[libc++][ranges] LWG3564: transform_view::iterator<true>::value_type and iterator_category should use const F&

[xiaoyang-sde]

Introduction

This patch implements LWG3564: transform_view::iterator<true>::value_type and iterator_category should use const F&.

transform_view's iterator currently obtained from a const transform_view invoke the transformation function as const, but the value_type and iterator_category determination uses non-const F&.

Reference

• [range.transform.iterator]
• LWG3564

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: Xiaoyang Liu (xiaoyang-sde)

Changes

Introduction

This patch implements LWG3564: transform_view::iterator<true>::value_type and iterator_category should use const F&.

transform_view's iterator currently obtained from a const transform_view invoke the transformation function as const, but the value_type and iterator_category determination uses non-const F&.

Reference

• [range.transform.iterator]
• LWG3564

---

Full diff: https://github.com/llvm/llvm-project/pull/91816.diff

4 Files Affected:

• (modified) libcxx/docs/Status/Cxx23Issues.csv (+1-1)
• (modified) libcxx/include/__ranges/transform_view.h (+6-6)
• (modified) libcxx/test/std/ranges/range.adaptors/range.transform/iterator/types.pass.cpp (+15)
• (modified) libcxx/test/std/ranges/range.adaptors/range.transform/types.h (+5)

diff --git a/libcxx/docs/Status/Cxx23Issues.csv b/libcxx/docs/Status/Cxx23Issues.csv
index 04f6e23375acd..fef60d289f86f 100644
--- a/libcxx/docs/Status/Cxx23Issues.csv
+++ b/libcxx/docs/Status/Cxx23Issues.csv
@@ -159,7 +159,7 @@
 "`3660 <https://wg21.link/LWG3660>`__","``iterator_traits<common_iterator>::pointer`` should conform to §[iterator.traits]","February 2022","|Complete|","14.0","|ranges|"
 "`3661 <https://wg21.link/LWG3661>`__","``constinit atomic<shared_ptr<T>> a(nullptr);`` should work","February 2022","",""
 "","","","","",""
-"`3564 <https://wg21.link/LWG3564>`__","``transform_view::iterator<true>::value_type`` and ``iterator_category`` should use ``const F&``","July 2022","","","|ranges|"
+"`3564 <https://wg21.link/LWG3564>`__","``transform_view::iterator<true>::value_type`` and ``iterator_category`` should use ``const F&``","July 2022","|Complete|","19.0","|ranges|"
 "`3617 <https://wg21.link/LWG3617>`__","``function``/``packaged_task`` deduction guides and deducing ``this``","July 2022","",""
 "`3656 <https://wg21.link/LWG3656>`__","Inconsistent bit operations returning a count","July 2022","|Complete|","15.0",""
 "`3659 <https://wg21.link/LWG3659>`__","Consider ``ATOMIC_FLAG_INIT`` undeprecation","July 2022","|Complete|","15.0"
diff --git a/libcxx/include/__ranges/transform_view.h b/libcxx/include/__ranges/transform_view.h
index dc3aaa59ed8c3..af973076e04f8 100644
--- a/libcxx/include/__ranges/transform_view.h
+++ b/libcxx/include/__ranges/transform_view.h
@@ -153,15 +153,15 @@ struct __transform_view_iterator_concept<_View> {
   using type = forward_iterator_tag;
 };
 
-template <class, class>
+template <class, class, bool>
 struct __transform_view_iterator_category_base {};
 
-template <forward_range _View, class _Fn>
-struct __transform_view_iterator_category_base<_View, _Fn> {
+template <forward_range _View, class _Fn, bool _Const>
+struct __transform_view_iterator_category_base<_View, _Fn, _Const> {
   using _Cat = typename iterator_traits<iterator_t<_View>>::iterator_category;
 
   using iterator_category =
-      conditional_t< is_reference_v<invoke_result_t<_Fn&, range_reference_t<_View>>>,
+      conditional_t< is_reference_v<invoke_result_t<__maybe_const<_Const, _Fn>&, range_reference_t<_View>>>,
                      conditional_t< derived_from<_Cat, contiguous_iterator_tag>, random_access_iterator_tag, _Cat >,
                      input_iterator_tag >;
 };
@@ -173,7 +173,7 @@ template <input_range _View, copy_constructible _Fn>
 #  endif
   requires __transform_view_constraints<_View, _Fn>
 template <bool _Const>
-class transform_view<_View, _Fn>::__iterator : public __transform_view_iterator_category_base<_View, _Fn> {
+class transform_view<_View, _Fn>::__iterator : public __transform_view_iterator_category_base<_View, _Fn, _Const> {
 
