mirror of https://gitee.com/bigwinds/arangodb
66 lines
1.8 KiB
C++
66 lines
1.8 KiB
C++
// Copyright Louis Dionne 2013-2016
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// Distributed under the Boost Software License, Version 1.0.
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// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
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#include <boost/hana/equal.hpp>
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#include <boost/hana/monadic_fold_left.hpp>
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#include <boost/hana/optional.hpp>
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#include <boost/hana/traits.hpp>
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#include <boost/hana/tuple.hpp>
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#include <boost/hana/type.hpp>
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#include <type_traits>
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namespace hana = boost::hana;
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auto builtin_common_t = hana::sfinae([](auto&& t, auto&& u) -> decltype(hana::type_c<
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std::decay_t<decltype(true ? hana::traits::declval(t) : hana::traits::declval(u))>
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>) { return {}; });
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template <typename ...T>
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struct common_type { };
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template <typename T, typename U>
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struct common_type<T, U>
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: std::conditional_t<std::is_same<std::decay_t<T>, T>{} &&
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std::is_same<std::decay_t<U>, U>{},
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decltype(builtin_common_t(hana::type_c<T>, hana::type_c<U>)),
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common_type<std::decay_t<T>, std::decay_t<U>>
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>
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{ };
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template <typename T1, typename ...Tn>
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struct common_type<T1, Tn...>
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: decltype(hana::monadic_fold_left<hana::optional_tag>(
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hana::tuple_t<Tn...>,
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hana::type_c<std::decay_t<T1>>,
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hana::sfinae(hana::metafunction<common_type>)
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))
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{ };
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template <typename ...Ts>
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using common_type_t = typename common_type<Ts...>::type;
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static_assert(std::is_same<
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common_type_t<char, short, char, short>,
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int
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>{}, "");
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static_assert(std::is_same<
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common_type_t<char, double, short, char, short, double>,
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double
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>{}, "");
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static_assert(std::is_same<
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common_type_t<char, short, float, short>,
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float
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>{}, "");
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static_assert(
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hana::sfinae(hana::metafunction<common_type>)(
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hana::type_c<int>, hana::type_c<int>, hana::type_c<int*>
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) == hana::nothing
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, "");
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int main() { }
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