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

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

@@ -0,0 +1,63 @@
+/*!
+@file
+Forward declares `boost::hana::flatten`.
+
+@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_FLATTEN_HPP
+#define BOOST_HANA_FWD_FLATTEN_HPP
+
+#include <boost/hana/config.hpp>
+#include <boost/hana/core/when.hpp>
+
+
+BOOST_HANA_NAMESPACE_BEGIN
+    //! Collapse two levels of monadic structure into a single level.
+    //! @ingroup group-Monad
+    //!
+    //! Given a monadic value wrapped into two levels of monad, `flatten`
+    //! removes one such level. An implementation of `flatten` must satisfy
+    //! @code
+    //!     flatten(xs) == chain(xs, id)
+    //! @endcode
+    //!
+    //! For `Sequence`s, this simply takes a `Sequence` of `Sequence`s, and
+    //! returns a (non-recursively) flattened `Sequence`.
+    //!
+    //!
+    //! Signature
+    //! ---------
+    //! For a `Monad` `M`, the signature of `flatten` is
+    //! @f$
+    //!     \mathtt{flatten} : M(M(T)) \to M(T)
+    //! @f$
+    //!
+    //! @param xs
+    //! A value with two levels of monadic structure, which should be
+    //! collapsed into a single level of structure.
+    //!
+    //!
+    //! Example
+    //! -------
+    //! @include example/flatten.cpp
+#ifdef BOOST_HANA_DOXYGEN_INVOKED
+    constexpr auto flatten = [](auto&& xs) {
+        return tag-dispatched;
+    };
+#else
+    template <typename M, typename = void>
+    struct flatten_impl : flatten_impl<M, when<true>> { };
+
+    struct flatten_t {
+        template <typename Xs>
+        constexpr auto operator()(Xs&& xs) const;
+    };
+
+    constexpr flatten_t flatten{};
+#endif
+BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_FWD_FLATTEN_HPP