[libc++][ranges] LWG3692: zip_view::iterator's operator<=> is overconstrained and changes of zip_view in P2165R4

The changes are nearly pure simplifications, so I think it's OK to do
them together in the same PR.

Actual test coverages were already added in
commit ad41d1e
(https://reviews.llvm.org/D141216). Thanks to @CaseyCarter!

Author: frederick-vs-ja

libcxx/docs/Status/Cxx23Issues.csv

@@ -168,7 +168,7 @@
 "`LWG3672 <https://wg21.link/LWG3672>`__","``common_iterator::operator->()`` should return by value","2022-07 (Virtual)","|Complete|","19.0",""
 "`LWG3683 <https://wg21.link/LWG3683>`__","``operator==`` for ``polymorphic_allocator`` cannot deduce template argument in common cases","2022-07 (Virtual)","|Complete|","20.0",""
 "`LWG3687 <https://wg21.link/LWG3687>`__","``expected<cv void, E>`` move constructor should move","2022-07 (Virtual)","|Complete|","16.0",""
-"`LWG3692 <https://wg21.link/LWG3692>`__","``zip_view::iterator``'s ``operator<=>`` is overconstrained","2022-07 (Virtual)","","",""
+"`LWG3692 <https://wg21.link/LWG3692>`__","``zip_view::iterator``'s ``operator<=>`` is overconstrained","2022-07 (Virtual)","|Complete|","20.0",""
 "`LWG3701 <https://wg21.link/LWG3701>`__","Make ``formatter<remove_cvref_t<const charT[N]>, charT>`` requirement explicit","2022-07 (Virtual)","|Complete|","15.0",""
 "`LWG3702 <https://wg21.link/LWG3702>`__","Should ``zip_transform_view::iterator`` remove ``operator<``","2022-07 (Virtual)","","",""
 "`LWG3703 <https://wg21.link/LWG3703>`__","Missing requirements for ``expected<T, E>`` requires ``is_void<T>``","2022-07 (Virtual)","|Complete|","16.0",""

libcxx/docs/Status/Cxx23Papers.csv

@@ -60,7 +60,7 @@
 "`P1642R11 <https://wg21.link/P1642R11>`__","Freestanding ``[utilities]``, ``[ranges]``, and ``[iterators]``","2022-07 (Virtual)","","",""
 "`P1899R3 <https://wg21.link/P1899R3>`__","``stride_view``","2022-07 (Virtual)","","",""
 "`P2093R14 <https://wg21.link/P2093R14>`__","Formatted output","2022-07 (Virtual)","|Complete|","18.0",""
-"`P2165R4 <https://wg21.link/P2165R4>`__","Compatibility between ``tuple``, ``pair`` and ``tuple-like`` objects","2022-07 (Virtual)","","",""
+"`P2165R4 <https://wg21.link/P2165R4>`__","Compatibility between ``tuple``, ``pair`` and ``tuple-like`` objects","2022-07 (Virtual)","|Partial|","","Only the part for ``zip_view`` is implemented."
 "`P2278R4 <https://wg21.link/P2278R4>`__","``cbegin`` should always return a constant iterator","2022-07 (Virtual)","","",""
 "`P2286R8 <https://wg21.link/P2286R8>`__","Formatting Ranges","2022-07 (Virtual)","|Complete|","16.0",""
 "`P2291R3 <https://wg21.link/P2291R3>`__","Add Constexpr Modifiers to Functions ``to_chars`` and ``from_chars`` for Integral Types in ``<charconv>`` Header","2022-07 (Virtual)","|Complete|","16.0",""

libcxx/include/__ranges/zip_view.h

@@ -36,7 +36,6 @@
 #include <__utility/forward.h>
 #include <__utility/integer_sequence.h>
 #include <__utility/move.h>
-#include <__utility/pair.h>
 #include <tuple>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
@@ -58,21 +57,11 @@ concept __zip_is_common =
     (!(bidirectional_range<_Ranges> && ...) && (common_range<_Ranges> && ...)) ||
     ((random_access_range<_Ranges> && ...) && (sized_range<_Ranges> && ...));
 
