[libcxx-commits] [libcxx] WIP [libc++][ranges] Applied [[nodiscard]] to `transform_view` (PR #204014)
via libcxx-commits
libcxx-commits at lists.llvm.org
Wed Jun 17 13:46:48 PDT 2026
https://github.com/lknknm updated https://github.com/llvm/llvm-project/pull/204014
>From e489902dd06c2ce3ce93b96c920c95d9ec8a039a Mon Sep 17 00:00:00 2001
From: Lucas Mellone <sxswt at protonmail.com>
Date: Tue, 16 Jun 2026 00:40:45 +0200
Subject: [PATCH 1/5] apply: nodiscard to transform_view.h
---
libcxx/include/__ranges/transform_view.h | 60 ++++++++++++++----------
1 file changed, 36 insertions(+), 24 deletions(-)
diff --git a/libcxx/include/__ranges/transform_view.h b/libcxx/include/__ranges/transform_view.h
index 98b38dd9b2aa1..7b4d5510376b2 100644
--- a/libcxx/include/__ranges/transform_view.h
+++ b/libcxx/include/__ranges/transform_view.h
@@ -88,43 +88,53 @@ class _LIBCPP_ABI_LLVM18_NO_UNIQUE_ADDRESS transform_view : public view_interfac
_LIBCPP_HIDE_FROM_ABI constexpr _LIBCPP_EXPLICIT_SINCE_CXX23 transform_view(_View __base, _Fn __func)
: __func_(std::in_place, std::move(__func)), __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_); }
- _LIBCPP_HIDE_FROM_ABI constexpr __iterator<false> begin() { return __iterator<false>{*this, ranges::begin(__base_)}; }
- _LIBCPP_HIDE_FROM_ABI constexpr __iterator<true> begin() const
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _View base() && { return std::move(__base_); }
+
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr __iterator<false> begin() {
+ return __iterator<false>{*this, ranges::begin(__base_)};
+ }
+
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr __iterator<true> begin() const
requires range<const _View> && __regular_invocable_with_range_ref<const _Fn&, const _View>
{
return __iterator<true>(*this, ranges::begin(__base_));
}
- _LIBCPP_HIDE_FROM_ABI constexpr __sentinel<false> end() { return __sentinel<false>(ranges::end(__base_)); }
- _LIBCPP_HIDE_FROM_ABI constexpr __iterator<false> end()
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr __sentinel<false> end() {
+ return __sentinel<false>(ranges::end(__base_));
+ }
+
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr __iterator<false> end()
requires common_range<_View>
{
return __iterator<false>(*this, ranges::end(__base_));
}
- _LIBCPP_HIDE_FROM_ABI constexpr __sentinel<true> end() const
+
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr __sentinel<true> end() const
requires range<const _View> && __regular_invocable_with_range_ref<const _Fn&, const _View>
{
return __sentinel<true>(ranges::end(__base_));
}
- _LIBCPP_HIDE_FROM_ABI constexpr __iterator<true> end() const
+
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr __iterator<true> end() const
requires common_range<const _View> && __regular_invocable_with_range_ref<const _Fn&, const _View>
{
return __iterator<true>(*this, 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_);
@@ -209,11 +219,11 @@ class transform_view<_View, _Fn>::__iterator
requires _Const && convertible_to<iterator_t<_View>, iterator_t<_Base>>
: __parent_(__i.__parent_), __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
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr decltype(auto) operator*() const
noexcept(noexcept(std::invoke(*__parent_->__func_, *__current_))) {
return std::invoke(*__parent_->__func_, *__current_);
}
@@ -248,21 +258,21 @@ class transform_view<_View, _Fn>::__iterator
return __tmp;
}
- _LIBCPP_HIDE_FROM_ABI constexpr __iterator& operator+=(difference_type __n)
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr __iterator& operator+=(difference_type __n)
requires random_access_range<_Base>
{
__current_ += __n;
return *this;
}
- _LIBCPP_HIDE_FROM_ABI constexpr __iterator& operator-=(difference_type __n)
