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+/*!
+@file
+Forward declares `boost::hana::at_key`.
+
+@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_FWD_AT_KEY_HPP
+#define BOOST_HANA_FWD_AT_KEY_HPP
+
+#include <boost/hana/config.hpp>
+#include <boost/hana/core/when.hpp>
+
+
+BOOST_HANA_NAMESPACE_BEGIN
+    //! Returns the value associated to the given key in a structure, or fail.
+    //! @ingroup group-Searchable
+    //!
+    //! Given a `key` and a `Searchable` structure, `at_key` returns the first
+    //! value whose key is equal to the given `key`, and fails at compile-time
+    //! if no such key exists. This requires the `key` to be compile-time
+    //! `Comparable`, exactly like for `find`. `at_key` satisfies the following:
+    //! @code
+    //!     at_key(xs, key) == find(xs, key).value()
+    //! @endcode
+    //!
+    //! If the `Searchable` actually stores the elements it contains, `at_key`
+    //! is required to return a lvalue reference, a lvalue reference to const
+    //! or a rvalue reference to the found value, where the type of reference
+    //! must match that of the structure passed to `at_key`. If the `Searchable`
+    //! does not store the elements it contains (i.e. it generates them on
+    //! demand), this requirement is dropped.
+    //!
+    //!
+    //! @param xs
+    //! The structure to be searched.
+    //!
+    //! @param key
+    //! A key to be searched for in the structure. The key has to be
+    //! `Comparable` with the other keys of the structure. In the current
+    //! version of the library, the comparison of `key` with any other key
+    //! of the structure must return a compile-time `Logical`.
+    //!
+    //!
+    //! Example
+    //! -------
+    //! @include example/at_key.cpp
+    //!
+    //!
+    //! Benchmarks
+    //! ----------
+    //! <div class="benchmark-chart"
+    //!      style="min-width: 310px; height: 400px; margin: 0 auto"
+    //!      data-dataset="benchmark.at_key.number_of_lookups.chart.json">
+    //! </div>
+#ifdef BOOST_HANA_DOXYGEN_INVOKED
+    constexpr auto at_key = [](auto&& xs, auto const& key) -> decltype(auto) {
+        return tag-dispatched;
+    };
+#else
+    template <typename S, typename = void>
+    struct at_key_impl : at_key_impl<S, when<true>> { };
+
+    struct at_key_t {
+        template <typename Xs, typename Key>
+        constexpr decltype(auto) operator()(Xs&& xs, Key const& key) const;
+    };
+
+    constexpr at_key_t at_key{};
+#endif
+BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_FWD_AT_KEY_HPP