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diff --git a/boost/hana/eval.hpp b/boost/hana/eval.hpp
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+/*!
+@file
+Defines `boost::hana::eval`.
+
+@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_EVAL_HPP
+#define BOOST_HANA_EVAL_HPP
+
+#include <boost/hana/fwd/eval.hpp>
+
+#include <boost/hana/config.hpp>
+#include <boost/hana/core/dispatch.hpp>
+#include <boost/hana/detail/wrong.hpp>
+#include <boost/hana/functional/id.hpp>
+
+
+BOOST_HANA_NAMESPACE_BEGIN
+    //! @cond
+    template <typename Expr>
+    constexpr decltype(auto) eval_t::operator()(Expr&& expr) const {
+        return eval_impl<typename hana::tag_of<Expr>::type>::apply(
+            static_cast<Expr&&>(expr)
+        );
+    }
+    //! @endcond
+
+    template <typename T, bool condition>
+    struct eval_impl<T, when<condition>> : default_ {
+        template <typename Expr>
+        static constexpr auto eval_helper(Expr&& expr, int)
+            -> decltype(static_cast<Expr&&>(expr)())
+        { return static_cast<Expr&&>(expr)(); }
+
+        template <typename Expr>
+        static constexpr auto eval_helper(Expr&& expr, long)
+            -> decltype(static_cast<Expr&&>(expr)(hana::id))
+        { return static_cast<Expr&&>(expr)(hana::id); }
+
+        template <typename Expr>
+        static constexpr auto eval_helper(Expr&&, ...) {
+            static_assert(detail::wrong<Expr>{},
+            "hana::eval(expr) requires the expression to be a hana::lazy, "
+            "a nullary Callable or a unary Callable that may be "
+            "called with hana::id");
+        }
+
+        template <typename Expr>
+        static constexpr decltype(auto) apply(Expr&& expr)
+        { return eval_helper(static_cast<Expr&&>(expr), int{}); }
+    };
+BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_EVAL_HPP