[libc++][spaceship] Implement `operator<=>` for `optional`

[H-G-Hristov]

Summary

Implements parts of P1614R2: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1614r2.html

• Implemented operator<=> for optional
• Updated "optional synopsis" to match the current draft https://eel.is/c++draft/optional closer
• Implemented https://cplusplus.github.io/LWG/issue3566
• Implemented https://cplusplus.github.io/LWG/issue3746

[H-G-Hristov]

• Updated SpaceshipProjects.csv

[H-G-Hristov]

Rebased

[H-G-Hristov]

• Fix CI

[H-G-Hristov]

Fix CI

[H-G-Hristov]

Run CI

[H-G-Hristov]

Run CI

[H-G-Hristov]

• Minor tweak

[H-G-Hristov]

Run CI

[H-G-Hristov]

Rebased

[philnik]

libcxx/docs/Status/SpaceshipProjects.csv

23–25

It looks like this patch implements LWG3566. If that's the case, please mark it as completed.

libcxx/include/optional

1316

Please clang-format the new code.

1419

This should just be an #else AFAICT.

1579

Please implement LWG3746.

libcxx/test/std/utilities/optional/optional.comp_with_t/compare.three_way.pass.cpp

26

Please add a SFINAE test for the three_way_comparable_with constraint.

29

Please also add tests for a user-defined type that is comparable to int.

libcxx/test/std/utilities/optional/optional.relops/compare.three_way.pass.cpp

27–30

Also, let's use two optionals to make sure it's not comparing addresses or something like that.

31

Please also add tests where one of the optionals has a value (both left and right).

[H-G-Hristov]

Thank you for the review.

• Addressed comments

[H-G-Hristov]

• Implemented LWG3746

[H-G-Hristov]

• Minor tweaks

[H-G-Hristov]

Ping @philnik

The CI issue is unrelated.

Sorry if personal pings are unacceptable!

[philnik]

In D146392#4301291, @H-G-Hristov wrote:

Ping @philnik

The CI issue is unrelated.

Sorry if personal pings are unacceptable!

That's fine (and even important in some cases), especially if the person requested changes.

LGTM with the last comment addressed.

libcxx/test/std/utilities/optional/optional.comp_with_t/compare.three_way.pass.cpp

29

I wanted you to test that it doesn't convert 'U' to 'T' :D (The other tests are also nice though)

35–76

I don't think any of this is necessary. You don't have to make it work like an int to make it comparable to an int.

[H-G-Hristov]

• Addressed comments

[H-G-Hristov]

Rebased

[H-G-Hristov]

Thank you for the review. I think I have addressed your comment. I just don't get what's going on with the CI. I haven't seen a green build for over a week.

[H-G-Hristov]

Rebased

[Mordante]

In D146392#4308372, @H-G-Hristov wrote:

Thank you for the review. I think I have addressed your comment. I just don't get what's going on with the CI. I haven't seen a green build for over a week.

The santizers are known to be buggy at the moment. The others should work again.

libcxx/include/optional

1419

Typically we use the same condition in the comments. I was confused you wanted to use operator<=> before C++20.

[H-G-Hristov]

• Addressed comments

[H-G-Hristov]

In D146392#4310020, @Mordante wrote:

In D146392#4308372, @H-G-Hristov wrote:

Thank you for the review. I think I have addressed your comment. I just don't get what's going on with the CI. I haven't seen a green build for over a week.

The santizers are known to be buggy at the moment. The others should work again.

@Mordante Thank you for the remark. It looks like I marked the comment as done but failed to do it.

In most of my revisions I see: merge_guards_bot fail and no other info or at least I don't know what to look for.

