[libc++][spaceship] Implement std::pair::operator<=>

Implements parts of P1614, including synth-three-way and three way comparison for std::pair.

Reviewed By: #libc, Quuxplusone, Mordante

Differential Revision: https://reviews.llvm.org/D107721

libcxx/docs/Status/SpaceshipProjects.csv

@@ -3,14 +3,14 @@ Section,Description,Dependencies,Assignee,Complete
 | `three_way_comparable_with <https://reviews.llvm.org/D103478>`_",None,Ruslan Arutyunyan,|In Progress|
 | `[cmp.result] <https://wg21.link/cmp.result>`_,| `compare_three_way_result <https://reviews.llvm.org/D103581>`_,None,Arthur O'Dwyer,|Complete|
 | `[expos.only.func] <https://wg21.link/expos.only.func>`_,"| `synth-three-way <https://reviews.llvm.org/D107721>`_
-| `synth-three-way-result <https://reviews.llvm.org/D107721>`_",[cmp.concept],Kent Ross,|In Progress|
+| `synth-three-way-result <https://reviews.llvm.org/D107721>`_",[cmp.concept],Kent Ross,|Complete|
 | `[comparisons.three.way] <https://wg21.link/comparisons.three.way>`_,| `compare_three_way <https://reviews.llvm.org/D80899>`_,[cmp.concept],Christopher Di Bella,|In Progress|
 | `[cmp.alg] <https://wg21.link/cmp.alg>`_,"| `strong_order <https://reviews.llvm.org/D107036>`_
 | `weak_order <https://reviews.llvm.org/D107036>`_
 | `partial_order <https://reviews.llvm.org/D107036>`_",None,Arthur O'Dwyer,|In Progress|
 | `[alg.three.way] <https://wg21.link/alg.three.way>`_,| `lexicographical_compare_three_way <https://reviews.llvm.org/D80902>`_,[comparisons.three.way],Christopher Di Bella,|In Progress|
 | `[coroutine.handle.compare] <https://wg21.link/coroutine.handle.compare>`_,| coroutine_handle,[comparisons.three.way],Unassigned,|Not Started|
-| `[pairs.spec] <https://wg21.link/pairs.spec>`_,| `pair <https://reviews.llvm.org/D107721>`_,[expos.only.func],Kent Ross,|In Progress|
+| `[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,[comparisons.three.way],Unassigned,|Not Started|
 | `[syserr.compare] <https://wg21.link/syserr.compare>`_,"| error_code
 | error_condition",None,Unassigned,|Not Started|

libcxx/include/CMakeLists.txt

@@ -104,6 +104,7 @@ set(files
   __compare/common_comparison_category.h
   __compare/compare_three_way_result.h
   __compare/ordering.h
+  __compare/synth_three_way.h
   __compare/three_way_comparable.h
   __concepts/arithmetic.h
   __concepts/assignable.h

libcxx/include/__compare/synth_three_way.h

@@ -0,0 +1,51 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___COMPARE_SYNTH_THREE_WAY_H
+#define _LIBCPP___COMPARE_SYNTH_THREE_WAY_H
+
+#include <__config>
+#include <__compare/ordering.h>
+#include <__compare/three_way_comparable.h>
+#include <__concepts/boolean_testable.h>
+#include <__utility/declval.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_SPACESHIP_OPERATOR) && !defined(_LIBCPP_HAS_NO_CONCEPTS)
+
+// [expos.only.func]
+
+_LIBCPP_HIDE_FROM_ABI inline constexpr auto __synth_three_way =
+  []<class _Tp, class _Up>(const _Tp& __t, const _Up& __u)
+    requires requires {
+      { __t < __u } -> __boolean_testable;
+      { __u < __t } -> __boolean_testable;
+    }
+  {
+    if constexpr (three_way_comparable_with<_Tp, _Up>) {
+      return __t <=> __u;
+    } else {
+      if (__t < __u) return weak_ordering::less;
+      if (__u < __t) return weak_ordering::greater;
+      return weak_ordering::equivalent;
+    }
+  };
+
+template <class _Tp, class _Up = _Tp>
+using __synth_three_way_result = decltype(__synth_three_way(declval<_Tp&>(), declval<_Up&>()));
+
+#endif // _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_SPACESHIP_OPERATOR) && !defined(_LIBCPP_HAS_NO_CONCEPTS)
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___COMPARE_SYNTH_THREE_WAY_H

