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/*!
@file
Defines `boost::hana::first`.
@copyright Louis Dionne 2013-2017
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_FIRST_HPP
#define BOOST_HANA_FIRST_HPP
#include <boost/hana/fwd/first.hpp>
#include <boost/hana/concept/product.hpp>
#include <boost/hana/config.hpp>
#include <boost/hana/core/dispatch.hpp>
BOOST_HANA_NAMESPACE_BEGIN
//! @cond
template <typename Pair>
constexpr decltype(auto) first_t::operator()(Pair&& pair) const {
using P = typename hana::tag_of<Pair>::type;
using First = BOOST_HANA_DISPATCH_IF(first_impl<P>,
hana::Product<P>::value
);
#ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
static_assert(hana::Product<P>::value,
"hana::first(pair) requires 'pair' to be a Product");
#endif
return First::apply(static_cast<Pair&&>(pair));
}
//! @endcond
template <typename P, bool condition>
struct first_impl<P, when<condition>> : default_ {
template <typename ...Args>
static constexpr auto apply(Args&& ...) = delete;
};
BOOST_HANA_NAMESPACE_END
#endif // !BOOST_HANA_FIRST_HPP
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