[ldionne]

LGTM. The CI is passing if you know where to look. Thanks a lot for the patch!

libcxx/docs/Status/SpaceshipProjects.csv

 | `[type.info] <https://wg21.link/type.info>`_,| `typeinfo <https://reviews.llvm.org/D130853>`_,None,Adrian Vogelsgesang,|Complete|
 | `[coroutine.handle.compare] <https://wg21.link/coroutine.handle.compare>`_,| `coroutine_handle <https://reviews.llvm.org/D109433>`_,[comparisons.three.way],Chuanqi Xu,|Complete|
 | `[pairs.spec] <https://wg21.link/pairs.spec>`_,| `pair <https://reviews.llvm.org/D107721>`_,[expos.only.func],Kent Ross,|Complete|
 | `[syserr.errcat.nonvirtuals] <https://wg21.link/syserr.errcat.nonvirtuals>`_,| `error_category <https://reviews.llvm.org/D131363>`_,[comparisons.three.way],Adrian Vogelsgesang,|Complete|
 | `[syserr.compare] <https://wg21.link/syserr.compare>`_,"| `error_code <https://reviews.llvm.org/D131371>`_
 | `error_condition <https://reviews.llvm.org/D131371>`_",None,Adrian Vogelsgesang,|Complete|
 | `[tuple.rel] <https://wg21.link/tuple.rel>`_,| `tuple <https://reviews.llvm.org/D108250>`_,[expos.only.func],Kent Ross,|Complete|
 "| `[optional.relops] <https://wg21.link/optional.relops>`_
 | `[optional.nullops] <https://wg21.link/optional.nullops>`_
-| `[optional.comp.with.t] <https://wg21.link/optional.comp.with.t>`_","| optional
-| nullopt",None,Kent Ross,|In Progress|
+| `[optional.comp.with.t] <https://wg21.link/optional.comp.with.t>`_","| `optional <https://reviews.llvm.org/D146392>`_
+| `nullopt <https://reviews.llvm.org/D146392>`_",None,Hristo Hristov,|Complete|
 "| `[variant.relops] <https://wg21.link/variant.relops>`_
 | `[variant.monostate.relops] <https://wg21.link/variant.monostate.relops>`_","| `monostate <https://reviews.llvm.org/D131372>`_
 | `variant <https://reviews.llvm.org/D131372>`_",None,Kent Ross,|Complete|
 | `[unique.ptr.special] <https://wg21.link/unique.ptr.special>`_,| `unique_ptr <https://reviews.llvm.org/D130838>`_,[comparisons.three.way],Adrian Vogelsgesang,|Complete|
 | `[util.smartptr.shared.cmp] <https://wg21.link/util.smartptr.shared.cmp>`_,| `shared_ptr <https://reviews.llvm.org/D130852>`_,[comparisons.three.way],Adrian Vogelsgesang,|Complete|
 | `[type.index.members] <https://wg21.link/type.index.members>`_,| `type_index <https://reviews.llvm.org/D131357>`_,None,Adrian Vogelsgesang,|Complete|
 | `[charconv.syn] <https://wg21.link/charconv.syn>`_,| `to_chars_result <https://reviews.llvm.org/D112366>`_,None,Mark de Wever,|Complete|
 | `[charconv.syn] <https://wg21.link/charconv.syn>`_,| `from_chars_result <https://reviews.llvm.org/D112366>`_,None,Mark de Wever,|Complete|

libcxx/include/optional

 #define _LIBCPP_OPTIONAL
 
 /*
     optional synopsis
 
 // C++1z
 
 namespace std {
-  // 23.6.3, optional for object types
+  // [optional.optional], class template optional
   template <class T> class optional;
 
-  // 23.6.4, no-value state indicator
+  // [optional.nullopt], no-value state indicator
   struct nullopt_t{see below };
   inline constexpr nullopt_t nullopt(unspecified );
 
-  // 23.6.5, class bad_optional_access
+  // [optional.bad.access], class bad_optional_access
   class bad_optional_access;
 
