libstdc++-v3/testsuite/20_util/unwrap_reference/1.cc

   1 // Copyright (C) 2018-2021 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 <type_traits>
  22
  23 #ifndef __cpp_lib_unwrap_ref
  24 # error "Feature-test macro for unwrap_reference missing in <type_traits>"
  25 #elif __cpp_lib_unwrap_ref != 201811L
  26 # error "Feature-test macro for unwrap_reference has wrong value in <type_traits>"
  27 #endif
  28
  29 template<typename T, typename U> struct expect_same;
  30 template<typename T> struct expect_same<T, T> : std::true_type { };
  31
  32 template<typename T, typename U = T>
  33   constexpr bool check()
  34   {
  35     using std::unwrap_reference;
  36     using T2 = typename unwrap_reference<T>::type;
  37     static_assert(expect_same<T2, std::unwrap_reference_t<T2>>::value);
  38     return expect_same<T2, U>::value;
  39   }
  40
  41 void
  42 test01()
  43 {
  44   static_assert( check<int>() );
  45   static_assert( check<const int>() );
  46   static_assert( check<const int&>() );
  47   static_assert( check<const int*>() );
  48   static_assert( check<const int*&>() );
  49
  50   // reference_wrapper types should get unwrapped:
  51   static_assert( check<std::reference_wrapper<int>, int&>() );
  52   static_assert( check<std::reference_wrapper<const int>, const int&>() );
  53   static_assert( check<std::reference_wrapper<long>, long&>() );
  54
  55   // But not cv-qualified reference_wrapper types:
  56   static_assert( check<const std::reference_wrapper<int>>() );
  57   static_assert( check<volatile std::reference_wrapper<int>>() );
  58   static_assert( check<const volatile std::reference_wrapper<int>>() );
  59
  60   // Or references to reference_wrapper types:
  61   static_assert( check<std::reference_wrapper<int>&>() );
  62   static_assert( check<std::reference_wrapper<int>&&>() );
  63   static_assert( check<const std::reference_wrapper<int>&>() );
  64 }