[libcxx-commits] [libcxx] [libcxx] Optimize `ranges::fold_left_with_iter` for segmented iterators (PR #177853)

[Connector Switch via libcxx-commits]

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@@ -80,18 +82,22 @@ concept __indirectly_binary_left_foldable =
 struct __fold_left_with_iter {
   template <input_iterator _Ip, sentinel_for<_Ip> _Sp, class _Tp, __indirectly_binary_left_foldable<_Tp, _Ip> _Fp>
   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI static constexpr auto operator()(_Ip __first, _Sp __last, _Tp __init, _Fp __f) {
-    using _Up = decay_t<invoke_result_t<_Fp&, _Tp, iter_reference_t<_Ip>>>;
+    using _Up        = decay_t<invoke_result_t<_Fp&, _Tp, iter_reference_t<_Ip>>>;
+    using __iter_ref = decltype(*__first);
 
     if (__first == __last) {
       return fold_left_with_iter_result<_Ip, _Up>{std::move(__first), _Up(std::move(__init))};
     }
 
     _Up __result = std::invoke(__f, std::move(__init), *__first);
-    for (++__first; __first != __last; ++__first) {
-      __result = std::invoke(__f, std::move(__result), *__first);
-    }
-
-    return fold_left_with_iter_result<_Ip, _Up>{std::move(__first), std::move(__result)};
+    ++__first;
+    auto __for_each_f = [&](__iter_ref __element) {
+      __result = std::invoke(__f, std::move(__result), std::forward<__iter_ref>(__element));
+    };
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c8ef wrote:

> Urgh. Could you make a patch to allow passing the functor directly, given that I've commented twice on this now?

Now we pass the functor directly. And also add the test coverage for previous buggy forward.

https://github.com/llvm/llvm-project/pull/177853