Address some review comments

- Templatize `test_basic_operations`.
- Assert noexcept-ness.
- Change notes in implementation status.

Author: frederick-vs-ja

libcxx/docs/Status/Cxx2cPapers.csv

@@ -42,7 +42,7 @@
 "`P2819R2 <https://wg21.link/P2819R2>`__","Add tuple protocol to complex","2023-11 (Kona)","|Complete|","19",""
 "`P2937R0 <https://wg21.link/P2937R0>`__","Freestanding: Remove ``strtok``","2023-11 (Kona)","","",""
 "`P2833R2 <https://wg21.link/P2833R2>`__","Freestanding Library: inout expected span","2023-11 (Kona)","","",""
-"`P2836R1 <https://wg21.link/P2836R1>`__","``std::basic_const_iterator`` should follow its underlying type's convertibility","2023-11 (Kona)","|Complete|","21","Implemented as a DR against C++23. (MSVC STL and libstdc++ will do the same.)"
+"`P2836R1 <https://wg21.link/P2836R1>`__","``std::basic_const_iterator`` should follow its underlying type's convertibility","2023-11 (Kona)","|Complete|","21","Implemented as a DR against C++23. (MSVC STL and libstdc++ do the same.)"
 "`P2264R7 <https://wg21.link/P2264R7>`__","Make ``assert()`` macro user friendly for C and C++","2023-11 (Kona)","","",""
 "`P1673R13 <https://wg21.link/P1673R13>`__","A free function linear algebra interface based on the BLAS","2023-11 (Kona)","","",""
 "","","","","",""

libcxx/include/__ranges/const_access.h

@@ -29,9 +29,9 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER >= 20
 
+namespace ranges {
 // [range.const]
 #  if _LIBCPP_STD_VER >= 23
-namespace ranges {
 template <input_range _Rp>
 _LIBCPP_HIDE_FROM_ABI constexpr auto& __possibly_const_range(_Rp& __rng) noexcept {
   if constexpr (input_range<const _Rp>) {
@@ -40,11 +40,9 @@ _LIBCPP_HIDE_FROM_ABI constexpr auto& __possibly_const_range(_Rp& __rng) noexcep
     return __rng;
   }
 }
-} // namespace ranges
 #  endif // _LIBCPP_STD_VER >= 23
 
 // [range.access.cbegin]
-namespace ranges {
 
 template <class _Type>
 concept __const_accessible_range = (!is_rvalue_reference_v<_Type&&> || enable_borrowed_range<remove_cv_t<_Type>>);
@@ -84,18 +82,15 @@ struct __fn {
 inline namespace __cpo {
 inline constexpr auto cbegin = __cbegin::__fn{};
 } // namespace __cpo
-} // namespace ranges
 
 #  if _LIBCPP_STD_VER >= 23
 // [range.range]
-namespace ranges {
 template <class _Rp>
 using const_iterator_t = decltype(ranges::cbegin(std::declval<_Rp&>()));
-} // namespace ranges
 #  endif // _LIBCPP_STD_VER >= 23
 
 // [range.access.cend]
-namespace ranges {
+
 namespace __cend {
 struct __fn {
 #  if _LIBCPP_STD_VER >= 23
@@ -131,18 +126,14 @@ struct __fn {
 inline namespace __cpo {
 inline constexpr auto cend = __cend::__fn{};
 } // namespace __cpo
-} // namespace ranges
 
 #  if _LIBCPP_STD_VER >= 23
 // [range.range]
-namespace ranges {
 template <class _Rp>
 using const_sentinel_t = decltype(ranges::cend(std::declval<_Rp&>()));
-} // namespace ranges
 #  endif
 
 // [range.access.crbegin]
-namespace ranges {
 namespace __crbegin {
 struct __fn {
 #  if _LIBCPP_STD_VER >= 23
@@ -178,10 +169,8 @@ struct __fn {
 inline namespace __cpo {
 inline constexpr auto crbegin = __crbegin::__fn{};
 } // namespace __cpo
-} // namespace ranges
 
