Improve the representation of <compare>'s zero-only type.

[rsmith]

Summary

• Use an empty struct instead of a member pointer to represent this type, so that we don't actually pass a zero member pointer at runtime.

• Mark the constructor as consteval to ensure that no code is emitted for it whenever possible.

• Add a honeypot constructor to reject all non-int arguments, so that the only argument that can arrive at the real constructor is the literal 0.

This results in better generated code, and rejecting invalid comparisons
against nullptr, 0L, and so on, while also rejecting invalid comparisons
against (1-1) and similar that would be allowed if we required an
integer constant expression with value 0.

[ldionne]

libcxx/test/std/language.support/cmp/cmp.categories.pre/zero_type.fail.cpp

1 ↗

(On Diff #282330)

Can you name this file zero_type.verify.cpp instead?

19 ↗

(On Diff #282330)

You could take both op and v here to avoid v coming from "nowhere".

[rsmith]

• Address review comments.
• Add some positive tests.

[ldionne]

@rsmith Would you like me to commit this?

[rsmith]

In D85051#2273847, @ldionne wrote:

@rsmith Would you like me to commit this?

Sorry for the delay :) Committed.

libcxx/include/__config

 #endif  // _LIBCPP_CXX03_LANG
 
 #ifdef _LIBCPP_CXX03_LANG
 #  define _LIBCPP_CONSTEXPR
 #else
 #  define _LIBCPP_CONSTEXPR constexpr
 #endif
 
+#ifndef __cpp_consteval
+#  define _LIBCPP_CONSTEVAL _LIBCPP_CONSTEXPR
+#else
+#  define _LIBCPP_CONSTEVAL consteval
+#endif
+
 #ifdef _LIBCPP_CXX03_LANG
 #  define _LIBCPP_DEFAULT {}
 #else
 #  define _LIBCPP_DEFAULT = default;
 #endif
 
 #ifdef _LIBCPP_CXX03_LANG
 #  define _LIBCPP_EQUAL_DELETE

libcxx/include/compare

enum class _LIBCPP_ENUM_VIS _OrdResult : signed char {
   __less = -1,
   __greater = 1
 };
 
 enum class _LIBCPP_ENUM_VIS _NCmpResult : signed char {
   __unordered = -127
 };
 
-struct _CmpUnspecifiedType;
-using _CmpUnspecifiedParam = void (_CmpUnspecifiedType::*)();
+struct _CmpUnspecifiedParam {
+  _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEVAL
+  _CmpUnspecifiedParam(int _CmpUnspecifiedParam::*) {}
+
+  template<typename _Tp, typename = _VSTD::enable_if_t<!_VSTD::is_same_v<_Tp, int>>>
+  _CmpUnspecifiedParam(_Tp) = delete;
+};
 
 class  weak_equality {
   _LIBCPP_INLINE_VISIBILITY
   constexpr explicit weak_equality(_EqResult __val) noexcept : __value_(__val) {}
 
 public:
   static const weak_equality equivalent;
   static const weak_equality nonequivalent;

libcxx/test/std/language.support/cmp/cmp.categories.pre/zero_type.verify.cpp

+// -*- 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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+
+// <compare>
+
+// Ensure we reject all cases where an argument other than a literal 0 is used
+// for a comparison against a comparison category type.
+
+#include <compare>
+
+#define TEST_OP(v, op)                                                         \
+  void(v op 0L);                                                               \
+  void(0L op v);                                                               \
+  void(v op nullptr);                                                          \
+  void(nullptr op v);                                                          \
+  void(v op(1 - 1));                                                           \
+  void((1 - 1) op v);
+
+template <typename T>
+void test_category(T v) {
+  TEST_OP(v, ==);  // expected-error 18 {{}}
+  TEST_OP(v, !=);  // expected-error 18 {{}}
+  TEST_OP(v, <);   // expected-error 18 {{}}
+  TEST_OP(v, <=);  // expected-error 18 {{}}
+  TEST_OP(v, >);   // expected-error 18 {{}}
+  TEST_OP(v, >=);  // expected-error 18 {{}}
+  TEST_OP(v, <=>); // expected-error 18 {{}}
+
+  void(v == 0);
+  void(0 == v);
+  void(v != 0);
+  void(0 != v);
+  void(v < 0);
+  void(0 < v);
+  void(v <= 0);
+  void(0 <= v);
+  void(v > 0);
+  void(0 > v);
+  void(v >= 0);
+  void(0 >= v);
+#ifndef _LIBCPP_HAS_NO_THREE_WAY_COMPARISON
+  void(v <=> 0); // expected-error 3 {{}}
+  void(0 <=> v); // expected-error 3 {{}}
+#endif
+}
+
+int main(int, char**) {
+  test_category(std::strong_ordering::equivalent);
+  test_category(std::weak_ordering::equivalent);
+  test_category(std::partial_ordering::equivalent);
+  return 0;
+}