1 files changed, 70 insertions, 0 deletions
diff --git a/libcxx/include/__compare/compare_weak_order_fallback.h b/libcxx/include/__compare/compare_weak_order_fallback.h
new file mode 100644
index 000000000000..0abd4f2dfbee
--- /dev/null
+++ b/libcxx/include/__compare/compare_weak_order_fallback.h
@@ -0,0 +1,70 @@
+//===----------------------------------------------------------------------===//
+//
+// 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_WEAK_ORDER_FALLBACK
+#define _LIBCPP___COMPARE_COMPARE_WEAK_ORDER_FALLBACK
+
+#include <__compare/ordering.h>
+#include <__compare/weak_order.h>
+#include <__config>
+#include <__utility/forward.h>
+#include <__utility/priority_tag.h>
+#include <type_traits>
+
+#ifndef _LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER
+#pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_CONCEPTS)
+
+// [cmp.alg]
+namespace __compare_weak_order_fallback {
+    struct __fn {
+        template<class _Tp, class _Up>
+            requires is_same_v<decay_t<_Tp>, decay_t<_Up>>
+        _LIBCPP_HIDE_FROM_ABI static constexpr auto
+        __go(_Tp&& __t, _Up&& __u, __priority_tag<1>)
+            noexcept(noexcept(_VSTD::weak_order(_VSTD::forward<_Tp>(__t), _VSTD::forward<_Up>(__u))))
+            -> decltype(      _VSTD::weak_order(_VSTD::forward<_Tp>(__t), _VSTD::forward<_Up>(__u)))
+            { return          _VSTD::weak_order(_VSTD::forward<_Tp>(__t), _VSTD::forward<_Up>(__u)); }
+
+        template<class _Tp, class _Up>
+            requires is_same_v<decay_t<_Tp>, decay_t<_Up>>
+        _LIBCPP_HIDE_FROM_ABI static constexpr auto
+        __go(_Tp&& __t, _Up&& __u, __priority_tag<0>)
+            noexcept(noexcept(_VSTD::forward<_Tp>(__t) == _VSTD::forward<_Up>(__u) ? weak_ordering::equivalent :
+                              _VSTD::forward<_Tp>(__t) < _VSTD::forward<_Up>(__u) ? weak_ordering::less :
+                              weak_ordering::greater))
+            -> decltype(      _VSTD::forward<_Tp>(__t) == _VSTD::forward<_Up>(__u) ? weak_ordering::equivalent :
+                              _VSTD::forward<_Tp>(__t) < _VSTD::forward<_Up>(__u) ? weak_ordering::less :
+                              weak_ordering::greater)
+        {
+            return            _VSTD::forward<_Tp>(__t) == _VSTD::forward<_Up>(__u) ? weak_ordering::equivalent :
+                              _VSTD::forward<_Tp>(__t) < _VSTD::forward<_Up>(__u) ? weak_ordering::less :
+                              weak_ordering::greater;
+        }
+
+        template<class _Tp, class _Up>
+        _LIBCPP_HIDE_FROM_ABI constexpr auto operator()(_Tp&& __t, _Up&& __u) const
+            noexcept(noexcept(__go(_VSTD::forward<_Tp>(__t), _VSTD::forward<_Up>(__u), __priority_tag<1>())))
+            -> decltype(      __go(_VSTD::forward<_Tp>(__t), _VSTD::forward<_Up>(__u), __priority_tag<1>()))
+            { return          __go(_VSTD::forward<_Tp>(__t), _VSTD::forward<_Up>(__u), __priority_tag<1>()); }
+    };
+} // namespace __compare_weak_order_fallback
+
+inline namespace __cpo {
+    inline constexpr auto compare_weak_order_fallback = __compare_weak_order_fallback::__fn{};
+} // namespace __cpo
+
+#endif // _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_CONCEPTS)
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___COMPARE_COMPARE_WEAK_ORDER_FALLBACK