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+/*!
+@file
+Defines `boost::hana::detail::ebo`.
+
+@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_EBO_HPP
+#define BOOST_HANA_DETAIL_EBO_HPP
+
+#include <boost/hana/config.hpp>
+#include <boost/hana/detail/intrinsics.hpp>
+
+
+namespace _hana {
+    //////////////////////////////////////////////////////////////////////////
+    // ebo<K, V>
+    //
+    // Building block to implement the Empty Base Optimization (EBO). We
+    // use a short name and define it in a short namespace to reduce
+    // symbol lengths, since this type is used as a building block for
+    // other widely used types such as `hana::pair`.
+    //
+    // When available, we use compiler intrinsics to reduce the number
+    // of instantiations.
+    //
+    // `ebo` provides a limited set of constructors to reduce instantiations.
+    // Also, the constructors are open-ended and they do not check for the
+    // validity of their arguments, again to reduce compile-time costs.
+    // Users of `ebo` should make sure that they only try to construct an
+    // `ebo` from a compatible value.
+    //
+    // EBOs can be indexed using an arbitrary type. The recommended usage is
+    // to define an integrap constant wrapper for the specific container using
+    // EBO, and then index using that wrapper:
+    //
+    //      template <int> struct idx; // wrapper for tuple
+    //      template <typename ...T>
+    //      struct tuple : ebo<idx<0>, T0>, ebo<idx<1>, T1>, ... { };
+    //
+    // The reason for defining one wrapper per container is to avoid any issues
+    // that can arise when using `ebo_get`, which casts to the base class. If
+    // `tuple` and `pair` are inheritting from `ebo`s with the same indexing
+    // scheme, trying to use `ebo_get` on a tuple of pairs will trigger an
+    // ambiguous base class conversion, since both tuple and pair inherit
+    // from `ebo`s with the same keys.
+    //////////////////////////////////////////////////////////////////////////
+    template <typename K, typename V, bool =
+        BOOST_HANA_TT_IS_EMPTY(V) && !BOOST_HANA_TT_IS_FINAL(V)
+    >
+    struct ebo;
+
+    // Specialize storage for empty types
+    template <typename K, typename V>
+    struct ebo<K, V, true> : V {
+        constexpr ebo() { }
+
+        template <typename T>
+        explicit constexpr ebo(T&& t)
+            : V(static_cast<T&&>(t))
+        { }
+    };
+
+    // Specialize storage for non-empty types
+    template <typename K, typename V>
+    struct ebo<K, V, false> {
+        constexpr ebo() : data_() { }
+
+        template <typename T>
+        explicit constexpr ebo(T&& t)
+            : data_(static_cast<T&&>(t))
+        { }
+
+        V data_;
+    };
+
+    //////////////////////////////////////////////////////////////////////////
+    // ebo_get
+    //////////////////////////////////////////////////////////////////////////
+    template <typename K, typename V>
+    constexpr V const& ebo_get(ebo<K, V, true> const& x)
+    { return x; }
+
+    template <typename K, typename V>
+    constexpr V& ebo_get(ebo<K, V, true>& x)
+    { return x; }
+
+    template <typename K, typename V>
+    constexpr V&& ebo_get(ebo<K, V, true>&& x)
+    { return static_cast<V&&>(x); }
+
+
+    template <typename K, typename V>
+    constexpr V const& ebo_get(ebo<K, V, false> const& x)
+    { return x.data_; }
+
+    template <typename K, typename V>
+    constexpr V& ebo_get(ebo<K, V, false>& x)
+    { return x.data_; }
+
+    template <typename K, typename V>
+    constexpr V&& ebo_get(ebo<K, V, false>&& x)
+    { return static_cast<V&&>(x.data_); }
+} // end namespace _hana
+
+BOOST_HANA_NAMESPACE_BEGIN
+    namespace detail {
+        using ::_hana::ebo;
+        using ::_hana::ebo_get;
+    }
+BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_DETAIL_EBO_HPP