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diff --git a/boost/hana/detail/first_unsatisfied_index.hpp b/boost/hana/detail/first_unsatisfied_index.hpp
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+++ b/boost/hana/detail/first_unsatisfied_index.hpp
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+/*!
+@file
+Defines `boost::hana::detail::first_unsatisfied_index`.
+
+@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_FIRST_UNSATISFIED_INDEX_HPP
+#define BOOST_HANA_DETAIL_FIRST_UNSATISFIED_INDEX_HPP
+
+#include <boost/hana/config.hpp>
+#include <boost/hana/integral_constant.hpp>
+#include <boost/hana/value.hpp>
+
+#include <utility>
+
+
+BOOST_HANA_NAMESPACE_BEGIN namespace detail {
+    template <bool, typename Pred, typename ...Xs>
+    struct find_tail_size;
+
+    template <typename Pred, typename X, typename ...Xs>
+    struct find_tail_size<true, Pred, X, Xs...> {
+        static constexpr int value = find_tail_size<
+            static_cast<bool>(hana::value<decltype(std::declval<Pred>()(std::declval<X>()))>()),
+            Pred, Xs...
+        >::value;
+    };
+
+    template <typename Pred>
+    struct find_tail_size<true, Pred> {
+        static constexpr int value = -1;
+    };
+
+    template <typename Pred, typename ...Xs>
+    struct find_tail_size<false, Pred, Xs...> {
+        static constexpr int value = sizeof...(Xs);
+    };
+
+    //! @ingroup group-details
+    //! Returns the index of the first element which does not satisfy `Pred`,
+    //! or `sizeof...(Xs)` if no such element exists.
+    template <typename Pred>
+    struct first_unsatisfied_index {
+        template <typename ...Xs>
+        constexpr auto operator()(Xs&& ...) const {
+            return hana::size_c<
+                sizeof...(Xs) - 1 - find_tail_size<true, Pred, Xs&&...>::value
+            >;
+        }
+    };
+} BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_DETAIL_FIRST_UNSATISFIED_INDEX_HPP