[libc++] Partly addresses LWG3798.

[Mordante]

Summary

LWG3798 Rvalue reference and iterator_category

The changes are only applied to ranges::transform_view, the other
views haven't been implemented yet.

libcxx/docs/Status/Cxx2b.rst

 
 Library Working Group Issues Status
 ====================================
 
 .. csv-table::
    :file: Cxx2bIssues.csv
    :header-rows: 1
    :widths: auto
+
+.. note::
+
+   .. [#note-LWG3798] LWG3798: ``join_with_view``, ``zip_transform_view``, and ``adjacent_transform_view`` haven't been done yet since these types aren't implemented yet.

libcxx/docs/Status/Cxx2bIssues.csv

 "`3781 <https://wg21.link/LWG3781>`__","The exposition-only alias templates ``cont-key-type`` and ``cont-mapped-type`` should be removed", "November 2022","","",""
 "`3782 <https://wg21.link/LWG3782>`__","Should ``<math.h>`` declare ``::lerp``?", "November 2022","","",""
 "`3784 <https://wg21.link/LWG3784>`__","std.compat should not provide ``::byte`` and its friends", "November 2022","","",""
 "`3785 <https://wg21.link/LWG3785>`__","``ranges::to`` is over-constrained on the destination type being a range", "November 2022","","","|ranges|"
 "`3788 <https://wg21.link/LWG3788>`__","``jthread::operator=(jthread&&)`` postconditions are unimplementable under self-assignment", "November 2022","","",""
 "`3792 <https://wg21.link/LWG3792>`__","``__cpp_lib_constexpr_algorithms`` should also be defined in ``<utility>``", "November 2022","|Complete|","16.0",""
 "`3795 <https://wg21.link/LWG3795>`__","Self-move-assignment of ``std::future`` and ``std::shared_future`` have unimplementable postconditions", "November 2022","","",""
 "`3796 <https://wg21.link/LWG3796>`__","``movable-box`` as member should use ``default-initialization`` instead of ``copy-initialization``", "November 2022","","","|ranges|"
-"`3798 <https://wg21.link/LWG3798>`__","Rvalue reference and ``iterator_category``", "November 2022","","",""
+"`3798 <https://wg21.link/LWG3798>`__","Rvalue reference and ``iterator_category``", "November 2022","|Partial| [#note-LWG3798]_","","|ranges|"
 "`3801 <https://wg21.link/LWG3801>`__","``cartesian_product_view::iterator::distance-from`` ignores the size of last underlying range", "November 2022","","","|ranges|"
 "`3814 <https://wg21.link/LWG3814>`__","Add freestanding items requested by NB comments", "November 2022","","",""
 "`3816 <https://wg21.link/LWG3816>`__","``flat_map`` and ``flat_multimap`` should impose sequence container requirements", "November 2022","","","|flat_containers|"
 "`3817 <https://wg21.link/LWG3817>`__","Missing preconditions on ``forward_list`` modifiers", "November 2022","","",""
 "`3818 <https://wg21.link/LWG3818>`__","Exposition-only concepts are not described in library intro", "November 2022","|Nothing to do|","",""
 "`3822 <https://wg21.link/LWG3822>`__","Avoiding normalization in ``filesystem::weakly_canonical``", "November 2022","","",""
 "`3823 <https://wg21.link/LWG3823>`__","Unnecessary precondition for ``is_aggregate``", "November 2022","","",""
 "`3824 <https://wg21.link/LWG3824>`__","Number of ``bind`` placeholders is underspecified", "November 2022","|Nothing to do|","",""
 "`3826 <https://wg21.link/LWG3826>`__","Redundant specification [for overload of yield_value]", "November 2022","","",""
 "","","","","",""
 "`3631 <https://wg21.link/LWG3631>`__","``basic_format_arg(T&&)`` should use ``remove_cvref_t<T>`` throughout","Not voted in","|Complete|","15.0",""
 "`3645 <https://wg21.link/LWG3645>`__","``resize_and_overwrite`` is overspecified to call its callback with lvalues","Not voted in","|Complete|","14.0",""
 "`3343 <https://wg21.link/LWG3343>`__","Ordering of calls to ``unlock()`` and ``notify_all()`` in Effects element of ``notify_all_at_thread_exit()`` should be reversed","Not Yet Adopted","|Complete|","16.0",""