libcxx/include/__utility/pair.h

@@ -9,6 +9,8 @@
 #ifndef _LIBCPP___UTILITY_PAIR_H
 #define _LIBCPP___UTILITY_PAIR_H
 
+#include <__compare/common_comparison_category.h>
+#include <__compare/synth_three_way.h>
 #include <__config>
 #include <__functional/unwrap_ref.h>
 #include <__tuple>
@@ -308,6 +310,8 @@ template<class _T1, class _T2>
 pair(_T1, _T2) -> pair<_T1, _T2>;
 #endif
 
+// [pairs.spec], specialized algorithms
+
 template <class _T1, class _T2>
 inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
 bool
@@ -316,6 +320,23 @@ operator==(const pair<_T1,_T2>& __x, const pair<_T1,_T2>& __y)
     return __x.first == __y.first && __x.second == __y.second;
 }
 
+#if _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_SPACESHIP_OPERATOR) && !defined(_LIBCPP_HAS_NO_CONCEPTS)
+
+template <class _T1, class _T2>
+_LIBCPP_HIDE_FROM_ABI constexpr
+common_comparison_category_t<
+        __synth_three_way_result<_T1>,
+        __synth_three_way_result<_T2> >
+operator<=>(const pair<_T1,_T2>& __x, const pair<_T1,_T2>& __y)
+{
+    if (auto __c = _VSTD::__synth_three_way(__x.first, __y.first); __c != 0) {
+      return __c;
+    }
+    return _VSTD::__synth_three_way(__x.second, __y.second);
+}
+
+#else // _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_SPACESHIP_OPERATOR) && !defined(_LIBCPP_HAS_NO_CONCEPTS)
+
 template <class _T1, class _T2>
 inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
 bool
@@ -356,6 +377,8 @@ operator<=(const pair<_T1,_T2>& __x, const pair<_T1,_T2>& __y)
     return !(__y < __x);
 }
 
+#endif // _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_SPACESHIP_OPERATOR) && !defined(_LIBCPP_HAS_NO_CONCEPTS)
+
 template <class _T1, class _T2>
 inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 typename enable_if

libcxx/include/module.modulemap

@@ -374,6 +374,7 @@ module std [system] {
       module common_comparison_category { private header "__compare/common_comparison_category.h" }
       module compare_three_way_result   { private header "__compare/compare_three_way_result.h"   }
       module ordering                   { private header "__compare/ordering.h"                   }
+      module synth_three_way            { private header "__compare/synth_three_way.h"            }
       module three_way_comparable       { private header "__compare/three_way_comparable.h"       }
     }
   }

libcxx/include/utility

@@ -96,11 +96,15 @@ struct pair
 };
 
 template <class T1, class T2> bool operator==(const pair<T1,T2>&, const pair<T1,T2>&); // constexpr in C++14
-template <class T1, class T2> bool operator!=(const pair<T1,T2>&, const pair<T1,T2>&); // constexpr in C++14
-template <class T1, class T2> bool operator< (const pair<T1,T2>&, const pair<T1,T2>&); // constexpr in C++14
-template <class T1, class T2> bool operator> (const pair<T1,T2>&, const pair<T1,T2>&); // constexpr in C++14
-template <class T1, class T2> bool operator>=(const pair<T1,T2>&, const pair<T1,T2>&); // constexpr in C++14
-template <class T1, class T2> bool operator<=(const pair<T1,T2>&, const pair<T1,T2>&); // constexpr in C++14
+template <class T1, class T2> bool operator!=(const pair<T1,T2>&, const pair<T1,T2>&); // constexpr in C++14, removed in C++20
+template <class T1, class T2> bool operator< (const pair<T1,T2>&, const pair<T1,T2>&); // constexpr in C++14, removed in C++20
+template <class T1, class T2> bool operator> (const pair<T1,T2>&, const pair<T1,T2>&); // constexpr in C++14, removed in C++20
+template <class T1, class T2> bool operator>=(const pair<T1,T2>&, const pair<T1,T2>&); // constexpr in C++14, removed in C++20
+template <class T1, class T2> bool operator<=(const pair<T1,T2>&, const pair<T1,T2>&); // constexpr in C++14, removed in C++20
+template <class T1, class T2>
+  constexpr common_comparison_type_t<synth-three-way-result<T1>,
+                                     synth-three-way-result<T2>>
+    operator<=>(const pair<T1,T2>&, const pair<T1,T2>&);                               // C++20
 
