add subtree-ish sources for 12.0.3

[ceph.git] / ceph / src / boost / libs / hana / include / boost / hana / fwd / front.hpp

diff --git a/ceph/src/boost/libs/hana/include/boost/hana/fwd/front.hpp b/ceph/src/boost/libs/hana/include/boost/hana/fwd/front.hpp
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+++ b/ceph/src/boost/libs/hana/include/boost/hana/fwd/front.hpp
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+/*!
+@file
+Forward declares `boost::hana::front`.
+
+@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_FRONT_HPP
+#define BOOST_HANA_FWD_FRONT_HPP
+
+#include <boost/hana/config.hpp>
+#include <boost/hana/core/when.hpp>
+
+
+BOOST_HANA_NAMESPACE_BEGIN
+    //! Returns the first element of a non-empty iterable.
+    //! @ingroup group-Iterable
+    //!
+    //! Given a non-empty Iterable `xs` with a linearization of `[x1, ..., xN]`,
+    //! `front(xs)` is equal to `x1`. If `xs` is empty, it is an error to
+    //! use this function. Equivalently, `front(xs)` must be equivalent to
+    //! `at_c<0>(xs)`, and that regardless of the value category of `xs`
+    //! (`front` must respect the reference semantics of `at`).
+    //!
+    //!
+    //! Example
+    //! -------
+    //! @include example/front.cpp
+#ifdef BOOST_HANA_DOXYGEN_INVOKED
+    constexpr auto front = [](auto&& xs) -> decltype(auto) {
+        return tag-dispatched;
+    };
+#else
+    template <typename It, typename = void>
+    struct front_impl : front_impl<It, when<true>> { };
+
+    struct front_t {
+        template <typename Xs>
+        constexpr decltype(auto) operator()(Xs&& xs) const;
+    };
+
+    constexpr front_t front{};
+#endif
+BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_FWD_FRONT_HPP