| 1 | // Copyright Louis Dionne 2013-2022 |
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| 2 | // Distributed under the Boost Software License, Version 1.0. |
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| 3 | // (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt) |
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| 4 | |
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| 5 | #include <boost/hana/assert.hpp> |
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| 6 | #include <boost/hana/equal.hpp> |
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| 7 | #include <boost/hana/monadic_fold_left.hpp> |
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| 8 | #include <boost/hana/optional.hpp> |
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| 9 | #include <boost/hana/traits.hpp> |
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| 10 | #include <boost/hana/tuple.hpp> |
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| 11 | #include <boost/hana/type.hpp> |
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| 12 | |
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| 13 | #include <type_traits> |
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| 14 | namespace hana = boost::hana; |
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| 15 | |
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| 16 | |
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| 17 | auto builtin_common_t = hana::sfinae([](auto&& t, auto&& u) -> hana::type< |
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| 18 | std::decay_t<decltype(true ? hana::traits::declval(t) : hana::traits::declval(u))> |
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| 19 | > { return {}; }); |
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| 20 | |
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| 21 | template <typename ...T> |
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| 22 | struct common_type { }; |
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| 23 | |
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| 24 | template <typename T, typename U> |
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| 25 | struct common_type<T, U> |
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| 26 | : std::conditional_t<std::is_same<std::decay_t<T>, T>{} && |
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| 27 | std::is_same<std::decay_t<U>, U>{}, |
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| 28 | decltype(builtin_common_t(hana::type_c<T>, hana::type_c<U>)), |
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| 29 | common_type<std::decay_t<T>, std::decay_t<U>> |
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| 30 | > |
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| 31 | { }; |
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| 32 | |
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| 33 | template <typename T1, typename ...Tn> |
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| 34 | struct common_type<T1, Tn...> |
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| 35 | : decltype(hana::monadic_fold_left<hana::optional_tag>( |
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| 36 | hana::tuple_t<Tn...>, |
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| 37 | hana::type_c<std::decay_t<T1>>, |
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| 38 | hana::sfinae(hana::metafunction<common_type>) |
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| 39 | )) |
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| 40 | { }; |
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| 41 | |
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| 42 | template <typename ...Ts> |
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| 43 | using common_type_t = typename common_type<Ts...>::type; |
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| 44 | |
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| 45 | BOOST_HANA_CONSTANT_CHECK( |
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| 46 | builtin_common_t(hana::type_c<int>, hana::type_c<float>) |
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| 47 | == |
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| 48 | hana::just(hana::type_c<float>) |
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| 49 | ); |
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| 50 | |
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| 51 | static_assert(std::is_same< |
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| 52 | common_type_t<char, short, char, short>, |
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| 53 | int |
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| 54 | >{}, "" ); |
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| 55 | |
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| 56 | static_assert(std::is_same< |
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| 57 | common_type_t<char, double, short, char, short, double>, |
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| 58 | double |
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| 59 | >{}, "" ); |
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| 60 | |
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| 61 | static_assert(std::is_same< |
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| 62 | common_type_t<char, short, float, short>, |
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| 63 | float |
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| 64 | >{}, "" ); |
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| 65 | |
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| 66 | static_assert( |
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| 67 | hana::sfinae(hana::metafunction<common_type>)( |
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| 68 | hana::type_c<int>, hana::type_c<int>, hana::type_c<int*> |
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| 69 | ) == hana::nothing |
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| 70 | , "" ); |
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| 71 | |
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| 72 | int main() { } |
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| 73 | |
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