 template <class T1, class T2> pair<V1, V2> make_pair(T1&&, T2&&);                // constexpr in C++14
 template <class T1, class T2>

libcxx/test/libcxx/diagnostics/detail.headers/compare/synth_three_way.module.verify.cpp

@@ -0,0 +1,16 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// 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: modules-build
+
+// WARNING: This test was generated by 'generate_private_header_tests.py'
+// and should not be edited manually.
+
+// expected-error@*:* {{use of private header from outside its module: '__compare/synth_three_way.h'}}
+#include <__compare/synth_three_way.h>

libcxx/test/libcxx/library/description/conventions/expos.only.func/synth_three_way.pass.cpp

@@ -0,0 +1,166 @@
+//===----------------------------------------------------------------------===//
+//
+// 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, libcpp-no-concepts
+// ADDITIONAL_COMPILE_FLAGS: -Wno-sign-compare
+
+// constexpr auto __synth_three_way = ...;
+
+#include <cassert>
+#include <compare>
+#include <limits>
+#include <utility> // Includes synth-three-way via std::pair::operator<=>
+
+#include "test_macros.h"
+
+template <typename T> concept can_synth_three_way = requires(T t) { std::__synth_three_way(t, t); };
+
+// A custom three-way result type
+struct CustomEquality {
+  friend constexpr bool operator==(const CustomEquality&, int) noexcept { return true; }
+  friend constexpr bool operator<(const CustomEquality&, int) noexcept { return false; }
+  friend constexpr bool operator<(int, const CustomEquality&) noexcept { return false; }
+};
+
+constexpr bool test() {
+  {
+    assert(std::__synth_three_way(1, 1) == std::strong_ordering::equal);
+    assert(std::__synth_three_way(2, 1) == std::strong_ordering::greater);
+    assert(std::__synth_three_way(1, 2) == std::strong_ordering::less);
+    ASSERT_SAME_TYPE(std::strong_ordering, std::__synth_three_way_result<int, int>);
+    ASSERT_SAME_TYPE(std::strong_ordering, std::__synth_three_way_result<short, long long int>);
+  }
+  {
+    constexpr double nan = std::numeric_limits<double>::quiet_NaN();
+    assert(std::__synth_three_way(1.0, 1.0) == std::partial_ordering::equivalent);
+    assert(std::__synth_three_way(2.0, 1.0) == std::partial_ordering::greater);
+    assert(std::__synth_three_way(1.0, 2.0) == std::partial_ordering::less);
+    assert(std::__synth_three_way(nan, nan) == std::partial_ordering::unordered);
+    ASSERT_SAME_TYPE(std::partial_ordering, std::__synth_three_way_result<double, double>);
+    ASSERT_SAME_TYPE(std::partial_ordering, std::__synth_three_way_result<double, float>);
+    ASSERT_SAME_TYPE(std::partial_ordering, std::__synth_three_way_result<double, int>);
+    ASSERT_SAME_TYPE(std::partial_ordering, std::__synth_three_way_result<float, short>);
+  }
+  {
+    struct StrongSpaceship {
+      int value;
+      constexpr bool operator==(const StrongSpaceship&) const = default;
+      constexpr std::strong_ordering operator<=>(const StrongSpaceship& other) const { return value <=> other.value; }
