libstdc++-v3/testsuite/20_util/function_objects/range.cmp/lwg3530.cc

   1 // Copyright (C) 2021-2024 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-do compile { target c++20 } }
  19
  20 #include <functional>
  21
  22 struct S {
  23   constexpr operator int*() const { return nullptr; }
  24 };
  25
  26 void operator!=(S const&, S const&) {}
  27 void operator>=(S const&, S const&) {}
  28
  29 // S can be compared via conversion to int*
  30 static_assert(S{} == S{});
  31 static_assert(S{} <= S{});
  32 // But concept not satisfied because operator!= returns void
  33 static_assert(!std::equality_comparable_with<S,S>);
  34 // But concept not satisfied because operator>= returns void
  35 static_assert(!std::totally_ordered<S>);
  36
  37 template<typename C, typename T>
  38   concept comparable = requires (const C& cmp, const T& t) { cmp(t, t); };
  39
  40 // LWG 3530 says [range.cmp] comparisons should not work for S
  41 static_assert( ! comparable<std::ranges::equal_to,      S> );
  42 static_assert( ! comparable<std::ranges::not_equal_to,  S> );
  43 static_assert( ! comparable<std::ranges::greater,       S> );
  44 static_assert( ! comparable<std::ranges::less,          S> );
  45 static_assert( ! comparable<std::ranges::greater_equal, S> );
  46 static_assert( ! comparable<std::ranges::less_equal,    S> );