[clang-tools-extra] [llvm] [clang] [libcxx] [libcxx] adds ranges::fold_left_with_iter and ranges::fold_left (PR #75259)

[Christopher Di Bella via cfe-commits]

================
@@ -0,0 +1,104 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
+
+// template <class I, class T>
+// struct in_value_result;
+
+#include <algorithm>
+#include <cassert>
+#include <type_traits>
+#include <utility>
+
+#include "MoveOnly.h"
+
+struct A {
+  explicit A(int);
+};
+// no implicit conversion
+static_assert(!std::is_constructible_v<std::ranges::in_value_result<A, A>, std::ranges::in_value_result<int, int>>);
+
+struct B {
+  B(int);
+};
+// implicit conversion
+static_assert(std::is_constructible_v<std::ranges::in_value_result<B, B>, std::ranges::in_value_result<int, int>>);
+static_assert(std::is_constructible_v<std::ranges::in_value_result<B, B>, std::ranges::in_value_result<int, int>&>);
+static_assert(
+    std::is_constructible_v<std::ranges::in_value_result<B, B>, const std::ranges::in_value_result<int, int>>);
+static_assert(
+    std::is_constructible_v<std::ranges::in_value_result<B, B>, const std::ranges::in_value_result<int, int>&>);
+
+struct C {
+  C(int&);
+};
+static_assert(!std::is_constructible_v<std::ranges::in_value_result<C, C>, std::ranges::in_value_result<int, int>&>);
+
+// has to be convertible via const&
+static_assert(std::is_convertible_v<std::ranges::in_value_result<int, int>&, std::ranges::in_value_result<long, long>>);
+static_assert(
+    std::is_convertible_v<const std::ranges::in_value_result<int, int>&, std::ranges::in_value_result<long, long>>);
+static_assert(
+    std::is_convertible_v<std::ranges::in_value_result<int, int>&&, std::ranges::in_value_result<long, long>>);
+static_assert(
+    std::is_convertible_v<const std::ranges::in_value_result<int, int>&&, std::ranges::in_value_result<long, long>>);
+
+// should be move constructible
+static_assert(std::is_move_constructible_v<std::ranges::in_value_result<MoveOnly, int>>);
+static_assert(std::is_move_constructible_v<std::ranges::in_value_result<int, MoveOnly>>);
+
+// should not copy constructible with move-only type
----------------
cjdb wrote:

> I fully agree that adding this to a project-wide style guide would be very useful (to be clear, I don't mean to imply it has to be a comprehensive huge guide, even if it documents just a few randomly chosen aspects, it's already helpful and we can always expand later). We do have a ["Coding standards"](https://libcxx.llvm.org/Contributing.html#coding-standards) section in the `Contributing` document -- we might want to add a new page and link it from that section. Thanks for taking this on!

Re ultranit: applied in #76534.

Re second comment: Not sure what this is pertaining to?

https://github.com/llvm/llvm-project/pull/75259