ceph/src/boost/libs/pfr/test/loophole_detection.cpp

   1 // Copyright (c) 2020 Antony Polukhin
   2 //
   3 // Distributed under the Boost Software License, Version 1.0. (See accompanying
   4 // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
   5
   6 // Detection of type loophole.
   7 // Inspired by the blog post: http://alexpolt.github.io/type-loophole.html
   8
   9 #include <type_traits>
  10
  11 #if defined(_MSC_VER) && _MSC_VER <= 1916
  12 #   error Compiler fails to do compile time computations for LoopHole. Fixed in later versions of the compiler
  13 // Error: boost/pfr/detail/core14_loophole.hpp(98): error C3779: 'boost::pfr::detail::loophole': a function that returns 'auto' cannot be used before it is defined
  14 #endif
  15
  16
  17 template <unsigned> struct tag{};
  18
  19 template <class T, unsigned N>
  20 struct loophole_t {
  21   friend auto loophole(tag<N>) { return T{}; };
  22 };
  23
  24 auto loophole(tag<0>);
  25
  26 int main() {
  27    sizeof(loophole_t<unsigned, 0>);
  28    static_assert( std::is_same<unsigned, decltype( loophole(tag<0>{}) ) >::value, "");
  29 }