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

   1 // Copyright Louis Dionne 2013-2017
   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/equal.hpp>
   7 #include <boost/hana/not.hpp>
   8 #include <boost/hana/optional.hpp>
   9 #include <boost/hana/traits.hpp>
  10 #include <boost/hana/type.hpp>
  11
  12 #include <type_traits>
  13 #include <utility>
  14 namespace hana = boost::hana;
  15
  16
  17 template <typename ...>
  18 using void_t = void;
  19
  20 template <typename T, typename = void>
  21 struct has_type : std::false_type { };
  22
  23 template <typename T>
  24 struct has_type<T, void_t<typename T::type>>
  25     : std::true_type
  26 { };
  27
  28 auto common_type_impl = hana::sfinae([](auto t, auto u) -> hana::type<
  29     decltype(true ? hana::traits::declval(t) : hana::traits::declval(u))
  30 > { return {}; });
  31
  32 template <typename T, typename U>
  33 using common_type = decltype(common_type_impl(hana::type_c<T>, hana::type_c<U>));
  34
  35 BOOST_HANA_CONSTANT_CHECK(
  36   common_type_impl(hana::type_c<int>, hana::type_c<float>)
  37     ==
  38   hana::just(hana::type_c<float>)
  39 );
  40
  41 static_assert(!has_type<common_type<int, int*>>{}, "");
  42 static_assert(std::is_same<common_type<int, float>::type, float>{}, "");
  43
  44 int main() { }