Squashed 'third_party/immer/' content from commit ad3e3556d

git-subtree-dir: third_party/immer
git-subtree-split: ad3e3556d38bb75966dd24c61a774970a7c7957e

immer/heap/free_list_node.hpp

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+//
+// immer: immutable data structures for C++
+// Copyright (C) 2016, 2017, 2018 Juan Pedro Bolivar Puente
+//
+// This software is distributed under the Boost Software License, Version 1.0.
+// See accompanying file LICENSE or copy at http://boost.org/LICENSE_1_0.txt
+//
+
+#pragma once
+
+#include <immer/heap/with_data.hpp>
+
+namespace immer {
+
+struct free_list_node
+{
+    free_list_node* next;
+};
+
+template <typename Base>
+struct with_free_list_node : with_data<free_list_node, Base>
+{};
+
+} // namespace immer