel_for<iterator_t<_Base>, iterator_t<_Base>>
{
return __x.__current_ - __y.__current_;
>From 8c1bb7ca3ce494344d17e1f41fd8317efe3809da Mon Sep 17 00:00:00 2001
From: Lucas Mellone <github.snugness349 at passinbox.com>
Date: Tue, 30 Jun 2026 12:46:41 +0200
Subject: [PATCH 3/3] add: range.elements.overview as per the 25.7.23.1.2.1
example in the Standard
---
.../range.elements/nodiscard.verify.cpp | 44 +++++++++++++++++++
1 file changed, 44 insertions(+)
create mode 100644 libcxx/test/libcxx/ranges/range.adaptors/range.elements/nodiscard.verify.cpp
diff --git a/libcxx/test/libcxx/ranges/range.adaptors/range.elements/nodiscard.verify.cpp b/libcxx/test/libcxx/ranges/range.adaptors/range.elements/nodiscard.verify.cpp
new file mode 100644
index 0000000000000..16759e6a53569
--- /dev/null
+++ b/libcxx/test/libcxx/ranges/range.adaptors/range.elements/nodiscard.verify.cpp
@@ -0,0 +1,44 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+
+// Test the libc++ extension that std::ranges::transform_view and std::views::transform are marked as [[nodiscard]].
+
+#include <ranges>
+#include <utility>
+#include <functional>
+#include <map>
+
+struct View : std::ranges::view_interface<View> {
+ int* begin();
+ const int* begin() const;
+ volatile int* end();
+ const volatile int* end() const;
+};
+static_assert(!std::ranges::common_range<View>);
+static_assert(!std::same_as<std::ranges::iterator_t<View>, std::ranges::iterator_t<const View>>);
+static_assert(!std::same_as<std::ranges::sentinel_t<View>, std::ranges::sentinel_t<const View>>);
+
+void test() {
+ auto historical_figures = std::map{
+ std::pair{"Lovelace"sv, 1815},
+ {"Turing"sv, 1912},
+ {"Babbage"sv, 1791},
+ {"Hamilton"sv, 1936}
+ };
+
+ // [range.elements.overview]
+
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::views::elements<0>(historical_figures);
+
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::views::elements<1>(historical_figures);
+}
+
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