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+/*!
+@file
+Forward declares `boost::hana::Hashable`.
+
+@copyright Louis Dionne 2016
+@copyright Jason Rice 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_CONCEPT_HASHABLE_HPP
+#define BOOST_HANA_FWD_CONCEPT_HASHABLE_HPP
+
+#include <boost/hana/config.hpp>
+
+
+BOOST_HANA_NAMESPACE_BEGIN
+    //! @ingroup group-concepts
+    //! @defgroup group-Hashable Hashable
+    //! The `Hashable` concept represents objects that can be normalized to
+    //! a type-level hash.
+    //!
+    //! In day to day programming, hashes are very important as a way to
+    //! efficiently lookup objects in maps. While the implementation of
+    //! maps is very different, the same idea of using hashes for efficient
+    //! lookup applies in metaprogramming. The `Hashable` concept represents
+    //! objects that can be summarized (possibly with loss of information) to
+    //! a type, in a way suitable for use in hash-based data structures. Of
+    //! course, in order for a hash to be well-behaved, it must obey some laws
+    //! that are explained below.
+    //!
+    //!
+    //! Minimal complete definition
+    //! ---------------------------
+    //! `hash`, satisfying the laws below
+    //!
+    //!
+    //! Laws
+    //! ----
+    //! First, `hana::hash` must return a `hana::type`. Furthermore, for any
+    //! two `Hashable` objects `x` and `y`, it must be the case that
+    //! @code
+    //!     x == y   implies   hash(x) == hash(y)
+    //! @endcode
+    //!
+    //! where `==` denotes `hana::equal`. In other words, any two objects that
+    //! compare equal (with `hana::equal`) must also have the same hash.
+    //! However, the reverse is not true, and two different objects may have
+    //! the same hash. This situation of two different objects having the same
+    //! hash is called a _collision_.
+    //!
+    //!
+    //! Concrete models
+    //! ---------------
+    //! `hana::integral_constant`, `hana::type`, `hana::string`
+    //!
+    //!
+    //! Free model for `IntegralConstant`s
+    //! ----------------------------------
+    //! Any `IntegralConstant` is `Hashable`, by normalizing its value to a
+    //! `hana::integral_constant`. The type of the value held in the normalized
+    //! `integral_constant` is `unsigned long long` for unsigned integral
+    //! types, and `signed long long` for signed integral types.
+    template <typename T>
+    struct Hashable;
+BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_FWD_CONCEPT_HASHABLE_HPP