[libcxx-commits] [libcxx] [libc++] Simplify most of the segmented iterator optimizations (PR #164797)

[Louis Dionne via libcxx-commits]

================
@@ -32,18 +29,23 @@ _LIBCPP_PUSH_MACROS
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-template <class _InputIterator,
-          class _Size,
-          class _Func,
-          class _Proj,
-          __enable_if_t<!__has_random_access_iterator_category<_InputIterator>::value &&
-                            _Or<integral_constant<bool, !__is_segmented_iterator_v<_InputIterator> >,
-                                _Not<__has_random_access_local_iterator<_InputIterator> > >::value,
-                        int> = 0>
+template <class _InputIterator, class _Size, class _Func, class _Proj>
 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _InputIterator
 __for_each_n(_InputIterator __first, _Size __orig_n, _Func& __f, _Proj& __proj) {
   typedef decltype(std::__convert_to_integral(__orig_n)) _IntegralSize;
   _IntegralSize __n = __orig_n;
+
+#ifndef _LIBCPP_CXX03_LANG
+  if constexpr (__is_segmented_iterator_v<_InputIterator>) {
+    using __local_iterator = typename __segmented_iterator_traits<_InputIterator>::__local_iterator;
+    if constexpr (__has_random_access_iterator_category<__local_iterator>::value) {
+      return std::__for_each_n_segment(__first, __orig_n, [&](__local_iterator __lfirst, __local_iterator __llast) {
+        std::__for_each(__lfirst, __llast, __f, __proj);
+      });
+    }
----------------
ldionne wrote:

```suggestion
    } else {
      use for_each_segment
    }
```

That preserves previous behaviour like my comment above.

https://github.com/llvm/llvm-project/pull/164797