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+/*!
+@file
+Defines `boost::hana::common` and `boost::hana::common_t`.
+
+@copyright Louis Dionne 2013-2016
+Distributed under the Boost Software License, Version 1.0.
+(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
+ */
+
+#ifndef BOOST_HANA_CORE_COMMON_HPP
+#define BOOST_HANA_CORE_COMMON_HPP
+
+#include <boost/hana/fwd/core/common.hpp>
+
+#include <boost/hana/concept/constant.hpp>
+#include <boost/hana/config.hpp>
+#include <boost/hana/core/when.hpp>
+#include <boost/hana/detail/canonical_constant.hpp>
+#include <boost/hana/detail/std_common_type.hpp>
+#include <boost/hana/detail/void_t.hpp>
+
+#include <type_traits>
+
+
+BOOST_HANA_NAMESPACE_BEGIN
+ //////////////////////////////////////////////////////////////////////////
+ // common
+ //////////////////////////////////////////////////////////////////////////
+ //! @cond
+ template <typename T, typename U, typename>
+ struct common : common<T, U, when<true>> { };
+ //! @endcond
+
+ template <typename T, typename U, bool condition>
+ struct common<T, U, when<condition>>
+ : detail::std_common_type<T, U>
+ { };
+
+ template <typename T>
+ struct common<T, T> {
+ using type = T;
+ };
+
+ //////////////////////////////////////////////////////////////////////////
+ // has_common
+ //////////////////////////////////////////////////////////////////////////
+ template <typename T, typename U, typename>
+ struct has_common : std::false_type { };
+
+ template <typename T, typename U>
+ struct has_common<T, U, detail::void_t<typename common<T, U>::type>>
+ : std::true_type
+ { };
+
+ //////////////////////////////////////////////////////////////////////////
+ // Provided common data types for Constants
+ //////////////////////////////////////////////////////////////////////////
+ namespace constant_detail {
+ //! @todo
+ //! This is an awful hack to avoid having
+ //! @code
+ //! common<integral_constant_tag<int>, integral_constant_tag<long>>
+ //! ==
+ //! CanonicalConstant<long>
+ //! @endcode
+ template <typename A, typename B, typename C>
+ struct which {
+ using type = detail::CanonicalConstant<C>;
+ };
+
+ template <template <typename ...> class A, typename T, typename U, typename C>
+ struct which<A<T>, A<U>, C> {
+ using type = A<C>;
+ };
+ }
+
+ template <typename A, typename B>
+ struct common<A, B, when<
+ hana::Constant<A>::value &&
+ hana::Constant<B>::value &&
+ has_common<typename A::value_type, typename B::value_type>::value
+ >> {
+ using type = typename constant_detail::which<
+ A, B,
+ typename common<typename A::value_type,
+ typename B::value_type>::type
+ >::type;
+ };
+
+ template <typename A, typename B>
+ struct common<A, B, when<
+ hana::Constant<A>::value &&
+ !hana::Constant<B>::value &&
+ has_common<typename A::value_type, B>::value
+ >> {
+ using type = typename common<typename A::value_type, B>::type;
+ };
+
+ template <typename A, typename B>
+ struct common<A, B, when<
+ !hana::Constant<A>::value &&
+ hana::Constant<B>::value &&
+ has_common<A, typename B::value_type>::value
+ >> {
+ using type = typename common<A, typename B::value_type>::type;
+ };
+BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_CORE_COMMON_HPP