[libcxx-commits] [libcxx] [libc++] Add `ranges::fold_left_first` and ranges::`fold_left_first_with_iter` (PR #121558)

[A. Jiang via libcxx-commits]

================
@@ -118,6 +121,54 @@ struct __fold_left {
 };
 
 inline constexpr auto fold_left = __fold_left();
+
+struct __fold_left_first_with_iter {
+  template <input_iterator _Ip, sentinel_for<_Ip> _Sp, __indirectly_binary_left_foldable<iter_value_t<_Ip>, _Ip> _Fp>
+  requires constructible_from<iter_value_t<_Ip>, iter_reference_t<_Ip>>
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI static constexpr auto operator()(_Ip __first, _Sp __last, _Fp __f) {
+    using _Up = decltype(fold_left(std::move(__first), __last, iter_value_t<_Ip>(*__first), __f));
+
+    // case of empty range
+    if (__first == __last) {
+      return fold_left_first_with_iter_result<_Ip, optional<_Up>>{std::move(__first), optional<_Up>()};
+    }
+
+    optional<_Up> __result(std::in_place, *__first);
+    for (++__first; __first != __last; ++__first) {
+      *__result = std::invoke(__f, std::move(*__result), *__first);
+    }
----------------
frederick-vs-ja wrote:

I think it's OK to construct a `_Up` object, use it in the loop, and finally constructs the `optional<_Up>`. This approach seemingly behaves better in debugging and constant evaluation, as less steps are executed.

```suggestion
    _Up __result(*__first);
    for (++__first; __first != __last; ++__first) {
      __result = std::invoke(__f, std::move(__result), *__first);
    }
```

https://github.com/llvm/llvm-project/pull/121558