+    };
+    assert(std::__synth_three_way(StrongSpaceship{1}, StrongSpaceship{1}) == std::strong_ordering::equal);
+    assert(std::__synth_three_way(StrongSpaceship{2}, StrongSpaceship{1}) == std::strong_ordering::greater);
+    assert(std::__synth_three_way(StrongSpaceship{1}, StrongSpaceship{2}) == std::strong_ordering::less);
+    ASSERT_SAME_TYPE(std::strong_ordering, std::__synth_three_way_result<StrongSpaceship, StrongSpaceship>);
+  }
+  {
+    struct WeakSpaceship {
+      int value;
+      constexpr bool operator==(const WeakSpaceship&) const = default;
+      constexpr std::weak_ordering operator<=>(const WeakSpaceship& other) const {
+        return value <=> other.value;
+      }
+    };
+    assert(std::__synth_three_way(WeakSpaceship{1}, WeakSpaceship{1}) == std::weak_ordering::equivalent);
+    assert(std::__synth_three_way(WeakSpaceship{2}, WeakSpaceship{1}) == std::weak_ordering::greater);
+    assert(std::__synth_three_way(WeakSpaceship{1}, WeakSpaceship{2}) == std::weak_ordering::less);
+    ASSERT_SAME_TYPE(std::weak_ordering, std::__synth_three_way_result<WeakSpaceship, WeakSpaceship>);
+  }
+  {
+    struct PartialSpaceship {
+      double value;
+      constexpr bool operator==(const PartialSpaceship&) const = default;
+      constexpr std::partial_ordering operator<=>(const PartialSpaceship& other) const {
+        return value <=> other.value;
+      }
+    };
+    constexpr double nan = std::numeric_limits<double>::quiet_NaN();
+    assert(std::__synth_three_way(PartialSpaceship{1.0}, PartialSpaceship{1.0}) == std::partial_ordering::equivalent);
+    assert(std::__synth_three_way(PartialSpaceship{2.0}, PartialSpaceship{1.0}) == std::partial_ordering::greater);
+    assert(std::__synth_three_way(PartialSpaceship{1.0}, PartialSpaceship{2.0}) == std::partial_ordering::less);
+    assert(std::__synth_three_way(PartialSpaceship{nan}, PartialSpaceship{nan}) == std::partial_ordering::unordered);
+    ASSERT_SAME_TYPE(std::partial_ordering, std::__synth_three_way_result<PartialSpaceship, PartialSpaceship>);
+  }
+  {
+    struct NoSpaceship {
+      int value;
+      constexpr bool operator==(const NoSpaceship&) const = default;
+      constexpr bool operator<(const NoSpaceship& other) const { return value < other.value; }
+    };
+    assert(std::__synth_three_way(NoSpaceship{1}, NoSpaceship{1}) == std::weak_ordering::equivalent);
+    assert(std::__synth_three_way(NoSpaceship{2}, NoSpaceship{1}) == std::weak_ordering::greater);
+    assert(std::__synth_three_way(NoSpaceship{1}, NoSpaceship{2}) == std::weak_ordering::less);
+    ASSERT_SAME_TYPE(std::weak_ordering, std::__synth_three_way_result<NoSpaceship, NoSpaceship>);
+  }
+  {
+    // Types with operator<=> but no operator== are not three_way_comparable and will fall back to operator< and
+    // compare as weakly ordered.
+    struct SpaceshipNoEquals {
+      constexpr std::strong_ordering operator<=>(const SpaceshipNoEquals&) const {
