[libcxx-commits] [libcxx] Make std::nullopt_t comparable (PR #195549)

[Mohamed Atef via libcxx-commits]

https://github.com/elhewaty updated https://github.com/llvm/llvm-project/pull/195549

>From 748eaa99ffcb499fa4ef42655d5ff892af913067 Mon Sep 17 00:00:00 2001
From: Mohamed Atef <mohamedatef1698 at gmail.com>
Date: Sun, 3 May 2026 23:22:57 +0300
Subject: [PATCH] [libcxx] Make std::nullopt_t comparable

---
 libcxx/docs/Status/Cxx2cIssues.csv            |  2 +-
 libcxx/include/optional                       |  3 ++
 .../optional.nullopt/nullopt_t.pass.cpp       | 49 +++++++++++++------
 3 files changed, 38 insertions(+), 16 deletions(-)

diff --git a/libcxx/docs/Status/Cxx2cIssues.csv b/libcxx/docs/Status/Cxx2cIssues.csv
index e825e9633bdb9..ef17e8261ebe2 100644
--- a/libcxx/docs/Status/Cxx2cIssues.csv
+++ b/libcxx/docs/Status/Cxx2cIssues.csv
@@ -309,7 +309,7 @@
 "`LWG4492 <https://wg21.link/LWG4492>`__","``std::generate`` and ``std::ranges::generate`` wording is unclear for parallel algorithms","2026-03 (Croydon)","","","`#189851 <https://github.com/llvm/llvm-project/issues/189851>`__",""
 "`LWG4493 <https://wg21.link/LWG4493>`__","Specification for some functions of bit reference types seems missing","2026-03 (Croydon)","","","`#189852 <https://github.com/llvm/llvm-project/issues/189852>`__",""
 "`LWG4496 <https://wg21.link/LWG4496>`__","Precedes vs Reachable in [meta.reflection]","2026-03 (Croydon)","","","`#189853 <https://github.com/llvm/llvm-project/issues/189853>`__",""
-"`LWG4497 <https://wg21.link/LWG4497>`__","``std::nullopt_t`` should be comparable","2026-03 (Croydon)","","","`#189854 <https://github.com/llvm/llvm-project/issues/189854>`__",""
+"`LWG4497 <https://wg21.link/LWG4497>`__","``std::nullopt_t`` should be comparable","2026-03 (Croydon)","|Complete|","23","`#189854 <https://github.com/llvm/llvm-project/issues/189854>`__",""
 "`LWG4499 <https://wg21.link/LWG4499>`__","``flat_set::insert_range`` specification may be problematic","2026-03 (Croydon)","","","`#189855 <https://github.com/llvm/llvm-project/issues/189855>`__",""
 "`LWG4500 <https://wg21.link/LWG4500>`__","``constant_wrapper`` wording problems","2026-03 (Croydon)","|Complete|","23","`#189856 <https://github.com/llvm/llvm-project/issues/189856>`__",""
 "`LWG4504 <https://wg21.link/LWG4504>`__","Wording problem in ``{simple_}counting_scope``","2026-03 (Croydon)","","","`#189857 <https://github.com/llvm/llvm-project/issues/189857>`__",""
diff --git a/libcxx/include/optional b/libcxx/include/optional
index f7e12f84beba0..7705ba49c49e7 100644
--- a/libcxx/include/optional
+++ b/libcxx/include/optional
@@ -362,6 +362,9 @@ struct nullopt_t {
     explicit __secret_tag() = default;
   };
   _LIBCPP_HIDE_FROM_ABI constexpr explicit nullopt_t(__secret_tag, __secret_tag) noexcept {}
+#    if _LIBCPP_STD_VER >= 26
+  _LIBCPP_HIDE_FROM_ABI constexpr strong_ordering operator<=>(const nullopt_t&) const noexcept = default;
+#    endif
 };
 
 inline constexpr nullopt_t nullopt{nullopt_t::__secret_tag{}, nullopt_t::__secret_tag{}};
diff --git a/libcxx/test/std/utilities/optional/optional.nullopt/nullopt_t.pass.cpp b/libcxx/test/std/utilities/optional/optional.nullopt/nullopt_t.pass.cpp
index 842b75893cbe4..a580348e0da9a 100644
--- a/libcxx/test/std/utilities/optional/optional.nullopt/nullopt_t.pass.cpp
+++ b/libcxx/test/std/utilities/optional/optional.nullopt/nullopt_t.pass.cpp
@@ -13,31 +13,50 @@
 // inline constexpr nullopt_t nullopt(unspecified);
 
 // [optional.nullopt]/2:
-//   Type nullopt_t shall not have a default constructor or an initializer-list
-//   constructor, and shall not be an aggregate.
+//   Type nullopt_t does not have a default constructor or an initializer-list
+//   constructor, and is not an aggregate. nullopt_t models copyable and
+//   three_way_comparable<strong_ordering>.
 
 #include <optional>
 #include <type_traits>
+#if _LIBCPP_STD_VER >= 26
+#  include <vector>
+#  include <ranges>
+#  include <cassert>
+#  include <algorithm>
+#endif
 
 #include "test_macros.h"
 
-using std::nullopt_t;
 using std::nullopt;
+using std::nullopt_t;
 
-constexpr bool test()
-{
-    nullopt_t foo{nullopt};
-    (void)foo;
-    return true;
+constexpr bool test() {
+  nullopt_t foo{nullopt};
+  (void)foo;
+  return true;
 }
 
-int main(int, char**)
-{
-    static_assert(std::is_empty_v<nullopt_t>);
-    static_assert(!std::is_default_constructible_v<nullopt_t>);
-
-    static_assert(std::is_same_v<const nullopt_t, decltype(nullopt)>);
-    static_assert(test());
+int main(int, char**) {
+  static_assert(std::is_empty_v<nullopt_t>);
+  static_assert(!std::is_default_constructible_v<nullopt_t>);
+
+  static_assert(std::is_same_v<const nullopt_t, decltype(nullopt)>);
+  static_assert(test());
+#if TEST_STD_VER >= 26
+  // Test comparisons between nullopt_t
+  static_assert(nullopt == nullopt);
+  static_assert(!(nullopt != nullopt));
+  static_assert(nullopt <= nullopt);
+  static_assert(nullopt >= nullopt);
+  static_assert((nullopt <=> nullopt) == std::strong_ordering::equal);
+
+  // Test ranges::find with nullopt
+  std::vector<std::optional<int>> v = {1, 2, nullopt, 4, 5};
+  auto itr                          = std::ranges::find(v, nullopt);
+  assert(itr != v.end());
+  assert(*itr == nullopt);
+#endif
 
   return 0;
 }