[clang] [clang-tools-extra] [llvm] [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
+static_assert(!std::is_copy_constructible_v<std::ranges::in_value_result<MoveOnly, int>>);
+static_assert(!std::is_copy_constructible_v<std::ranges::in_value_result<int, MoveOnly>>);
+
+struct NotConvertible {};
+// conversions should not work if there is no conversion
+static_assert(
+    !std::is_convertible_v<std::ranges::in_value_result<NotConvertible, int>, std::ranges::in_value_result<int, int>>);
+static_assert(
+    !std::is_convertible_v<std::ranges::in_value_result<int, NotConvertible>, std::ranges::in_value_result<int, int>>);
+
+template <class T>
+struct ConvertibleFrom {
+  constexpr ConvertibleFrom(T c) : content{c} {}
+  T content;
+};
+
+constexpr bool test() {
+  {
+    std::ranges::in_value_result<int, double> res{10, 0.};
+    assert(res.in == 10);
+    assert(res.value == 0.);
+    std::ranges::in_value_result<ConvertibleFrom<int>, ConvertibleFrom<double>> res2 = res;
+    assert(res2.in.content == 10);
+    assert(res2.value.content == 0.);
+  }
----------------
cjdb wrote:

I've applied this in #76534.

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