libstdc++-v3/testsuite/18_support/comparisons/object/lwg3530.cc

   1 // Copyright (C) 2020-2022 Free Software Foundation, Inc.
   2 //
   3 // This file is part of the GNU ISO C++ Library.  This library is free
   4 // software; you can redistribute it and/or modify it under the
   5 // terms of the GNU General Public License as published by the
   6 // Free Software Foundation; either version 3, or (at your option)
   7 // any later version.
   8
   9 // This library is distributed in the hope that it will be useful,
  10 // but WITHOUT ANY WARRANTY; without even the implied warranty of
  11 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  12 // GNU General Public License for more details.
  13
  14 // You should have received a copy of the GNU General Public License along
  15 // with this library; see the file COPYING3.  If not see
  16 // <http://www.gnu.org/licenses/>.
  17
  18 // { dg-options "-std=gnu++2a" }
  19 // { dg-do compile { target c++2a } }
  20
  21 #include <compare>
  22
  23 template<typename C, typename T, typename U>
  24   concept comparable = requires (const C& cmp, const T& t, const U& u) {
  25     cmp(t, u);
  26   };
  27
  28 void
  29 test01()
  30 {
  31   struct X
  32   {
  33     operator int() const { return 0; }
  34     operator long*() const { return nullptr; }
  35   } x;
  36
  37   // Not three-way-comparable because of ambiguous conversion to int or long*:
  38   static_assert( ! std::three_way_comparable<X> );
  39
  40   // And therefore X is not three-way-comparable-with anything else
  41   // (because std::three_way_comparable_with<X, Y> requires that both
  42   // three_way_comparable<X> and three_way_comparable<Y> are true).
  43   static_assert( ! std::three_way_comparable_with<X, long*> );
  44
  45   long l;
  46   // But <=> is valid and resolves to a builtin operator comparing pointers:
  47   [[maybe_unused]] auto c = &l <=> x;
  48
  49   // But LWG 3530 says std::compare_three_way should not be usable:
  50   static_assert( ! comparable<std::compare_three_way, long*, X> );
  51   static_assert( ! comparable<std::compare_three_way, X, long*> );
  52 }