Add enable_borrowed_range<optional<T&>> and corresponding test

Author: smallp-o-p

libcxx/include/optional

@@ -210,6 +210,7 @@ namespace std {
 #  include <__iterator/wrap_iter.h>
 #  include <__memory/addressof.h>
 #  include <__memory/construct_at.h>
+#  include <__ranges/enable_borrowed_range.h>
 #  include <__ranges/enable_view.h>
 #  include <__tuple/sfinae_helpers.h>
 #  include <__type_traits/add_pointer.h>
@@ -594,6 +595,10 @@ constexpr bool ranges::enable_view<optional<_Tp>> = true;
 
 template <class _Tp>
 constexpr range_format format_kind<optional<_Tp>> = range_format::disabled;
+
+template <class _Tp>
+constexpr bool ranges::enable_borrowed_range<optional<_Tp&>> = true;
+
 #    endif
 
 #    if _LIBCPP_STD_VER >= 20

libcxx/test/std/utilities/optional/optional.iterator/borrowed_range.pass.cpp

@@ -0,0 +1,74 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: std-at-least-c++26
+
+// <optional>
+
+// template <class T> class optional<T&>::iterator;
+// template <class T> class optional<T&>::const_iterator;
+// template <class T>
+// constexpr bool ranges::enable_borrowed_range<optional<T&>> = true;
+
+#include <cassert>
+#include <optional>
+#include <ranges>
+
+template <typename T>
+constexpr bool enable_borrowed_range() {
+  {
+    assert(std::ranges::enable_borrowed_range<std::optional<T&>>);
+  }
+  return true;
+}
+
+template <typename T>
+constexpr bool borrowed_range() {
+  if (std::ranges::range<std::optional<T&>>) {
+    assert(std::ranges::borrowed_range<std::optional<T&>>);
+  } else {
+    assert(!std::ranges::borrowed_range<std::optional<T&>>);
+    return false;
+  }
+
+  return true;
+}
+
+constexpr bool test_enable_borrowed_range() {
+  assert(enable_borrowed_range<int>());
+  assert(enable_borrowed_range<const int>());
+  assert(enable_borrowed_range<int[]>());
+  assert(enable_borrowed_range<int[10]>());
+  assert(enable_borrowed_range<int()>());
+
+  return true;
+}
+
+constexpr bool test_borrowed_range() {
+  assert(borrowed_range<int>());
+  assert(borrowed_range<const int>());
+  assert(!borrowed_range<int[]>());
+  assert(!borrowed_range<int[10]>());
+  assert(!borrowed_range<int()>());
+
+  return true;
+}
+
+int main(int, char**) {
+  {
+    static_assert(test_enable_borrowed_range());
+    assert(test_enable_borrowed_range());
+  }
+
+  {
+    static_assert(test_borrowed_range());
+    assert(test_enable_borrowed_range());
+  }
+
+  return 0;
+}