   using _Parent = __maybe_const<_Const, transform_view>;
   using _Base   = __maybe_const<_Const, _View>;
@@ -190,7 +190,7 @@ class transform_view<_View, _Fn>::__iterator : public __transform_view_iterator_
   iterator_t<_Base> __current_ = iterator_t<_Base>();
 
   using iterator_concept = typename __transform_view_iterator_concept<_View>::type;
-  using value_type       = remove_cvref_t<invoke_result_t<_Fn&, range_reference_t<_Base>>>;
+  using value_type       = remove_cvref_t<invoke_result_t<__maybe_const<_Const, _Fn>&, range_reference_t<_Base>>>;
   using difference_type  = range_difference_t<_Base>;
 
   _LIBCPP_HIDE_FROM_ABI __iterator()
diff --git a/libcxx/test/std/ranges/range.adaptors/range.transform/iterator/types.pass.cpp b/libcxx/test/std/ranges/range.adaptors/range.transform/iterator/types.pass.cpp
index 25d8936f4e4eb..17340467f856b 100644
--- a/libcxx/test/std/ranges/range.adaptors/range.transform/iterator/types.pass.cpp
+++ b/libcxx/test/std/ranges/range.adaptors/range.transform/iterator/types.pass.cpp
@@ -61,6 +61,21 @@ constexpr bool test() {
     static_assert(std::same_as<typename TIter::value_type, int>);
     static_assert(std::same_as<typename TIter::difference_type, std::ptrdiff_t>);
   }
+  {
+    // Member typedefs for random access iterator, LWG3564 mutable overload
+    using TView = std::ranges::transform_view<RandomAccessView, PlusOneMutableOverload>;
+    using TIter = std::ranges::iterator_t<TView>;
+    static_assert(std::same_as<typename TIter::iterator_concept, std::random_access_iterator_tag>);
+    static_assert(std::same_as<typename TIter::iterator_category, std::random_access_iterator_tag>);
+    static_assert(std::same_as<typename TIter::value_type, int>);
+    static_assert(std::same_as<typename TIter::difference_type, std::ptrdiff_t>);
+
+    using CTIter = std::ranges::iterator_t<const TView>;
+    static_assert(std::same_as<typename CTIter::iterator_concept, std::random_access_iterator_tag>);
+    static_assert(std::same_as<typename CTIter::iterator_category, std::input_iterator_tag>); // Note: this is now input_iterator_tag.
+    static_assert(std::same_as<typename CTIter::value_type, int>);
+    static_assert(std::same_as<typename CTIter::difference_type, std::ptrdiff_t>);
+  }
   {
     // Member typedefs for bidirectional iterator.
     using TView = std::ranges::transform_view<BidirectionalView, Increment>;
diff --git a/libcxx/test/std/ranges/range.adaptors/range.transform/types.h b/libcxx/test/std/ranges/range.adaptors/range.transform/types.h
index 14f85722a8c19..9268a977b8109 100644
--- a/libcxx/test/std/ranges/range.adaptors/range.transform/types.h
+++ b/libcxx/test/std/ranges/range.adaptors/range.transform/types.h
@@ -137,6 +137,11 @@ struct PlusOne {
   constexpr int operator()(int x) const { return x + 1; }
 };
 
+struct PlusOneMutableOverload {
+  constexpr int operator()(int x) const { return x + 1; }
+  constexpr int& operator()(int& x) { return ++x; }
+};
+
 struct Increment {
   constexpr int& operator()(int& x) { return ++x; }
 };

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[ldionne]

Thanks for the patch and sorry for the review delay. Let's move this to LLVM 20, rebase to get the CI running and merge once the CI is green. LGTM.

[xiaoyang-sde]

Thanks for the patch and sorry for the review delay. Let's move this to LLVM 20, rebase to get the CI running and merge once the CI is green. LGTM.

Feel free to merge this patch if it looks good!

[mordante]

Thanks for the patch and sorry for the review delay. Let's move this to LLVM 20, rebase to get the CI running and merge once the CI is green. LGTM.

Feel free to merge this patch if it looks good!

There is a new merge conflict. Can you rebase this patch?

[ldionne]

This can be merged once CI is green.

[xiaoyang-sde]

This can be merged once CI is green.

Thanks for resolving the merge conflict! Could you please restart the failed tests? It seems that these are false negatives.

[mordante]

This can be merged once CI is green.

Thanks for resolving the merge conflict! Could you please restart the failed tests? It seems that these are false negatives.

Done and merged. Thanks for your patch!