-  // 23.6.6, relational operators
+  // [optional.relops], relational operators
   template <class T, class U>
     constexpr bool operator==(const optional<T>&, const optional<U>&);
   template <class T, class U>
     constexpr bool operator!=(const optional<T>&, const optional<U>&);
   template <class T, class U>
     constexpr bool operator<(const optional<T>&, const optional<U>&);
   template <class T, class U>
     constexpr bool operator>(const optional<T>&, const optional<U>&);
   template <class T, class U>
     constexpr bool operator<=(const optional<T>&, const optional<U>&);
   template <class T, class U>
     constexpr bool operator>=(const optional<T>&, const optional<U>&);
+  template<class T, three_way_comparable_with<T> U>
+    constexpr compare_three_way_result_t<T, U>
+      operator<=>(const optional<T>&, const optional<U>&); // since C++20
 
-  // 23.6.7 comparison with nullopt
-  template <class T> constexpr bool operator==(const optional<T>&, nullopt_t) noexcept;
-  template <class T> constexpr bool operator==(nullopt_t, const optional<T>&) noexcept;
-  template <class T> constexpr bool operator!=(const optional<T>&, nullopt_t) noexcept;
-  template <class T> constexpr bool operator!=(nullopt_t, const optional<T>&) noexcept;
-  template <class T> constexpr bool operator<(const optional<T>&, nullopt_t) noexcept;
-  template <class T> constexpr bool operator<(nullopt_t, const optional<T>&) noexcept;
-  template <class T> constexpr bool operator<=(const optional<T>&, nullopt_t) noexcept;
-  template <class T> constexpr bool operator<=(nullopt_t, const optional<T>&) noexcept;
-  template <class T> constexpr bool operator>(const optional<T>&, nullopt_t) noexcept;
-  template <class T> constexpr bool operator>(nullopt_t, const optional<T>&) noexcept;
-  template <class T> constexpr bool operator>=(const optional<T>&, nullopt_t) noexcept;
-  template <class T> constexpr bool operator>=(nullopt_t, const optional<T>&) noexcept;
+  // [optional.nullops], comparison with nullopt
+  template<class T> constexpr bool operator==(const optional<T>&, nullopt_t) noexcept;
+  template<class T> constexpr bool operator==(nullopt_t, const optional<T>&) noexcept; // until C++17
+  template<class T> constexpr bool operator!=(const optional<T>&, nullopt_t) noexcept; // until C++17
+  template<class T> constexpr bool operator!=(nullopt_t, const optional<T>&) noexcept; // until C++17
+  template<class T> constexpr bool operator<(const optional<T>&, nullopt_t) noexcept;  // until C++17
+  template<class T> constexpr bool operator<(nullopt_t, const optional<T>&) noexcept;  // until C++17
+  template<class T> constexpr bool operator<=(const optional<T>&, nullopt_t) noexcept; // until C++17
+  template<class T> constexpr bool operator<=(nullopt_t, const optional<T>&) noexcept; // until C++17
+  template<class T> constexpr bool operator>(const optional<T>&, nullopt_t) noexcept;  // until C++17
+  template<class T> constexpr bool operator>(nullopt_t, const optional<T>&) noexcept;  // until C++17
+  template<class T> constexpr bool operator>=(const optional<T>&, nullopt_t) noexcept; // until C++17
+  template<class T> constexpr bool operator>=(nullopt_t, const optional<T>&) noexcept; // until C++17
+  template<class T>
+    constexpr strong_ordering operator<=>(const optional<T>&, nullopt_t) noexcept;     // since C++20
 