+        return std::strong_ordering::equivalent;
+      }
+    };
+    assert(std::__synth_three_way(SpaceshipNoEquals{}, SpaceshipNoEquals{}) == std::weak_ordering::equivalent);
+    ASSERT_SAME_TYPE(std::weak_ordering, std::__synth_three_way_result<SpaceshipNoEquals, SpaceshipNoEquals>);
+  }
+  {
+    // Custom three-way-comparison result types cannot satisfy standard concepts (and therefore synth-three-way)
+    // because they are not understood by std::common_comparison_category, but they can still be used in
+    // the same way as standard orderings to do comparisons, and thus can be used by synth-three-way to yield a
+    // weakly-ordered result.
+    struct CustomSpaceship {
+      constexpr CustomEquality operator<=>(const CustomSpaceship&) const { return CustomEquality(); }
+    };
+    assert((CustomSpaceship() <=> CustomSpaceship()) == 0);
+    assert(!(CustomSpaceship() < CustomSpaceship()));
+    assert(std::__synth_three_way(CustomSpaceship(), CustomSpaceship()) == std::weak_ordering::equivalent);
+    ASSERT_SAME_TYPE(std::weak_ordering, std::__synth_three_way_result<CustomSpaceship, CustomSpaceship>);
+  }
+  // SFINAE tests demonstrating synth-three-way needs three_way_comparable or operator<.
+  {
+    struct NoRelative {
+      constexpr bool operator==(const NoRelative&) const;
+    };
+    static_assert(!can_synth_three_way<NoRelative>);
+  }
+  {
+    struct NoLessThan {
+      constexpr bool operator==(const NoLessThan&) const;
+      constexpr bool operator>(const NoLessThan&) const;
+      constexpr bool operator>=(const NoLessThan&) const;
+      constexpr bool operator<=(const NoLessThan&) const;
+    };
+    static_assert(!can_synth_three_way<NoLessThan>);
+  }
+  {
+    assert(std::__synth_three_way(1, 1U) == std::weak_ordering::equivalent);
+    assert(std::__synth_three_way(-1, 0U) == std::weak_ordering::greater);
+    // Even with the warning suppressed (-Wno-sign-compare) there should still be no <=> operator
+    // between signed and unsigned types, so we should end up with a synthesized weak ordering.
+    ASSERT_SAME_TYPE(std::weak_ordering, std::__synth_three_way_result<int, unsigned int>);
+    // When an unsigned type can be narrowed to a larger signed type, <=> should be defined and we
+    // should get a strong ordering. (This probably does not raise a warning due to safe narrowing.)
+    assert((static_cast<long long int>(-1) <=> static_cast<unsigned char>(0)) == std::strong_ordering::less);
+    assert(std::__synth_three_way(static_cast<long long int>(-1),
+                                  static_cast<unsigned char>(0)) == std::strong_ordering::less);
+    ASSERT_SAME_TYPE(std::strong_ordering, std::__synth_three_way_result<long long int, unsigned char>);
+  }
+
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+
+  {
+    constexpr double nan = std::numeric_limits<double>::quiet_NaN();
+    assert(std::__synth_three_way(nan, 1.0) == std::partial_ordering::unordered);
+  }
+
+  return 0;
+}

