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

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

@@ -0,0 +1,58 @@
+/*!
+@file
+Forward declares `boost::hana::eval`.
+
+@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_EVAL_HPP
+#define BOOST_HANA_FWD_EVAL_HPP
+
+#include <boost/hana/config.hpp>
+#include <boost/hana/core/when.hpp>
+
+
+BOOST_HANA_NAMESPACE_BEGIN
+    //! Evaluate a lazy value and return it.
+    //! @relates hana::lazy
+    //!
+    //! Given a lazy expression `expr`, `eval` evaluates `expr` and returns
+    //! the result as a normal value. However, for convenience, `eval` can
+    //! also be used with nullary and unary function objects. Specifically,
+    //! if `expr` is not a `hana::lazy`, it is called with no arguments at
+    //! all and the result of that call (`expr()`) is returned. Otherwise,
+    //! if `expr()` is ill-formed, then `expr(hana::id)` is returned instead.
+    //! If that expression is ill-formed, then a compile-time error is
+    //! triggered.
+    //!
+    //! The reason for allowing nullary callables in `eval` is because this
+    //! allows using nullary lambdas as lazy branches to `eval_if`, which
+    //! is convenient. The reason for allowing unary callables and calling
+    //! them with `hana::id` is because this allows deferring the
+    //! compile-time evaluation of selected expressions inside the callable.
+    //! How this can be achieved is documented by `hana::eval_if`.
+    //!
+    //!
+    //! Example
+    //! -------
+    //! @include example/eval.cpp
+#ifdef BOOST_HANA_DOXYGEN_INVOKED
+    constexpr auto eval = [](auto&& see_documentation) -> decltype(auto) {
+        return tag-dispatched;
+    };
+#else
+    template <typename T, typename = void>
+    struct eval_impl : eval_impl<T, when<true>> { };
+
+    struct eval_t {
+        template <typename Expr>
+        constexpr decltype(auto) operator()(Expr&& expr) const;
+    };
+
+    constexpr eval_t eval{};
+#endif
+BOOST_HANA_NAMESPACE_END
+
+#endif // !BOOST_HANA_FWD_EVAL_HPP