-  // 23.6.8, comparison with T
-  template <class T, class U> constexpr bool operator==(const optional<T>&, const U&);
-  template <class T, class U> constexpr bool operator==(const T&, const optional<U>&);
-  template <class T, class U> constexpr bool operator!=(const optional<T>&, const U&);
-  template <class T, class U> constexpr bool operator!=(const T&, const optional<U>&);
-  template <class T, class U> constexpr bool operator<(const optional<T>&, const U&);
-  template <class T, class U> constexpr bool operator<(const T&, const optional<U>&);
-  template <class T, class U> constexpr bool operator<=(const optional<T>&, const U&);
-  template <class T, class U> constexpr bool operator<=(const T&, const optional<U>&);
-  template <class T, class U> constexpr bool operator>(const optional<T>&, const U&);
-  template <class T, class U> constexpr bool operator>(const T&, const optional<U>&);
-  template <class T, class U> constexpr bool operator>=(const optional<T>&, const U&);
-  template <class T, class U> constexpr bool operator>=(const T&, const optional<U>&);
+  // [optional.comp.with.t], comparison with T
+  template<class T, class U> constexpr bool operator==(const optional<T>&, const U&);
+  template<class T, class U> constexpr bool operator==(const T&, const optional<U>&);
+  template<class T, class U> constexpr bool operator!=(const optional<T>&, const U&);
+  template<class T, class U> constexpr bool operator!=(const T&, const optional<U>&);
+  template<class T, class U> constexpr bool operator<(const optional<T>&, const U&);
+  template<class T, class U> constexpr bool operator<(const T&, const optional<U>&);
+  template<class T, class U> constexpr bool operator<=(const optional<T>&, const U&);
+  template<class T, class U> constexpr bool operator<=(const T&, const optional<U>&);
+  template<class T, class U> constexpr bool operator>(const optional<T>&, const U&);
+  template<class T, class U> constexpr bool operator>(const T&, const optional<U>&);
+  template<class T, class U> constexpr bool operator>=(const optional<T>&, const U&);
+  template<class T, class U> constexpr bool operator>=(const T&, const optional<U>&);
+  template<class T, class U>
+      requires (!is-derived-from-optional<U>) && three_way_comparable_with<T, U>
+    constexpr compare_three_way_result_t<T, U>
+      operator<=>(const optional<T>&, const U&);                                       // since C++20
 
-  // 23.6.9, specialized algorithms
-  template <class T> void swap(optional<T>&, optional<T>&) noexcept(see below ); // constexpr in C++20
-  template <class T> constexpr optional<see below > make_optional(T&&);
-  template <class T, class... Args>
+  // [optional.specalg], specialized algorithms
+  template<class T>
+    void swap(optional<T>&, optional<T>&) noexcept(see below ); // constexpr in C++20
+
+  template<class T>
+    constexpr optional<see below > make_optional(T&&);
+  template<class T, class... Args>
     constexpr optional<T> make_optional(Args&&... args);
-  template <class T, class U, class... Args>
+  template<class T, class U, class... Args>
     constexpr optional<T> make_optional(initializer_list<U> il, Args&&... args);
 
-  // 23.6.10, hash support
-  template <class T> struct hash;
-  template <class T> struct hash<optional<T>>;
+  // [optional.hash], hash support
+  template<class T> struct hash;
+  template<class T> struct hash<optional<T>>;
 
-  template <class T> class optional {
+  template<class T>
+  class optional {
   public:
     using value_type = T;
 
-    // 23.6.3.1, constructors
+    // [optional.ctor], constructors
     constexpr optional() noexcept;
     constexpr optional(nullopt_t) noexcept;
     constexpr optional(const optional &);
     constexpr optional(optional &&) noexcept(see below);
-    template <class... Args> constexpr explicit optional(in_place_t, Args &&...);
-    template <class U, class... Args>
+    template<class... Args>
+      constexpr explicit optional(in_place_t, Args &&...);
+    template<class U, class... Args>
       constexpr explicit optional(in_place_t, initializer_list<U>, Args &&...);
-    template <class U = T>
+    template<class U = T>
       constexpr explicit(see-below) optional(U &&);
-    template <class U>
-      explicit(see-below) optional(const optional<U> &);         // constexpr in C++20
-    template <class U>
-      explicit(see-below) optional(optional<U> &&);              // constexpr in C++20
+    template<class U>
+      explicit(see-below) optional(const optional<U> &);                          // constexpr in C++20
+    template<class U>
+      explicit(see-below) optional(optional<U> &&);                               // constexpr in C++20
 