-template <typename _Tp, typename _Up>
-auto __tuple_or_pair_test() -> pair<_Tp, _Up>;
-
-template <typename... _Types>
-  requires(sizeof...(_Types) != 2)
-auto __tuple_or_pair_test() -> tuple<_Types...>;
-
-template <class... _Types>
-using __tuple_or_pair = decltype(__tuple_or_pair_test<_Types...>());
-
 template <class _Fun, class _Tuple>
 _LIBCPP_HIDE_FROM_ABI constexpr auto __tuple_transform(_Fun&& __f, _Tuple&& __tuple) {
   return std::apply(
       [&]<class... _Types>(_Types&&... __elements) {
-        return __tuple_or_pair<invoke_result_t<_Fun&, _Types>...>(
+        return tuple<invoke_result_t<_Fun&, _Types>...>(
             std::invoke(__f, std::forward<_Types>(__elements))...);
       },
       std::forward<_Tuple>(__tuple));
@@ -88,7 +77,7 @@ _LIBCPP_HIDE_FROM_ABI constexpr void __tuple_for_each(_Fun&& __f, _Tuple&& __tup
 }
 
 template <class _Fun, class _Tuple1, class _Tuple2, size_t... _Indices>
-_LIBCPP_HIDE_FROM_ABI constexpr __tuple_or_pair<
+_LIBCPP_HIDE_FROM_ABI constexpr tuple<
     invoke_result_t<_Fun&,
                     typename tuple_element<_Indices, remove_cvref_t<_Tuple1>>::type,
                     typename tuple_element<_Indices, remove_cvref_t<_Tuple2>>::type>...>
@@ -250,10 +239,10 @@ template <input_range... _Views>
   requires(view<_Views> && ...) && (sizeof...(_Views) > 0)
 template <bool _Const>
 class zip_view<_Views...>::__iterator : public __zip_view_iterator_category_base<_Const, _Views...> {
-  __tuple_or_pair<iterator_t<__maybe_const<_Const, _Views>>...> __current_;
+  tuple<iterator_t<__maybe_const<_Const, _Views>>...> __current_;
 
   _LIBCPP_HIDE_FROM_ABI constexpr explicit __iterator(
-      __tuple_or_pair<iterator_t<__maybe_const<_Const, _Views>>...> __current)
+      tuple<iterator_t<__maybe_const<_Const, _Views>>...> __current)
       : __current_(std::move(__current)) {}
 
   template <bool>
@@ -266,7 +255,7 @@ class zip_view<_Views...>::__iterator : public __zip_view_iterator_category_base
 
 public:
   using iterator_concept = decltype(__get_zip_view_iterator_tag<_Const, _Views...>());
-  using value_type       = __tuple_or_pair<range_value_t<__maybe_const<_Const, _Views>>...>;
+  using value_type       = tuple<range_value_t<__maybe_const<_Const, _Views>>...>;
   using difference_type  = common_type_t<range_difference_t<__maybe_const<_Const, _Views>>...>;
 
   _LIBCPP_HIDE_FROM_ABI __iterator() = default;
@@ -340,33 +329,8 @@ class zip_view<_Views...>::__iterator : public __zip_view_iterator_category_base
     }
   }
 
-  _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator<(const __iterator& __x, const __iterator& __y)
-    requires __zip_all_random_access<_Const, _Views...>
-  {
-    return __x.__current_ < __y.__current_;
-  }
-
-  _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator>(const __iterator& __x, const __iterator& __y)
-    requires __zip_all_random_access<_Const, _Views...>
-  {
-    return __y < __x;
-  }
-
-  _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator<=(const __iterator& __x, const __iterator& __y)
-    requires __zip_all_random_access<_Const, _Views...>
-  {
-    return !(__y < __x);
-  }
-
-  _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator>=(const __iterator& __x, const __iterator& __y)
-    requires __zip_all_random_access<_Const, _Views...>
-  {
-    return !(__x < __y);
-  }
-
   _LIBCPP_HIDE_FROM_ABI friend constexpr auto operator<=>(const __iterator& __x, const __iterator& __y)
-    requires __zip_all_random_access<_Const, _Views...> &&
-             (three_way_comparable<iterator_t<__maybe_const<_Const, _Views>>> && ...)
+    requires __zip_all_random_access<_Const, _Views...>
   {
     return __x.__current_ <=> __y.__current_;
   }
@@ -427,10 +391,10 @@ template <input_range... _Views>
   requires(view<_Views> && ...) && (sizeof...(_Views) > 0)
 template <bool _Const>
 class zip_view<_Views...>::__sentinel {
-  __tuple_or_pair<sentinel_t<__maybe_const<_Const, _Views>>...> __end_;
+  tuple<sentinel_t<__maybe_const<_Const, _Views>>...> __end_;
 
