libstdc++-v3/testsuite/20_util/reference_wrapper/lwg2993.cc

   1 // Copyright (C) 2018-2022 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-do compile { target c++11 } }
  19
  20 #include <functional>
  21
  22 // LWG 2993. reference_wrapper<T> conversion from T&&
  23
  24 static_assert(std::is_convertible<std::reference_wrapper<int>,
  25                                   std::reference_wrapper<const int>>::value,
  26               "LWG 2993 enables qualification conversions");
  27
  28 // The comments below are taken from the issue discussion and describe the
  29 // behaviour before the resolution of LWG 2993. There should be no errors now.
  30
  31 struct convertible_from_int { convertible_from_int(int) { } };
  32
  33 void
  34 test01()
  35 {
  36
  37   void meow(std::reference_wrapper<int>); //#1
  38   void meow(convertible_from_int); //#2
  39   // error, ambiguous; would unambiguously call #2 if #1 instead took int&
  40   meow(0);
  41 }
  42
  43 void
  44 test02()
  45 {
  46   std::reference_wrapper<int> purr(); // { dg-warning "empty parentheses" }
  47
  48   // error, ambiguous: ICS exists from int prvalue to
  49   // reference_wrapper<int> and from reference_wrapper<int> to int
  50   auto x = true ? purr() : 0;
  51
  52   // error: no member 'type' because the conditional
  53   // expression is ill-formed
  54   using t = typename std::common_type<std::reference_wrapper<int>, int>::type;
  55 }