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr __iterator& operator-=(difference_type __n)
requires random_access_range<_Base>
{
__current_ -= __n;
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
noexcept(noexcept(std::invoke(*__parent_->__func_, __current_[__n])))
requires random_access_range<_Base>
{
@@ -305,25 +315,26 @@ class transform_view<_View, _Fn>::__iterator
return __x.__current_ <=> __y.__current_;
}
- _LIBCPP_HIDE_FROM_ABI friend constexpr __iterator operator+(__iterator __i, difference_type __n)
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI friend constexpr __iterator operator+(__iterator __i, difference_type __n)
requires random_access_range<_Base>
{
return __iterator{*__i.__parent_, __i.__current_ + __n};
}
- _LIBCPP_HIDE_FROM_ABI friend constexpr __iterator operator+(difference_type __n, __iterator __i)
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI friend constexpr __iterator operator+(difference_type __n, __iterator __i)
requires random_access_range<_Base>
{
return __iterator{*__i.__parent_, __i.__current_ + __n};
}
- _LIBCPP_HIDE_FROM_ABI friend constexpr __iterator operator-(__iterator __i, difference_type __n)
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI friend constexpr __iterator operator-(__iterator __i, difference_type __n)
requires random_access_range<_Base>
{
return __iterator{*__i.__parent_, __i.__current_ - __n};
}
- _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_;
@@ -361,24 +372,25 @@ class transform_view<_View, _Fn>::__sentinel {
requires _Const && convertible_to<sentinel_t<_View>, sentinel_t<_Base>>
: __end_(std::move(__i.__end_)) {}
- _LIBCPP_HIDE_FROM_ABI constexpr sentinel_t<_Base> base() const { return __end_; }
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr sentinel_t<_Base> base() const { return __end_; }
template <bool _OtherConst>
requires sentinel_for<sentinel_t<_Base>, iterator_t<__maybe_const<_OtherConst, _View>>>
- _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(const __iterator<_OtherConst>& __x, const __sentinel& __y) {
+ [[nodiscard]] _LIBCPP_HIDE_FROM_ABI friend constexpr bool
+ operator==(const __iterator<_OtherConst>& __x, const __sentinel& __y) {
return __x.__current_ == __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 __iterator<_OtherConst>& __x, const __sentinel& __y) {
return __x.__current_ - __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_ - __y.__current_;
}
>From 2c54d6695b7e670e65481b6969c752bd931f9e67 Mon Sep 17 00:00:00 2001
From: Lucas Mellone <sxswt at protonmail.com>
Date: Tue, 16 Jun 2026 22:06:37 +0200
Subject: [PATCH 2/5] fix: remove [[nodiscard]] from operator+= and operator-=
to avoid false positives
---
libcxx/include/__ranges/transform_view.h | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/libcxx/include/__ranges/transform_view.h b/libcxx/include/__ranges/transform_view.h
index 7b4d5510376b2..380346c1979c6 100644
--- a/libcxx/include/__ranges/transform_view.h
+++ b/libcxx/include/__ranges/transform_view.h
@@ -258,14 +258,14 @@ class transform_view<_View, _Fn>::__iterator
return __tmp;
}
- [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr __iterator& operator+=(difference_type __n)
+ _LIBCPP_HIDE_FROM_ABI constexpr __iterator& operator+=(difference_type __n)
requires random_access_range<_Base>
{
__current_ += __n;
return *this;
}
- [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr __iterator& operator-=(difference_type __n)
+ _LIBCPP_HIDE_FROM_ABI constexpr __iterator& operator-=(difference_type __n)
requires random_access_range<_Base>
{
__current_ -= __n;
>From 7611d0c56f5d8fe8fb7de2a11d46615c60097fe9 Mon Sep 17 00:00:00 2001
From: Lucas Mellone <lucas at zuru.com>
Date: Wed, 17 Jun 2026 15:00:47 +0200
Subject: [PATCH 3/5] fix: remove [[nodiscard]] at it operator==
