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/monadic_fold_left.hpp>
8 #include <boost/hana/optional.hpp>
9 #include <boost/hana/traits.hpp>
10 #include <boost/hana/tuple.hpp>
11 #include <boost/hana/type.hpp>
12
13 #include <type_traits>
14 namespace hana = boost::hana;
15
16
17 auto builtin_common_t = hana::sfinae([](auto&& t, auto&& u) -> hana::type<
18 std::decay_t<decltype(true ? hana::traits::declval(t) : hana::traits::declval(u))>
__anonc46602680102(auto&& t, auto&& u) 19 > { return {}; });
20
21 template <typename ...T>
22 struct common_type { };
23
24 template <typename T, typename U>
25 struct common_type<T, U>
26 : std::conditional_t<std::is_same<std::decay_t<T>, T>{} &&
27 std::is_same<std::decay_t<U>, U>{},
28 decltype(builtin_common_t(hana::type_c<T>, hana::type_c<U>)),
29 common_type<std::decay_t<T>, std::decay_t<U>>
30 >
31 { };
32
33 template <typename T1, typename ...Tn>
34 struct common_type<T1, Tn...>
35 : decltype(hana::monadic_fold_left<hana::optional_tag>(
36 hana::tuple_t<Tn...>,
37 hana::type_c<std::decay_t<T1>>,
38 hana::sfinae(hana::metafunction<common_type>)
39 ))
40 { };
41
42 template <typename ...Ts>
43 using common_type_t = typename common_type<Ts...>::type;
44
45 BOOST_HANA_CONSTANT_CHECK(
46 builtin_common_t(hana::type_c<int>, hana::type_c<float>)
47 ==
48 hana::just(hana::type_c<float>)
49 );
50
51 static_assert(std::is_same<
52 common_type_t<char, short, char, short>,
53 int
54 >{}, "");
55
56 static_assert(std::is_same<
57 common_type_t<char, double, short, char, short, double>,
58 double
59 >{}, "");
60
61 static_assert(std::is_same<
62 common_type_t<char, short, float, short>,
63 float
64 >{}, "");
65
66 static_assert(
67 hana::sfinae(hana::metafunction<common_type>)(
68 hana::type_c<int>, hana::type_c<int>, hana::type_c<int*>
69 ) == hana::nothing
70 , "");
71
main()72 int main() { }
73