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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
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