libstdc++-v3/testsuite/20_util/result_of/sfinae_friendly_2.cc

   1 // { dg-do compile { target c++11 } }
   2
   3 // Copyright (C) 2012-2020 Free Software Foundation, Inc.
   4 //
   5 // This file is part of the GNU ISO C++ Library.  This library is free
   6 // software; you can redistribute it and/or modify it under the
   7 // terms of the GNU General Public License as published by the
   8 // Free Software Foundation; either version 3, or (at your option)
   9 // any later version.
  10
  11 // This library is distributed in the hope that it will be useful,
  12 // but WITHOUT ANY WARRANTY; without even the implied warranty of
  13 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  14 // GNU General Public License for more details.
  15
  16 // You should have received a copy of the GNU General Public License along
  17 // with this library; see the file COPYING3.  If not see
  18 // <http://www.gnu.org/licenses/>.
  19
  20 // Taken from N3436:
  21
  22 #include <type_traits>
  23 #include <string>
  24
  25 struct eat { template<typename T> eat(T const &) {} };
  26 struct not_incrementable {};
  27
  28 struct inc {
  29  template<typename T>
  30  auto operator()(T t) const -> decltype(t++)
  31  { return t++; }
  32 };
  33
  34 template<typename A>
  35 typename std::result_of<inc(A)>::type // sfinae here
  36 try_inc(A a) {
  37   return inc()(a);
  38 }
  39
  40 not_incrementable
  41 try_inc(eat) {
  42   return not_incrementable();
  43 }
  44
  45 template<typename>
  46 struct never { static const bool value = false; };
  47
  48 template<typename T>
  49 struct Fail
  50 {
  51   static_assert(never<T>::value, "duh");
  52   typedef int type;
  53 };
  54
  55 struct Fun
  56 {
  57   template<typename T>
  58   typename Fail<T>::type operator()(T)
  59   { return 0; }
  60 };
  61
  62 template<typename T>
  63 typename std::result_of<Fun(T)>::type foo(T)
  64 { return 0; }
  65
  66 template<typename>
  67 int foo(...)
  68 { return 0; }
  69
  70 void result_of_sfinae() {
  71   int x = try_inc(1); // OK
  72   not_incrementable y = try_inc(std::string("foo")); // OK, not_incrementable
  73   (void) x;
  74   (void) y;
  75 }