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/*!
@file
Defines `boost::hana::detail::type_at`.
@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_DETAIL_TYPE_AT_HPP
#define BOOST_HANA_DETAIL_TYPE_AT_HPP
#include <boost/hana/config.hpp>
#include <cstddef>
#include <utility>
// If possible, use an intrinsic provided by Clang
#if defined(__has_builtin)
# if __has_builtin(__type_pack_element)
# define BOOST_HANA_USE_TYPE_PACK_ELEMENT_INTRINSIC
# endif
#endif
BOOST_HANA_NAMESPACE_BEGIN namespace detail {
namespace td {
template <std::size_t I, typename T>
struct elt { using type = T; };
template <typename Indices, typename ...T>
struct indexer;
template <std::size_t ...I, typename ...T>
struct indexer<std::index_sequence<I...>, T...>
: elt<I, T>...
{ };
template <std::size_t I, typename T>
elt<I, T> get_elt(elt<I, T> const&);
}
//! @ingroup group-details
//! Classic MPL-style metafunction returning the nth element of a type
//! parameter pack.
template <std::size_t n, typename ...T>
struct type_at {
#if defined(BOOST_HANA_USE_TYPE_PACK_ELEMENT_INTRINSIC)
using type = __type_pack_element<n, T...>;
#else
using Indexer = td::indexer<std::make_index_sequence<sizeof...(T)>, T...>;
using type = typename decltype(td::get_elt<n>(Indexer{}))::type;
#endif
};
} BOOST_HANA_NAMESPACE_END
#endif // !BOOST_HANA_DETAIL_TYPE_AT_HPP
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