[libc++] Implement operator<=> for error_category

Implements part of P1614R2 "The Mothership has Landed"

Differential Revision: https://reviews.llvm.org/D131363

Author: vogelsgesang

libcxx/docs/Status/SpaceshipProjects.csv

@@ -15,7 +15,7 @@ Section,Description,Dependencies,Assignee,Complete
 | `[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,|In Progress|
+| `[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,|In Progress|
 | `[tuple.rel] <https://wg21.link/tuple.rel>`_,| `tuple <https://reviews.llvm.org/D108250>`_,[expos.only.func],Kent Ross,|Complete|

libcxx/include/system_error

@@ -32,8 +32,9 @@ public:
     virtual string message(int ev) const = 0;
 
     bool operator==(const error_category& rhs) const noexcept;
-    bool operator!=(const error_category& rhs) const noexcept;
-    bool operator<(const error_category& rhs) const noexcept;
+    bool operator!=(const error_category& rhs) const noexcept;              // removed in C++20
+    bool operator<(const error_category& rhs) const noexcept;               // removed in C++20
+    strong_ordering operator<=>(const error_category& rhs) const noexcept;  // C++20
 };
 
 const error_category& generic_category() noexcept;
@@ -147,6 +148,7 @@ template <> struct hash<std::error_condition>;
 #include <__errc>
 #include <__functional/hash.h>
 #include <__functional/unary_function.h>
+#include <__memory/addressof.h>
 #include <stdexcept>
 #include <string>
 #include <type_traits>
@@ -223,12 +225,21 @@ public:
     _LIBCPP_INLINE_VISIBILITY
     bool operator==(const error_category& __rhs) const _NOEXCEPT {return this == &__rhs;}
 
+#if _LIBCPP_STD_VER > 17
+
+    _LIBCPP_HIDE_FROM_ABI
+    strong_ordering operator<=>(const error_category& __rhs) const noexcept {return compare_three_way()(this, std::addressof(__rhs));}
+
+#else // _LIBCPP_STD_VER > 17
+
     _LIBCPP_INLINE_VISIBILITY
     bool operator!=(const error_category& __rhs) const _NOEXCEPT {return !(*this == __rhs);}
 
     _LIBCPP_INLINE_VISIBILITY
     bool operator< (const error_category& __rhs) const _NOEXCEPT {return this < &__rhs;}
 
+#endif // _LIBCPP_STD_VER > 17
+
     friend class _LIBCPP_HIDDEN __do_message;
 };
 

libcxx/test/std/diagnostics/syserr/syserr.errcat/syserr.errcat.nonvirtuals/cmp.pass.cpp

@@ -0,0 +1,36 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+
+// <system_error>
+
+// class error_category
+
+// strong_ordering operator<=>(const error_category& rhs) const noexcept;
+
+#include <system_error>
+#include <cassert>
+
+#include "test_macros.h"
+#include "test_comparisons.h"
+
+int main(int, char**) {
+  AssertOrderAreNoexcept<std::error_category>();
+  AssertOrderReturn<std::strong_ordering, std::error_category>();
+
+  const std::error_category& e_cat1 = std::generic_category();
+  const std::error_category& e_cat2 = std::generic_category();
+  const std::error_category& e_cat3 = std::system_category();
+
+  assert(testOrder(e_cat1, e_cat2, std::strong_ordering::equal));
+
+  bool isLess = e_cat1 < e_cat3;
+  assert(testOrder(e_cat1, e_cat3, isLess ? std::strong_ordering::less : std::strong_ordering::greater));
+
+  return 0;
+}