-    // 23.6.3.2, destructor
+    // [optional.dtor], destructor
     ~optional(); // constexpr in C++20
 
-    // 23.6.3.3, assignment
-    optional &operator=(nullopt_t) noexcept;                     // constexpr in C++20
+    // [optional.assign], assignment
+    optional &operator=(nullopt_t) noexcept;                                      // constexpr in C++20
     constexpr optional &operator=(const optional &);
     constexpr optional &operator=(optional &&) noexcept(see below);
-    template <class U = T> optional &operator=(U &&);            // constexpr in C++20
-    template <class U> optional &operator=(const optional<U> &); // constexpr in C++20
-    template <class U> optional &operator=(optional<U> &&);      // constexpr in C++20
-    template <class... Args> T& emplace(Args &&...);             // constexpr in C++20
-    template <class U, class... Args>
-      T& emplace(initializer_list<U>, Args &&...);               // constexpr in C++20
+    template<class U = T> optional &operator=(U &&);                              // constexpr in C++20
+    template<class U> optional &operator=(const optional<U> &);                   // constexpr in C++20
+    template<class U> optional &operator=(optional<U> &&);                        // constexpr in C++20
+    template<class... Args> T& emplace(Args &&...);                               // constexpr in C++20
+    template<class U, class... Args> T& emplace(initializer_list<U>, Args &&...); // constexpr in C++20
 
-    // 23.6.3.4, swap
+    // [optional.swap], swap
     void swap(optional &) noexcept(see below ); // constexpr in C++20
 
-    // 23.6.3.5, observers
+    // [optional.observe], observers
     constexpr T const *operator->() const;
     constexpr T *operator->();
     constexpr T const &operator*() const &;
     constexpr T &operator*() &;
     constexpr T &&operator*() &&;
     constexpr const T &&operator*() const &&;
     constexpr explicit operator bool() const noexcept;
     constexpr bool has_value() const noexcept;
     constexpr T const &value() const &;
     constexpr T &value() &;
     constexpr T &&value() &&;
     constexpr const T &&value() const &&;
-    template <class U> constexpr T value_or(U &&) const &;
-    template <class U> constexpr T value_or(U &&) &&;
+    template<class U> constexpr T value_or(U &&) const &;
+    template<class U> constexpr T value_or(U &&) &&;
 
     // [optional.monadic], monadic operations
     template<class F> constexpr auto and_then(F&& f) &;         // since C++23
     template<class F> constexpr auto and_then(F&& f) &&;        // since C++23
     template<class F> constexpr auto and_then(F&& f) const&;    // since C++23
     template<class F> constexpr auto and_then(F&& f) const&&;   // since C++23
     template<class F> constexpr auto transform(F&& f) &;        // since C++23
     template<class F> constexpr auto transform(F&& f) &&;       // since C++23
     template<class F> constexpr auto transform(F&& f) const&;   // since C++23
     template<class F> constexpr auto transform(F&& f) const&&;  // since C++23
     template<class F> constexpr optional or_else(F&& f) &&;     // since C++23
     template<class F> constexpr optional or_else(F&& f) const&; // since C++23
 
-    // 23.6.3.6, modifiers
-    void reset() noexcept; // constexpr in C++20
+    // [optional.mod], modifiers
+    void reset() noexcept;                                      // constexpr in C++20
 
   private:
-    T *val; // exposition only
+    T *val;         // exposition only
   };
 
   template<class T>
     optional(T) -> optional<T>;
 
 } // namespace std
 
 */
 
 #include <__assert> // all public C++ headers provide the assertion handler
 #include <__availability>
+#include <__compare/compare_three_way_result.h>
+#include <__compare/three_way_comparable.h>
 #include <__concepts/invocable.h>
 #include <__config>
 #include <__functional/hash.h>
 #include <__functional/invoke.h>
 #include <__functional/unary_function.h>
 #include <__memory/addressof.h>
 #include <__memory/construct_at.h>
 #include <__tuple_dir/sfinae_helpers.h>

