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>
 
@@ -112,10 +113,16 @@ TEST_CONSTEXPR_CXX20 bool test() {
 
 #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}};
+    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++); });
   }
+
+  if (!TEST_IS_CONSTANT_EVALUATED) {
+    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
 
   return true;