   _LIBCPP_HIDE_FROM_ABI constexpr explicit __sentinel(
-      __tuple_or_pair<sentinel_t<__maybe_const<_Const, _Views>>...> __end)
+      tuple<sentinel_t<__maybe_const<_Const, _Views>>...> __end)
       : __end_(__end) {}
 
   friend class zip_view<_Views...>;

libcxx/test/std/ranges/range.adaptors/range.zip/cpo.pass.cpp

@@ -63,11 +63,7 @@ constexpr bool test() {
         std::ranges::zip_view<std::ranges::zip_view<SizedRandomAccessView, SizedRandomAccessView>>> decltype(auto) v2 =
         std::views::zip(v);
 
-#ifdef _LIBCPP_VERSION // libc++ doesn't implement P2165R4 yet
-    static_assert(std::is_same_v<std::ranges::range_reference_t<decltype(v2)>, std::tuple<std::pair<int&, int&>>>);
-#else
     static_assert(std::is_same_v<std::ranges::range_reference_t<decltype(v2)>, std::tuple<std::tuple<int&, int&>>>);
-#endif
   }
   return true;
 }

libcxx/test/std/ranges/range.adaptors/range.zip/ctor.default.pass.cpp

@@ -49,12 +49,8 @@ constexpr bool test() {
     using View = std::ranges::zip_view<DefaultConstructibleView, DefaultConstructibleView>;
     View v = View(); // the default constructor is not explicit
     assert(v.size() == 3);
-    auto it = v.begin();
-#ifdef _LIBCPP_VERSION // libc++ doesn't implement P2165R4 yet
-    using Value = std::pair<const int&, const int&>;
-#else
+    auto it     = v.begin();
     using Value = std::tuple<const int&, const int&>;
-#endif
     assert(*it++ == Value(buff[0], buff[0]));
     assert(*it++ == Value(buff[1], buff[1]));
     assert(*it == Value(buff[2], buff[2]));

libcxx/test/std/ranges/range.adaptors/range.zip/iterator/compare.pass.cpp

@@ -10,17 +10,8 @@
 
 // friend constexpr bool operator==(const iterator& x, const iterator& y)
 //   requires (equality_comparable<iterator_t<maybe-const<Const, Views>>> && ...);
-// friend constexpr bool operator<(const iterator& x, const iterator& y)
-//   requires all-random-access<Const, Views...>;
-// friend constexpr bool operator>(const iterator& x, const iterator& y)
-//   requires all-random-access<Const, Views...>;
-// friend constexpr bool operator<=(const iterator& x, const iterator& y)
-//   requires all-random-access<Const, Views...>;
-// friend constexpr bool operator>=(const iterator& x, const iterator& y)
-//   requires all-random-access<Const, Views...>;
 // friend constexpr auto operator<=>(const iterator& x, const iterator& y)
-//   requires all-random-access<Const, Views...> &&
-//            (three_way_comparable<iterator_t<maybe-const<Const, Views>>> && ...);
+//   requires all-random-access<Const, Views...>;
 
 #include <ranges>
 #include <compare>
@@ -165,12 +156,7 @@ constexpr bool test() {
     using Subrange = std::ranges::subrange<It>;
     static_assert(!std::three_way_comparable<It>);
     using R = std::ranges::zip_view<Subrange, Subrange>;
-#ifdef _LIBCPP_VERSION
-    // libc++ hasn't implemented LWG-3692 "zip_view::iterator's operator<=> is overconstrained"
-    static_assert(!std::three_way_comparable<std::ranges::iterator_t<R>>);
-#else
     static_assert(std::three_way_comparable<std::ranges::iterator_t<R>>);
-#endif
 
     int a[] = {1, 2, 3, 4};
     int b[] = {5, 6, 7, 8, 9};

libcxx/test/std/ranges/range.adaptors/range.zip/iterator/deref.pass.cpp

@@ -42,11 +42,7 @@ constexpr bool test() {
     auto [x, y] = *it;
     assert(&x == &(a[0]));
     assert(&y == &(b[0]));
-#ifdef _LIBCPP_VERSION // libc++ doesn't implement P2165R4 yet
-    static_assert(std::is_same_v<decltype(*it), std::pair<int&, double&>>);
-#else
     static_assert(std::is_same_v<decltype(*it), std::tuple<int&, double&>>);
-#endif
 