libcxx/include/__iterator/iterator_traits.h

typename common_reference_t<decltype(*__i++)&&,
                                 typename indirectly_readable_traits<_Ip>::value_type&>;
     requires signed_integral<typename incrementable_traits<_Ip>::difference_type>;
   };
 
 template<class _Ip>
 concept __cpp17_forward_iterator =
   __cpp17_input_iterator<_Ip> &&
   constructible_from<_Ip> &&
-  is_lvalue_reference_v<iter_reference_t<_Ip>> &&
+  is_reference_v<iter_reference_t<_Ip>> &&
   same_as<remove_cvref_t<iter_reference_t<_Ip>>,
           typename indirectly_readable_traits<_Ip>::value_type> &&
   requires(_Ip __i) {
     {  __i++ } -> convertible_to<_Ip const&>;
     { *__i++ } -> same_as<iter_reference_t<_Ip>>;
   };
 
 template<class _Ip>

libcxx/include/__ranges/transform_view.h

 template<class, class>
 struct __transform_view_iterator_category_base {};
 
 template<forward_range _View, class _Fn>
 struct __transform_view_iterator_category_base<_View, _Fn> {
   using _Cat = typename iterator_traits<iterator_t<_View>>::iterator_category;
 
   using iterator_category = conditional_t<
-    is_lvalue_reference_v<invoke_result_t<_Fn&, range_reference_t<_View>>>,
+    is_reference_v<invoke_result_t<_Fn&, range_reference_t<_View>>>,
     conditional_t<
       derived_from<_Cat, contiguous_iterator_tag>,
       random_access_iterator_tag,
       _Cat
     >,
     input_iterator_tag
   >;
 };

libcxx/test/libcxx/iterators/iterator.requirements/iterator.assoc.types/iterator.traits/legacy_bidirectional_iterator.compile.pass.cpp

 // <forward_list>
 static_assert(!std::__iterator_traits_detail::__cpp17_bidirectional_iterator<std::forward_list<int>::iterator>);
 static_assert(!std::__iterator_traits_detail::__cpp17_bidirectional_iterator<std::forward_list<int>::const_iterator>);
 
 // <iterator>
 static_assert(!std::__iterator_traits_detail::__cpp17_bidirectional_iterator<std::back_insert_iterator<std::vector<int>>>);
 static_assert(!std::__iterator_traits_detail::__cpp17_bidirectional_iterator<std::front_insert_iterator<std::vector<int>>>);
 static_assert(!std::__iterator_traits_detail::__cpp17_bidirectional_iterator<std::insert_iterator<std::vector<int>>>);
-static_assert(!std::__iterator_traits_detail::__cpp17_bidirectional_iterator<std::move_iterator<int*>>);
+static_assert(std::__iterator_traits_detail::__cpp17_bidirectional_iterator<std::move_iterator<int*>>);
 
 // <list>
 static_assert(std::__iterator_traits_detail::__cpp17_bidirectional_iterator<std::list<int>::iterator>);
 static_assert(std::__iterator_traits_detail::__cpp17_bidirectional_iterator<std::list<int>::const_iterator>);
 static_assert(std::__iterator_traits_detail::__cpp17_bidirectional_iterator<std::list<int>::reverse_iterator>);
 static_assert(std::__iterator_traits_detail::__cpp17_bidirectional_iterator<std::list<int>::const_reverse_iterator>);
 
 // <map>

libcxx/test/libcxx/iterators/iterator.requirements/iterator.assoc.types/iterator.traits/legacy_forward_iterator.compile.pass.cpp