libcxx/test/std/utilities/utility/pairs/pairs.spec/three_way_comparison.pass.cpp

@@ -0,0 +1,115 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// <utility>
+
+// template <class T1, class T2> struct pair
+
+// template <class T1, class T2> bool operator<=>(const pair<T1,T2>&, const pair<T1,T2>&);
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17, libcpp-no-concepts
+
+#include <cassert>
+#include <compare>
+#include <limits>
+#include <type_traits> // std::is_constant_evaluated
+#include <utility>
+
+#include "test_macros.h"
+
+template<class T> concept HasEqual = requires(T t) { t == t; };
+template<class T> concept HasLess = requires(T t) { t < t; };
+template<class T> concept HasSpaceship = requires(T t) { t <=> t; };
+
+constexpr bool test() {
+  {
+    // Pairs of types that both have strong ordering should compare with strong ordering.
+    using P = std::pair<int, int>;
+    ASSERT_SAME_TYPE(decltype(P() <=> P()), std::strong_ordering);
+    assert((P(1, 1) <=> P(1, 2)) == std::strong_ordering::less);
+    assert((P(2, 1) <=> P(1, 2)) == std::strong_ordering::greater);
+    assert((P(0, 0) <=> P(0, 0)) == std::strong_ordering::equal);
+  }
+  {
+    // Pairs of int and a type with no spaceship operator should compare with weak ordering.
+    struct NoSpaceship {
+      int value;
+      constexpr bool operator==(const NoSpaceship&) const = default;
+      constexpr bool operator<(const NoSpaceship& other) const { return value < other.value; }
+    };
+    using P = std::pair<int, NoSpaceship>;
+    ASSERT_SAME_TYPE(decltype(P() <=> P()), std::weak_ordering);
+    assert((P(1, {1}) <=> P(1, {2})) == std::weak_ordering::less);
+    assert((P(2, {1}) <=> P(1, {2})) == std::weak_ordering::greater);
+    assert((P(0, {0}) <=> P(0, {0})) == std::weak_ordering::equivalent);
+  }
+  {
+    // Pairs of int (strongly ordered) and double (partially ordered) should compare with partial ordering.
+    using P = std::pair<int, double>;
+    constexpr double nan = std::numeric_limits<double>::quiet_NaN();
+    ASSERT_SAME_TYPE(decltype(P() <=> P()), std::partial_ordering);
+    assert((P(1, 1.0) <=> P(1, 2.0)) == std::partial_ordering::less);
+    assert((P(1, 1.0) <=> P(1, 1.0)) == std::partial_ordering::equivalent);
+    assert((P(1, -0.0) <=> P(1, 0.0)) == std::partial_ordering::equivalent);
+    assert((P(1, 2.0) <=> P(1, 1.0)) == std::partial_ordering::greater);
+    assert((P(1, nan) <=> P(2, nan)) == std::partial_ordering::less);
+    assert((P(2, nan) <=> P(1, nan)) == std::partial_ordering::greater);
+    assert((P(1, nan) <=> P(1, nan)) == std::partial_ordering::unordered);
+  }
+  {
+    using P = std::pair<double, int>;
+    constexpr double nan = std::numeric_limits<double>::quiet_NaN();
+    ASSERT_SAME_TYPE(decltype(P() <=> P()), std::partial_ordering);
+    assert((P(2.0, 1) <=> P(1.0, 2)) == std::partial_ordering::greater);
+    assert((P(1.0, 1) <=> P(1.0, 2)) == std::partial_ordering::less);
+    assert((P(nan, 1) <=> P(nan, 2)) == std::partial_ordering::unordered);
+  }
+  {
+    struct NoRelative {
+      constexpr bool operator==(const NoRelative&) const;
+    };
+    static_assert(HasEqual<std::pair<int, NoRelative>>);
+    static_assert(!HasLess<std::pair<int, NoRelative>>);
+    static_assert(!HasSpaceship<std::pair<int, NoRelative>>);
+  }
+  {
+    struct NoLessThan {
+      constexpr bool operator==(const NoLessThan&) const;
+      constexpr bool operator>(const NoLessThan&) const;
+    };
+    static_assert(HasEqual<std::pair<int, NoLessThan>>);
+    static_assert(!HasLess<std::pair<int, NoLessThan>>);
+    static_assert(!HasSpaceship<std::pair<int, NoLessThan>>);
+  }
+
+#ifdef TEST_COMPILER_GCC
+  // GCC cannot evaluate NaN @ non-NaN constexpr, so test that runtime-only.
+  if (!std::is_constant_evaluated())
+#endif
+  {
+    {
+      using P = std::pair<int, double>;
+      constexpr double nan = std::numeric_limits<double>::quiet_NaN();
+      assert((P(1, 2.0) <=> P(1, nan)) == std::partial_ordering::unordered);
+    }
+    {
+      using P = std::pair<double, int>;
+      constexpr double nan = std::numeric_limits<double>::quiet_NaN();
+      assert((P(1.0, 1) <=> P(nan, 2)) == std::partial_ordering::unordered);
+    }
+  }
+
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+
+  return 0;
+}