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diff --git a/boost/hana/detail/unpack_flatten.hpp b/boost/hana/detail/unpack_flatten.hpp
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+++ b/boost/hana/detail/unpack_flatten.hpp
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+/*!
+@file
+Defines `boost::hana::detail::unpack_flatten`.
+
+@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_UNPACK_FLATTEN_HPP
+#define BOOST_HANA_DETAIL_UNPACK_FLATTEN_HPP
+
+#include <boost/hana/at.hpp>
+#include <boost/hana/config.hpp>
+#include <boost/hana/detail/algorithm.hpp>
+#include <boost/hana/detail/array.hpp>
+#include <boost/hana/length.hpp>
+#include <boost/hana/unpack.hpp>
+
+#include <cstddef>
+#include <utility>
+
+
+BOOST_HANA_NAMESPACE_BEGIN namespace detail {
+    template <std::size_t ...Lengths>
+    struct flatten_indices {
+        // avoid empty arrays by appending 0 to `lengths`
+        static constexpr std::size_t lengths[] = {Lengths..., 0};
+        static constexpr auto flat_length =
+            detail::accumulate(lengths, lengths + sizeof...(Lengths), 0);
+
+        template <bool Inner>
+        static constexpr auto compute() {
+            detail::array<std::size_t, flat_length> indices{};
+            for (std::size_t index = 0, i = 0; i < sizeof...(Lengths); ++i)
+                for (std::size_t j = 0; j < lengths[i]; ++j, ++index)
+                    indices[index] = (Inner ? i : j);
+            return indices;
+        }
+
+        static constexpr auto inner = compute<true>();
+        static constexpr auto outer = compute<false>();
+
+        template <typename Xs, typename F, std::size_t ...i>
+        static constexpr decltype(auto)
+        apply(Xs&& xs, F&& f, std::index_sequence<i...>) {
+            return static_cast<F&&>(f)(
+                hana::at_c<outer[i]>(hana::at_c<inner[i]>(
+                    static_cast<Xs&&>(xs)
+                ))...
+            );
+        }
+    };
+
+    struct make_flatten_indices {
+        template <typename ...Xs>
+        auto operator()(Xs const& ...xs) const -> detail::flatten_indices<
+            decltype(hana::length(xs))::value...
+        >;
+    };
+
+    template <typename Xs, typename F>
+    constexpr decltype(auto) unpack_flatten(Xs&& xs, F&& f) {
+        using Indices = decltype(hana::unpack(xs, make_flatten_indices{}));
+        return Indices::apply(static_cast<Xs&&>(xs), static_cast<F&&>(f),
+                        std::make_index_sequence<Indices::flat_length>{});
+    }
+} BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_DETAIL_UNPACK_FLATTEN_HPP