 // <forward_list>
 static_assert(std::__iterator_traits_detail::__cpp17_forward_iterator<std::forward_list<int>::iterator>);
 static_assert(std::__iterator_traits_detail::__cpp17_forward_iterator<std::forward_list<int>::const_iterator>);
 
 // <iterator>
 static_assert(!std::__iterator_traits_detail::__cpp17_forward_iterator<std::back_insert_iterator<std::vector<int>>>);
 static_assert(!std::__iterator_traits_detail::__cpp17_forward_iterator<std::front_insert_iterator<std::vector<int>>>);
 static_assert(!std::__iterator_traits_detail::__cpp17_forward_iterator<std::insert_iterator<std::vector<int>>>);
-static_assert(!std::__iterator_traits_detail::__cpp17_forward_iterator<std::move_iterator<int*>>);
+static_assert(std::__iterator_traits_detail::__cpp17_forward_iterator<std::move_iterator<int*>>);
 
 // <list>
 static_assert(std::__iterator_traits_detail::__cpp17_forward_iterator<std::list<int>::iterator>);
 static_assert(std::__iterator_traits_detail::__cpp17_forward_iterator<std::list<int>::const_iterator>);
 static_assert(std::__iterator_traits_detail::__cpp17_forward_iterator<std::list<int>::reverse_iterator>);
 static_assert(std::__iterator_traits_detail::__cpp17_forward_iterator<std::list<int>::const_reverse_iterator>);
 
 // <map>

libcxx/test/libcxx/iterators/iterator.requirements/iterator.assoc.types/iterator.traits/legacy_random_access_iterator.compile.pass.cpp

 // <forward_list>
 static_assert(!std::__iterator_traits_detail::__cpp17_random_access_iterator<std::forward_list<int>::iterator>);
 static_assert(!std::__iterator_traits_detail::__cpp17_random_access_iterator<std::forward_list<int>::const_iterator>);
 
 // <iterator>
 static_assert(!std::__iterator_traits_detail::__cpp17_random_access_iterator<std::back_insert_iterator<std::vector<int>>>);
 static_assert(!std::__iterator_traits_detail::__cpp17_random_access_iterator<std::front_insert_iterator<std::vector<int>>>);
 static_assert(!std::__iterator_traits_detail::__cpp17_random_access_iterator<std::insert_iterator<std::vector<int>>>);
-static_assert(!std::__iterator_traits_detail::__cpp17_random_access_iterator<std::move_iterator<int*>>);
+static_assert(std::__iterator_traits_detail::__cpp17_random_access_iterator<std::move_iterator<int*>>);
 
 // <list>
 static_assert(!std::__iterator_traits_detail::__cpp17_random_access_iterator<std::list<int>::iterator>);
 static_assert(!std::__iterator_traits_detail::__cpp17_random_access_iterator<std::list<int>::const_iterator>);
 static_assert(!std::__iterator_traits_detail::__cpp17_random_access_iterator<std::list<int>::reverse_iterator>);
 static_assert(!std::__iterator_traits_detail::__cpp17_random_access_iterator<std::list<int>::const_reverse_iterator>);
 
 // <map>

libcxx/test/std/ranges/range.adaptors/range.transform/iterator/types.pass.cpp

constexpr bool test() {
     using TView = std::ranges::transform_view<RandomAccessView, Increment>;
     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>);
   }
   {
+    // Member typedefs for random access iterator, LWG3798 rvalue reference.
+    using TView = std::ranges::transform_view<RandomAccessView, IncrementRvalueRef>;
+    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>);
+  }
+  {
     // Member typedefs for random access iterator/not-lvalue-ref.
     using TView = std::ranges::transform_view<RandomAccessView, PlusOneMutable>;
     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::input_iterator_tag>); // Note: this is now input_iterator_tag.
     static_assert(std::same_as<typename TIter::value_type, int>);
     static_assert(std::same_as<typename TIter::difference_type, std::ptrdiff_t>);
   }