stabilize build system: depends, installer, boost/bdb fixes, cross targets groundwork

depends/x86_64-pc-linux-gnu/include/boost/hana/fwd/plus.hpp

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+/*!
+@file
+Forward declares `boost::hana::plus`.
+
+@copyright Louis Dionne 2013-2017
+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_PLUS_HPP
+#define BOOST_HANA_FWD_PLUS_HPP
+
+#include <boost/hana/config.hpp>
+#include <boost/hana/core/when.hpp>
+
+
+BOOST_HANA_NAMESPACE_BEGIN
+    //! Associative binary operation on a `Monoid`.
+    //! @ingroup group-Monoid
+    //!
+    //! @param x, y
+    //! Two objects to combine with the `Monoid`'s binary operation.
+    //!
+    //!
+    //! Cross-type version of the method
+    //! --------------------------------
+    //! The `plus` method is "overloaded" to handle distinct data types
+    //! with certain properties. Specifically, `plus` is defined for
+    //! _distinct_ data types `A` and `B` such that
+    //! 1. `A` and `B` share a common data type `C`, as determined by the
+    //!    `common` metafunction
+    //! 2. `A`, `B` and `C` are all `Monoid`s when taken individually
+    //! 3. `to<C> : A -> B` and `to<C> : B -> C` are `Monoid`-embeddings, as
+    //!    determined by the `is_embedding` metafunction.
+    //!
+    //! The definition of `plus` for data types satisfying the above
+    //! properties is obtained by setting
+    //! @code
+    //!     plus(x, y) = plus(to<C>(x), to<C>(y))
+    //! @endcode
+    //!
+    //!
+    //! Example
+    //! -------
+    //! @include example/plus.cpp
+#ifdef BOOST_HANA_DOXYGEN_INVOKED
+    constexpr auto plus = [](auto&& x, auto&& y) -> decltype(auto) {
+        return tag-dispatched;
+    };
+#else
+    template <typename T, typename U, typename = void>
+    struct plus_impl : plus_impl<T, U, when<true>> { };
+
+    struct plus_t {
+        template <typename X, typename Y>
+        constexpr decltype(auto) operator()(X&& x, Y&& y) const;
+    };
+
+    constexpr plus_t plus{};
+#endif
+BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_FWD_PLUS_HPP