[libc++] Extend __default_three_way_comparator to any types that only implement operator<=>

Author: philnik777

libcxx/include/__utility/default_three_way_comparator.h

@@ -27,9 +27,11 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 template <class _LHS, class _RHS, class = void>
 struct __default_three_way_comparator;
 
-template <class _Tp>
-struct __default_three_way_comparator<_Tp, _Tp, __enable_if_t<is_arithmetic<_Tp>::value> > {
-  _LIBCPP_HIDE_FROM_ABI static int operator()(_Tp __lhs, _Tp __rhs) {
+template <class _LHS, class _RHS>
+struct __default_three_way_comparator<_LHS,
+                                      _RHS,
+                                      __enable_if_t<is_arithmetic<_LHS>::value && is_arithmetic<_RHS>::value> > {
+  _LIBCPP_HIDE_FROM_ABI static int operator()(_LHS __lhs, _RHS __rhs) {
     if (__lhs < __rhs)
       return -1;
     if (__lhs > __rhs)
@@ -38,6 +40,24 @@ struct __default_three_way_comparator<_Tp, _Tp, __enable_if_t<is_arithmetic<_Tp>
   }
 };
 
+#if _LIBCPP_STD_VER >= 20 && __has_builtin(__builtin_lt_synthesises_from_spaceship)
+template <class _LHS, class _RHS>
+struct __default_three_way_comparator<
+    _LHS,
+    _RHS,
+    __enable_if_t<(!is_arithmetic<_LHS>::value || !is_arithmetic<_RHS>::value) &&
+                  __builtin_lt_synthesises_from_spaceship(const _LHS&, const _RHS&)>> {
+  _LIBCPP_HIDE_FROM_ABI static int operator()(const _LHS& __lhs, const _RHS& __rhs) {
+    auto __res = __lhs <=> __rhs;
+    if (__res < 0)
+      return -1;
+    if (__res > 0)
+      return 1;
+    return 0;
+  }
+};
+#endif
+
 template <class _LHS, class _RHS, bool = true>
 inline const bool __has_default_three_way_comparator_v = false;
 

libcxx/test/libcxx/utilities/utility/has_default_three_way.compile.pass.cpp

@@ -0,0 +1,35 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+
+#include <__utility/default_three_way_comparator.h>
+#include <string>
+#include <vector>
+
+static_assert(std::__has_default_three_way_comparator_v<int, int>);
+static_assert(std::__has_default_three_way_comparator_v<int, long>);
+static_assert(std::__has_default_three_way_comparator_v<long, int>);
+static_assert(std::__has_default_three_way_comparator_v<long, long>);
+static_assert(std::__has_default_three_way_comparator_v<const std::string&, const std::string&>);
+
+#if __has_builtin(__builtin_lt_synthesises_from_spaceship)
+static_assert(std::__has_default_three_way_comparator_v<const std::string&, const std::string_view&>);
+static_assert(std::__has_default_three_way_comparator_v<const std::string&, const char*>);
+static_assert(!std::__has_default_three_way_comparator_v<const std::string&, const wchar_t*>);
+
+static_assert(std::__has_default_three_way_comparator_v<const std::vector<int>&, const std::vector<int>&>);
+
+struct MyStruct {
+  int i;
+
+  friend auto operator<=>(MyStruct, MyStruct) = default;
+};
+
+static_assert(std::__has_default_three_way_comparator_v<const MyStruct&, const MyStruct&>);
+#endif