     x = 5;
     y = 0.1;
@@ -70,11 +66,7 @@ constexpr bool test() {
     auto it = v.begin();
     assert(&(std::get<0>(*it)) == &(a[0]));
     assert(&(std::get<1>(*it)) == &(a[0]));
-#ifdef _LIBCPP_VERSION // libc++ doesn't implement P2165R4 yet
-    static_assert(std::is_same_v<decltype(*it), std::pair<int&, int const&>>);
-#else
     static_assert(std::is_same_v<decltype(*it), std::tuple<int&, int const&>>);
-#endif
   }
   return true;
 }

libcxx/test/std/ranges/range.adaptors/range.zip/iterator/member_types.compile.pass.cpp

@@ -65,19 +65,15 @@ struct ConstVeryDifferentRange {
 void test() {
   int buffer[] = {1, 2, 3, 4};
   {
-    // 2 views should have pair value_type
+    // 2 views should have 2-tuple value_type
     // random_access_iterator_tag
     std::ranges::zip_view v(buffer, buffer);
     using Iter = decltype(v.begin());
 
     static_assert(std::is_same_v<Iter::iterator_concept, std::random_access_iterator_tag>);
     static_assert(std::is_same_v<Iter::iterator_category, std::input_iterator_tag>);
     static_assert(std::is_same_v<Iter::difference_type, std::ptrdiff_t>);
-#ifdef _LIBCPP_VERSION // libc++ doesn't implement P2165R4 yet
-    static_assert(std::is_same_v<Iter::value_type, std::pair<int, int>>);
-#else
     static_assert(std::is_same_v<Iter::value_type, std::tuple<int, int>>);
-#endif
     static_assert(HasIterCategory<Iter>);
   }
 
@@ -124,11 +120,7 @@ void test() {
     static_assert(std::is_same_v<Iter::iterator_concept, std::random_access_iterator_tag>);
     static_assert(std::is_same_v<Iter::iterator_category, std::input_iterator_tag>);
     static_assert(std::is_same_v<Iter::difference_type, std::ptrdiff_t>);
-#ifdef _LIBCPP_VERSION // libc++ doesn't implement P2165R4 yet
-    static_assert(std::is_same_v<Iter::value_type, std::pair<int, std::pair<int, int>>>);
-#else
     static_assert(std::is_same_v<Iter::value_type, std::tuple<int, std::tuple<int, int>>>);
-#endif
     static_assert(HasIterCategory<Iter>);
   }
 
@@ -169,11 +161,7 @@ void test() {
     // value_type of multiple views with different value_type
     std::ranges::zip_view v{foos, bars};
     using Iter = decltype(v.begin());
-#ifdef _LIBCPP_VERSION // libc++ doesn't implement P2165R4 yet
-    static_assert(std::is_same_v<Iter::value_type, std::pair<Foo, Bar>>);
-#else
     static_assert(std::is_same_v<Iter::value_type, std::tuple<Foo, Bar>>);
-#endif
   }
 
   {

libcxx/test/std/ranges/range.adaptors/range.zip/iterator/subscript.pass.cpp

@@ -27,11 +27,7 @@ constexpr bool test() {
     assert(it[2] == *(it + 2));
     assert(it[4] == *(it + 4));
 
-#ifdef _LIBCPP_VERSION // libc++ doesn't implement P2165R4 yet
-    static_assert(std::is_same_v<decltype(it[2]), std::pair<int&, int>>);
-#else
     static_assert(std::is_same_v<decltype(it[2]), std::tuple<int&, int>>);
-#endif
   }
 
   {
@@ -42,11 +38,7 @@ constexpr bool test() {
     assert(it[2] == *(it + 2));
     assert(it[4] == *(it + 4));
 
-#ifdef _LIBCPP_VERSION // libc++ doesn't implement P2165R4 yet
-    static_assert(std::is_same_v<decltype(it[2]), std::pair<int&, int&>>);
-#else
     static_assert(std::is_same_v<decltype(it[2]), std::tuple<int&, int&>>);
-#endif
   }
 
   {