Address ldionne's comments

Author: winner245

libcxx/include/__algorithm/for_each.h

@@ -9,6 +9,7 @@
 
 #ifndef _LIBCPP___ALGORITHM_FOR_EACH_H
 #define _LIBCPP___ALGORITHM_FOR_EACH_H
+
 #include <__algorithm/for_each_segment.h>
 #include <__config>
 #include <__iterator/segmented_iterator.h>

libcxx/include/__algorithm/for_each_n.h

@@ -15,7 +15,9 @@
 #include <__config>
 #include <__iterator/iterator_traits.h>
 #include <__iterator/segmented_iterator.h>
+#include <__type_traits/disjunction.h>
 #include <__type_traits/enable_if.h>
+#include <__type_traits/negation.h>
 #include <__utility/convert_to_integral.h>
 #include <__utility/move.h>
 
@@ -32,11 +34,8 @@ template <class _InputIterator,
           class _Size,
           class _Func,
           __enable_if_t<!__has_random_access_iterator_category<_InputIterator>::value &&
-                            (!__is_segmented_iterator<_InputIterator>::value
-                             //   || !__has_random_access_iterator_category<
-                             //      typename __segmented_iterator_traits<_InputIterator>::__local_iterator>::value
-                             ), // TODO: __segmented_iterator_traits<_InputIterator> results in template instantiation
-                                // during SFINAE, which is a hard error to be fixed. Once fixed, we should uncomment.
+                            _Or< _Not<__is_segmented_iterator<_InputIterator> >,
+                                 _Not<__has_random_access_local_iterator<_InputIterator> > >::value,
                         int> = 0>
 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _InputIterator
 __for_each_n(_InputIterator __first, _Size __orig_n, _Func& __f) {

libcxx/include/__iterator/segmented_iterator.h

@@ -42,6 +42,7 @@
 
 #include <__config>
 #include <__cstddef/size_t.h>
+#include <__iterator/iterator_traits.h>
 #include <__type_traits/integral_constant.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
@@ -74,6 +75,11 @@ struct __has_specialization<_Tp, sizeof(_Tp) * 0> : true_type {};
 template <class _Iterator>
 using __is_segmented_iterator _LIBCPP_NODEBUG = __has_specialization<__segmented_iterator_traits<_Iterator> >;
 
+template <class _SegmentedIterator>
+struct __has_random_access_local_iterator
+    : __has_random_access_iterator_category<
+          typename __segmented_iterator_traits< _SegmentedIterator >::__local_iterator > {};
+
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___SEGMENTED_ITERATOR_H

libcxx/test/std/algorithms/alg.nonmodifying/alg.foreach/for_each_n.pass.cpp

@@ -18,6 +18,7 @@
 #include <deque>
 #include <functional>
 #include <iterator>
+#include <list>
 #include <ranges>
 #include <vector>
 
@@ -46,15 +47,24 @@ struct deque_test {
 };
 
 /*TEST_CONSTEXPR_CXX26*/
-void test_segmented_deque_iterator() { // TODO: Mark as TEST_CONSTEXPR_CXX26 once std::deque is constexpr
-  // check that segmented deque iterators work properly
-  int sizes[] = {0, 1, 2, 1023, 1024, 1025, 2047, 2048, 2049};
-  for (const int size : sizes) {
-    std::deque<int> d(size);
-    int index = 0;
-
-    std::for_each_n(d.begin(), d.size(), deque_test(d, index));
+void test_deque_and_join_view_iterators() { // TODO: Mark as TEST_CONSTEXPR_CXX26 once std::deque is constexpr
+  {                                         // Verify that segmented deque iterators work properly
+    int sizes[] = {0, 1, 2, 1023, 1024, 1025, 2047, 2048, 2049};
+    for (const int size : sizes) {
+      std::deque<int> d(size);
+      int index = 0;
+
+      std::for_each_n(d.begin(), d.size(), deque_test(d, index));
+    }
+  }
+#if TEST_STD_VER >= 20
+  { // Verify that join_view of lists work properly. Note that join_view of (non-random access) lists does
+    // not produce segmented iterators.
+    std::list<std::list<int>> lst = {{}, {0}, {1, 2}, {}, {3, 4, 5}, {6, 7, 8, 9}, {10}, {11, 12, 13}};
+    auto v                        = lst | std::views::join;
+    std::for_each_n(v.begin(), std::ranges::distance(v), [i = 0](int& a) mutable { assert(a == i++); });
   }
+#endif
 }
 
 TEST_CONSTEXPR_CXX20 bool test() {
@@ -108,11 +118,11 @@ TEST_CONSTEXPR_CXX20 bool test() {
   }
 
   if (!TEST_IS_CONSTANT_EVALUATED) // TODO: Use TEST_STD_AT_LEAST_26_OR_RUNTIME_EVALUATED when std::deque is made constexpr
-    test_segmented_deque_iterator();
+    test_deque_and_join_view_iterators();
 
 #if TEST_STD_VER >= 20
-  {
-    std::vector<std::vector<int>> vec = {{0}, {1, 2}, {3, 4, 5}, {6, 7, 8, 9}, {10}, {11, 12, 13}};
+  { // join_views of (random-access) vectors yield segmented iterators
+    std::vector<std::vector<int>> vec = {{}, {0}, {1, 2}, {}, {3, 4, 5}, {6, 7, 8, 9}, {10}, {11, 12, 13}};
     auto v                            = vec | std::views::join;
     std::for_each_n(v.begin(), std::ranges::distance(v), [i = 0](int& a) mutable { assert(a == i++); });
   }