 {
     if (!static_cast<bool>(__y))
         return true;
     if (!static_cast<bool>(__x))
         return false;
     return *__x >= *__y;
 }
 
+#if _LIBCPP_STD_VER >= 20
+
+template<class _Tp, three_way_comparable_with<_Tp> _Up>
+_LIBCPP_HIDE_FROM_ABI
+constexpr compare_three_way_result_t<_Tp, _Up>
+operator<=>(const optional<_Tp>& __x, const optional<_Up>& __y)
+{
+    if (__x && __y)
+        return *__x <=> *__y;
+    return __x.has_value() <=> __y.has_value();
+}
+
+#endif
+
 // Comparisons with nullopt
 template <class _Tp>
 _LIBCPP_INLINE_VISIBILITY constexpr
 bool
 operator==(const optional<_Tp>& __x, nullopt_t) noexcept
 {
     return !static_cast<bool>(__x);
 }
 
+#if _LIBCPP_STD_VER <= 17
+
 template <class _Tp>
 _LIBCPP_INLINE_VISIBILITY constexpr
 bool
 operator==(nullopt_t, const optional<_Tp>& __x) noexcept
 {
     return !static_cast<bool>(__x);
 }
 

 template <class _Tp>
 _LIBCPP_INLINE_VISIBILITY constexpr
 bool
 operator>=(nullopt_t, const optional<_Tp>& __x) noexcept
 {
     return !static_cast<bool>(__x);
 }
 
+#elif _LIBCPP_STD_VER >= 20
+
+template<class _Tp>
+_LIBCPP_HIDE_FROM_ABI
+constexpr strong_ordering
+operator<=>(const optional<_Tp>& __x, nullopt_t) noexcept
+{
+    return __x.has_value() <=> false;
+}
+
+#endif
+
 // Comparisons with T
 template <class _Tp, class _Up>
 _LIBCPP_INLINE_VISIBILITY constexpr
 enable_if_t<
     is_convertible_v<decltype(std::declval<const _Tp&>() ==
         std::declval<const _Up&>()), bool>,
     bool
 >
is_convertible_v<decltype(std::declval<const _Tp&>() >=
         std::declval<const _Up&>()), bool>,
     bool
 >
 operator>=(const _Tp& __v, const optional<_Up>& __x)
 {
     return static_cast<bool>(__x) ? __v >= *__x : true;
 }
 
+#if _LIBCPP_STD_VER >= 20
+
+template<class _Tp, class _Up>
+    requires (!__is_std_optional<_Up>::value) && three_way_comparable_with<_Tp, _Up>
+_LIBCPP_HIDE_FROM_ABI
+constexpr compare_three_way_result_t<_Tp, _Up>
+operator<=>(const optional<_Tp>& __x, const _Up& __v)
+{
+    return __x.has_value() ? *__x <=> __v : strong_ordering::less;
+}
+
+#endif
+
 
 template <class _Tp>
 inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_SINCE_CXX20
 enable_if_t<
     is_move_constructible_v<_Tp> && is_swappable_v<_Tp>,
     void
 >
 swap(optional<_Tp>& __x, optional<_Tp>& __y) noexcept(noexcept(__x.swap(__y)))

libcxx/test/std/utilities/optional/optional.comp_with_t/compare.three_way.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// 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
+
+// <optional>
+
+// [optional.comp.with.t], comparison with T
+
+// template<class T, class U>
+//     requires (!is-derived-from-optional<U>) && three_way_comparable_with<T, U>
+//   constexpr compare_three_way_result_t<T, U>
+//     operator<=>(const optional<T>&, const U&);
+
+#include <cassert>
+#include <compare>
+#include <optional>
+
+#include "test_comparisons.h"
+
+constexpr bool test() {
+  // [optional.comp.with.t], comparison with T
+  {
+    int t{2};
+    std::optional<int> op{3};
+    assert((t <=> op) == std::strong_ordering::less);
+    assert(testOrder(t, op, std::strong_ordering::less));
+  }
+  {
+    int t{3};
+    std::optional<int> op{2};
+    assert((t <=> op) == std::strong_ordering::greater);
+    assert(testOrder(t, op, std::strong_ordering::greater));
+  }
+  {
+    int t{3};
+    std::optional<int> op{3};
+    assert((t <=> op) == std::strong_ordering::equal);
+    assert(testOrder(t, op, std::strong_ordering::equal));
+  }
+
+  return true;
+}
+
+int main() {
+  assert(test());
+  static_assert(test());
+  return 0;
+}