---
libcxx/include/__ranges/transform_view.h | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)
diff --git a/libcxx/include/__ranges/transform_view.h b/libcxx/include/__ranges/transform_view.h
index 380346c1979c6..25254cc684bd9 100644
--- a/libcxx/include/__ranges/transform_view.h
+++ b/libcxx/include/__ranges/transform_view.h
@@ -376,8 +376,7 @@ class transform_view<_View, _Fn>::__sentinel {
template <bool _OtherConst>
requires sentinel_for<sentinel_t<_Base>, iterator_t<__maybe_const<_OtherConst, _View>>>
- [[nodiscard]] _LIBCPP_HIDE_FROM_ABI friend constexpr bool
- operator==(const __iterator<_OtherConst>& __x, const __sentinel& __y) {
+ _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(const __iterator<_OtherConst>& __x, const __sentinel& __y) {
return __x.__current_ == __y.__end_;
}
>From 6b9f7268e8e1ce9636faf835a18c280078800c29 Mon Sep 17 00:00:00 2001
From: Lucas Mellone <lucas at zuru.com>
Date: Wed, 17 Jun 2026 17:31:19 +0200
Subject: [PATCH 4/5] rename: adaptor.nodiscard.verify.cpp to
nodiscard.verify.cpp
---
.../{adaptor.nodiscard.verify.cpp => nodiscard.verify.cpp} | 0
1 file changed, 0 insertions(+), 0 deletions(-)
rename libcxx/test/libcxx/ranges/range.adaptors/range.transform/{adaptor.nodiscard.verify.cpp => nodiscard.verify.cpp} (100%)
diff --git a/libcxx/test/libcxx/ranges/range.adaptors/range.transform/adaptor.nodiscard.verify.cpp b/libcxx/test/libcxx/ranges/range.adaptors/range.transform/nodiscard.verify.cpp
similarity index 100%
rename from libcxx/test/libcxx/ranges/range.adaptors/range.transform/adaptor.nodiscard.verify.cpp
rename to libcxx/test/libcxx/ranges/range.adaptors/range.transform/nodiscard.verify.cpp
>From 1e9875bda87483c2f3c2e9daa447dc84b57d1a56 Mon Sep 17 00:00:00 2001
From: Lucas Mellone <sxswt at protonmail.com>
Date: Wed, 17 Jun 2026 22:46:31 +0200
Subject: [PATCH 5/5] WIP: format existing tests with the addition of the
TestView
---
.../range.transform/nodiscard.verify.cpp | 30 +++++++++++++++----
1 file changed, 25 insertions(+), 5 deletions(-)
diff --git a/libcxx/test/libcxx/ranges/range.adaptors/range.transform/nodiscard.verify.cpp b/libcxx/test/libcxx/ranges/range.adaptors/range.transform/nodiscard.verify.cpp
index 0a5fbf72a83ee..b41206cf55b7d 100644
--- a/libcxx/test/libcxx/ranges/range.adaptors/range.transform/nodiscard.verify.cpp
+++ b/libcxx/test/libcxx/ranges/range.adaptors/range.transform/nodiscard.verify.cpp
@@ -11,13 +11,33 @@
// Test the libc++ extension that std::views::transform is marked as [[nodiscard]].
#include <ranges>
+#include <functional>
+
+struct TestView : std::ranges::view_interface<TestView> {
+ int* begin();
+ char* begin() const;
+ const int* end();
+ const char* end() const;
+};
void test() {
int range[] = {1, 2, 3};
- auto f = [](int i) { return i; };
+ auto f = [](int i) { return i; };
+
+ auto identity_view = TestView{} | std::views::transform(std::identity{});
+
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ identity_view.begin();
+
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::views::transform(f);
+
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::views::transform(range, f);
+
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ range | std::views::transform(f);
- std::views::transform(f); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
- std::views::transform(range, f); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
- range | std::views::transform(f); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
- std::views::all | std::views::transform(f); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+ // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+ std::views::all | std::views::transform(f);
}
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