 // [range.access.crend]
-namespace ranges {
 namespace __crend {
 struct __fn {
 #  if _LIBCPP_STD_VER >= 23
@@ -217,11 +206,9 @@ struct __fn {
 inline namespace __cpo {
 inline constexpr auto crend = __crend::__fn{};
 } // namespace __cpo
-} // namespace ranges
 
 // [range.prim.cdata]
 
-namespace ranges {
 namespace __cdata {
 struct __fn {
 #  if _LIBCPP_STD_VER >= 23

libcxx/test/std/iterators/const.iterators/iterator.pass.cpp

@@ -73,13 +73,23 @@ constexpr bool test_p2836r1() {
   return true;
 }
 
-constexpr bool test_basic_operations() {
-  struct S {
-    int x;
-  };
-  S arr[10]                           = {};
-  std::basic_const_iterator<S*> first = arr;
-  std::basic_const_iterator<S*> last  = arr + 10;
+struct S {
+  int x;
+};
+
+template <class It>
+constexpr void test_basic_operations() {
+  S arr[10]{};
+
+  std::basic_const_iterator<It> first = It{arr};
+  std::basic_const_iterator<It> last  = It{arr + 10};
+
+  ASSERT_NOEXCEPT(first.base());
+  assert(first.base() == It{arr});
+  assert(last.base() == It{arr + 10});
+
+  static_assert(noexcept(iter_move(first)) == noexcept(std::ranges::iter_move(first.base())));
+  static_assert(noexcept(std::ranges::iter_move(first)) == noexcept(std::ranges::iter_move(first.base())));
 
   for (auto it = first; it != last; ++it) {
     (void)*it;
@@ -88,33 +98,46 @@ constexpr bool test_basic_operations() {
   }
   static_assert(!std::is_invocable_v<decltype(std::ranges::iter_swap), decltype(first), decltype(first)>);
 
-  assert(++first == arr + 1);
-  assert(--first == arr + 0);
-  assert(first++ == arr + 0);
-  assert(first-- == arr + 1);
-
-  assert(first + 3 == arr + 3);
-  assert(last - 1 == arr + 9);
-
-  first += 3;
-  assert(first == arr + 3);
-  first -= 2;
-  assert(first == arr + 1);
-  --first;
-
-  assert(first < last);
-  assert(last > first);
-  assert(first <= last);
-  assert(last >= first);
-
-  assert(first < arr + 1);
-  assert(arr + 1 > first);
-  assert(first <= arr + 1);
-  assert(arr + 1 >= first);
-
-  assert((first <=> last) < 0);
-  assert((first <=> arr + 1) < 0);
-  assert((arr + 1 <=> first) > 0);
+  if constexpr (std::bidirectional_iterator<It>) {
+    assert(++first == It{arr + 1});
+    assert(--first == It{arr + 0});
+    assert(first++ == It{arr + 0});
+    assert(first-- == It{arr + 1});
+  }
+
+  if constexpr (std::random_access_iterator<It>) {
+    assert(first + 3 == It{arr + 3});
+    assert(last - 1 == It{arr + 9});
+
+    first += 3;
+    assert(first == It{arr + 3});
+    first -= 2;
+    assert(first == It{arr + 1});
+    --first;
+
+    assert(first < last);
+    assert(last > first);
+    assert(first <= last);
+    assert(last >= first);
+
+    assert(first < It{arr + 1});
+    assert(It{arr + 1} > first);
+    assert(first <= It{arr + 1});
+    assert(It{arr + 1} >= first);
+
+    if constexpr (std::three_way_comparable<It>) {
+      assert((first <=> last) < 0);
+      assert((first <=> It{arr + 1}) < 0);
+      assert((It{arr + 1} <=> first) > 0);
+    }
+  }
+}
+
+constexpr bool test_basic_operations() {
+  test_basic_operations<S*>();
+  test_basic_operations<forward_iterator<S*>>();
+  test_basic_operations<bidirectional_iterator<S*>>();
+  test_basic_operations<random_access_iterator<S*>>();
 
   return true;
 }