libcxx/test/std/utilities/optional/optional.nullops/compare.three_way.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// 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
+
+// <optional>
+
+// [optional.nullops], comparison with nullopt
+
+// template<class T>
+//   constexpr strong_ordering operator<=>(const optional<T>&, nullopt_t) noexcept;
+
+#include <cassert>
+#include <compare>
+#include <optional>
+
+#include "test_comparisons.h"
+
+constexpr bool test() {
+  // [optional.nullops], comparison with nullopt
+  {
+    std::optional<int> op{1};
+    assert((std::nullopt <=> op) == std::strong_ordering::less);
+    assert(testOrder(std::nullopt, op, std::strong_ordering::less));
+  }
+  {
+    std::optional<int> op{1};
+    assert((op <=> std::nullopt) == std::strong_ordering::greater);
+    assert(testOrder(op, std::nullopt, std::strong_ordering::greater));
+  }
+  {
+    std::optional<int> op;
+    assert((std::nullopt <=> op) == std::strong_ordering::equal);
+    assert(testOrder(std::nullopt, op, std::strong_ordering::equal));
+    assert((op <=> std::nullopt) == std::strong_ordering::equal);
+    assert(testOrder(op, std::nullopt, std::strong_ordering::equal));
+  }
+
+  return true;
+}
+
+int main() {
+  assert(test());
+  static_assert(test());
+  return 0;
+}

libcxx/test/std/utilities/optional/optional.relops/compare.three_way.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// 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
+
+// <optional>
+
+// [optional.relops], relational operators
+
+// template<class T, three_way_comparable_with<T> U>
+//   constexpr compare_three_way_result_t<T, U>
+//     operator<=>(const optional<T>&, const optional<U>&);
+
+#include <cassert>
+#include <compare>
+#include <optional>
+
+#include "test_comparisons.h"
+
+constexpr bool test() {
+  // [optional.relops], relational operators
+  std::optional<int> op;
+
+  {
+    std::optional<int> op1{2};
+    std::optional<int> op2{3};
+    assert((op <=> op2) == std::strong_ordering::less);
+    assert(testOrder(op, op2, std::strong_ordering::less));
+    assert((op1 <=> op2) == std::strong_ordering::less);
+    assert(testOrder(op1, op2, std::strong_ordering::less));
+  }
+  {
+    std::optional<int> op1{3};
+    std::optional<int> op2{2};
+    assert((op1 <=> op) == std::strong_ordering::greater);
+    assert(testOrder(op1, op, std::strong_ordering::greater));
+    assert((op1 <=> op2) == std::strong_ordering::greater);
+    assert(testOrder(op1, op2, std::strong_ordering::greater));
+  }
+  {
+    assert((op <=> op) == std::strong_ordering::equal);
+    assert(testOrder(op, op, std::strong_ordering::equal));
+
+    std::optional<int> op1{3};
+    std::optional<int> op2{3};
+    assert((op1 <=> op1) == std::strong_ordering::equal);
+    assert(testOrder(op1, op1, std::strong_ordering::equal));
+    assert((op1 <=> op2) == std::strong_ordering::equal);
+    assert(testOrder(op1, op2, std::strong_ordering::equal));
+  }
+
+  return true;
+}
+
+int main() {
+  assert(test());
+  static_assert(test());
+  return 0;
+}