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+/*!
+@file
+Forward declares `boost::hana::is_a` and `boost::hana::is_an`.
+
+@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_FWD_CORE_IS_A_HPP
+#define BOOST_HANA_FWD_CORE_IS_A_HPP
+
+#include <boost/hana/config.hpp>
+
+
+BOOST_HANA_NAMESPACE_BEGIN
+    //! @ingroup group-core
+    //! Returns whether the tag of an object matches a given tag.
+    //!
+    //! Given a tag `Tag` and a C++ type `T`, `is_a<Tag, T>` is a compile-time
+    //! Logical representing whether the tag of `T` is exactly `Tag`. In other
+    //! words, it is equivalent to
+    //! @code
+    //!     std::is_same<Tag, tag_of<T>::type>
+    //! @endcode
+    //!
+    //! For convenience, an alternate syntax is provided for using `is_a`.
+    //! Specifically, `is_a<Tag>` is a function object returning whether the
+    //! argument it is passed has the given tag. In other words,
+    //! @code
+    //!     is_a<Tag>(x) == is_a<Tag, decltype(x)>
+    //! @endcode
+    //!
+    //!
+    //! Example
+    //! -------
+    //! @include example/core/is_a.cpp
+#ifdef BOOST_HANA_DOXYGEN_INVOKED
+    template <typename Tag, typename optional_T>
+    constexpr auto is_a = see-documentation;
+#else
+    template <typename Tag, typename ...T>
+    struct is_a_t;
+
+    template <typename Tag, typename ...T>
+    constexpr is_a_t<Tag, T...> is_a{};
+#endif
+
+    //! @ingroup group-core
+    //! Equivalent to `is_a`; provided for consistency with the rules of the
+    //! English language.
+#ifdef BOOST_HANA_DOXYGEN_INVOKED
+    template <typename Tag, typename ...T>
+    constexpr auto is_an = is_a<Tag, T...>;
+#else
+    template <typename Tag, typename ...T>
+    constexpr is_a_t<Tag, T...> is_an{};
+#endif
+BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_FWD_CORE_IS_A_HPP