ceph/src/boost/libs/hana/example/optional/sfinae_friendly_metafunctions.cpp

   1 // Copyright Louis Dionne 2013-2016
   2 // Distributed under the Boost Software License, Version 1.0.
   3 // (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
   4
   5 #include <boost/hana/assert.hpp>
   6 #include <boost/hana/not.hpp>
   7 #include <boost/hana/optional.hpp>
   8 #include <boost/hana/traits.hpp>
   9 #include <boost/hana/type.hpp>
  10
  11 #include <type_traits>
  12 #include <utility>
  13 namespace hana = boost::hana;
  14
  15
  16 template <typename ...>
  17 using void_t = void;
  18
  19 template <typename T, typename = void>
  20 struct has_type : std::false_type { };
  21
  22 template <typename T>
  23 struct has_type<T, void_t<typename T::type>>
  24     : std::true_type
  25 { };
  26
  27 auto common_type_impl = hana::sfinae([](auto t, auto u) -> decltype(hana::type_c<
  28     decltype(true ? hana::traits::declval(t) : hana::traits::declval(u))
  29 >) { return {}; });
  30
  31 template <typename T, typename U>
  32 using common_type2 = decltype(common_type_impl(hana::type_c<T>, hana::type_c<U>));
  33
  34 static_assert(!has_type<common_type2<int, int*>>{}, "");
  35 static_assert(std::is_same<common_type2<int, float>::type, float>{}, "");
  36
  37 int main() { }