Consistently extend segmented iterator optimization to ranges::for_each

Author: winner245

libcxx/docs/ReleaseNotes/21.rst

@@ -61,7 +61,7 @@ Improvements and New Features
 - Updated formatting library to Unicode 16.0.0.
 
 - The ``std::ranges::for_each`` and ``std::ranges::for_each_n`` algorithms have been optimized for segmented iterators,
-  resulting in performance improvements of up to 21.2x for ``std::deque::iterator`` segmented inputs and 17.9x for
+  resulting in performance improvements of up to 21.3x for ``std::deque::iterator`` segmented inputs and 24.9x for
   ``join_view`` of ``vector<vector<T>>``.
 
 Deprecations and Removals

libcxx/include/__algorithm/ranges_for_each.h

@@ -10,7 +10,9 @@
 #define _LIBCPP___ALGORITHM_RANGES_FOR_EACH_H
 
 #include <__algorithm/for_each.h>
+#include <__algorithm/for_each_n.h>
 #include <__algorithm/in_fun_result.h>
+#include <__concepts/assignable.h>
 #include <__config>
 #include <__functional/identity.h>
 #include <__functional/invoke.h>
@@ -42,11 +44,14 @@ struct __for_each {
   template <class _Iter, class _Sent, class _Proj, class _Func>
   _LIBCPP_HIDE_FROM_ABI constexpr static for_each_result<_Iter, _Func>
   __for_each_impl(_Iter __first, _Sent __last, _Func& __func, _Proj& __proj) {
-    if constexpr (random_access_iterator<_Iter> && sized_sentinel_for<_Sent, _Iter>) {
-      auto __n   = __last - __first;
-      auto __end = __first + __n;
-      auto __f   = [&](auto&& __val) { std::invoke(__func, std::invoke(__proj, __val)); };
-      std::__for_each(__first, __end, __f);
+    if constexpr (std::assignable_from<_Iter&, _Sent>) {
+      _Iter __end = std::move(__last);
+      std::for_each(__first, __end, [&](auto&& __val) { std::invoke(__func, std::invoke(__proj, __val)); });
+      return {std::move(__end), std::move(__func)};
+    } else if constexpr (sized_sentinel_for<_Sent, _Iter>) {
+      auto __end = std::for_each_n(__first, __last - __first, [&](auto&& __val) {
+        std::invoke(__func, std::invoke(__proj, __val));
+      });
       return {std::move(__end), std::move(__func)};
     } else {
       for (; __first != __last; ++__first)