--- /dev/null
+//===----------------------------------------------------------------------===//
+//
+// 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_COMPARE_THREE_WAY_RESULT_H
+#define _LIBCPP___COMPARE_COMPARE_THREE_WAY_RESULT_H
+
+#include <__config>
+#include <type_traits>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER > 17
+
+template<class, class, class>
+struct _LIBCPP_HIDE_FROM_ABI __compare_three_way_result { };
+
+template<class _Tp, class _Up>
+struct _LIBCPP_HIDE_FROM_ABI __compare_three_way_result<_Tp, _Up, decltype(
+ declval<__make_const_lvalue_ref<_Tp>>() <=> declval<__make_const_lvalue_ref<_Up>>(), void()
+)> {
+ using type = decltype(declval<__make_const_lvalue_ref<_Tp>>() <=> declval<__make_const_lvalue_ref<_Up>>());
+};
+
+template<class _Tp, class _Up = _Tp>
+struct _LIBCPP_TEMPLATE_VIS compare_three_way_result : __compare_three_way_result<_Tp, _Up, void> { };
+
+template<class _Tp, class _Up = _Tp>
+using compare_three_way_result_t = typename compare_three_way_result<_Tp, _Up>::type;
+
+#endif // _LIBCPP_STD_VER > 17
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___COMPARE_COMPARE_THREE_WAY_RESULT_H
--- /dev/null
+//===----------------------------------------------------------------------===//
+//
+// 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
+// UNSUPPORTED: libcpp-no-concepts
+
+// <compare>
+
+// template<class T, class U = T> struct compare_three_way_result;
+// template<class T, class U = T>
+// using compare_three_way_result_t = typename compare_three_way_result<T, U>::type;
+
+#include <compare>
+
+#include "test_macros.h"
+
+template<class T>
+concept has_no_nested_type = !requires { typename T::type; };
+
+ASSERT_SAME_TYPE(std::compare_three_way_result_t<int>, std::strong_ordering);
+ASSERT_SAME_TYPE(std::compare_three_way_result_t<float>, std::partial_ordering);
+ASSERT_SAME_TYPE(std::compare_three_way_result_t<unsigned>, std::strong_ordering);
+
+ASSERT_SAME_TYPE(std::compare_three_way_result_t<int, int>, std::strong_ordering);
+ASSERT_SAME_TYPE(std::compare_three_way_result_t<int, float>, std::partial_ordering);
+ASSERT_SAME_TYPE(std::compare_three_way_result_t<float, int>, std::partial_ordering);
+ASSERT_SAME_TYPE(std::compare_three_way_result_t<float, float>, std::partial_ordering);
+ASSERT_SAME_TYPE(std::compare_three_way_result_t<float, unsigned>, std::partial_ordering);
+ASSERT_SAME_TYPE(std::compare_three_way_result_t<unsigned, float>, std::partial_ordering);
+ASSERT_SAME_TYPE(std::compare_three_way_result_t<unsigned, unsigned>, std::strong_ordering);
+
+ASSERT_SAME_TYPE(std::compare_three_way_result_t<const int&>, std::strong_ordering);
+ASSERT_SAME_TYPE(std::compare_three_way_result_t<const int&, int>, std::strong_ordering);
+ASSERT_SAME_TYPE(std::compare_three_way_result_t<const int*>, std::strong_ordering);
+ASSERT_SAME_TYPE(std::compare_three_way_result_t<const int*, int*>, std::strong_ordering);
+
+static_assert(has_no_nested_type<std::compare_three_way_result<void>>);
+static_assert(has_no_nested_type<std::compare_three_way_result<void, void>>);
+static_assert(has_no_nested_type<std::compare_three_way_result<int, void>>);
+static_assert(has_no_nested_type<std::compare_three_way_result<int, int*>>);
+static_assert(has_no_nested_type<std::compare_three_way_result<int, unsigned>>);
+static_assert(has_no_nested_type<std::compare_three_way_result<unsigned, int>>);
+
+struct A {
+ float operator<=>(const A&) const; // a non-comparison-category type is OK
+};
+ASSERT_SAME_TYPE(std::compare_three_way_result_t<A>, float);
+ASSERT_SAME_TYPE(std::compare_three_way_result_t<A, A>, float);
+
+struct B {
+ using T = int(&)();
+ T operator<=>(const B&) const; // no decay takes place either
+};
+ASSERT_SAME_TYPE(std::compare_three_way_result_t<B>, int(&)());
+ASSERT_SAME_TYPE(std::compare_three_way_result_t<B, B>, int(&)());
+
+struct C {
+ std::strong_ordering operator<=>(C&); // C isn't const-comparable
+};
+static_assert(has_no_nested_type<std::compare_three_way_result<C>>);
+static_assert(has_no_nested_type<std::compare_three_way_result<C&>>);
+static_assert(has_no_nested_type<std::compare_three_way_result<C&&>>);
+
+static_assert(has_no_nested_type<std::compare_three_way_result<C, C>>);
+static_assert(has_no_nested_type<std::compare_three_way_result<C&, C&>>);
+static_assert(has_no_nested_type<std::compare_three_way_result<C&&, C&&>>);
+
+struct D {
+ std::strong_ordering operator<=>(D&) &;
+ std::strong_ordering operator<=>(D&&) &&;
+ std::weak_ordering operator<=>(const D&) const&; // comparison is always done by const&
+ std::strong_ordering operator<=>(const D&&) const&&;
+};
+ASSERT_SAME_TYPE(std::compare_three_way_result_t<D>, std::weak_ordering);
+ASSERT_SAME_TYPE(std::compare_three_way_result_t<D&>, std::weak_ordering);
+ASSERT_SAME_TYPE(std::compare_three_way_result_t<D&&>, std::weak_ordering);
+ASSERT_SAME_TYPE(std::compare_three_way_result_t<const D&>, std::weak_ordering);
+ASSERT_SAME_TYPE(std::compare_three_way_result_t<const D&&>, std::weak_ordering);
+
+ASSERT_SAME_TYPE(std::compare_three_way_result_t<D, D>, std::weak_ordering);
+ASSERT_SAME_TYPE(std::compare_three_way_result_t<D&, D&>, std::weak_ordering);
+ASSERT_SAME_TYPE(std::compare_three_way_result_t<D&&, D&&>, std::weak_ordering);
+ASSERT_SAME_TYPE(std::compare_three_way_result_t<const D&, const D&>, std::weak_ordering);
+ASSERT_SAME_TYPE(std::compare_three_way_result_t<const D&&, const D&&>, std::weak_ordering);