Squashed 'third_party/immer/' content from commit ad3e3556d

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

immer/refcount/enable_intrusive_ptr.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/refcount/no_refcount_policy.hpp>
+
+namespace immer {
+
+template <typename Deriv, typename RefcountPolicy>
+class enable_intrusive_ptr
+{
+    mutable RefcountPolicy refcount_data_;
+
+public:
+    enable_intrusive_ptr()
+        : refcount_data_{disowned{}}
+    {}
+
+    friend void intrusive_ptr_add_ref(const Deriv* x)
+    {
+        x->refcount_data_.inc();
+    }
+
+    friend void intrusive_ptr_release(const Deriv* x)
+    {
+        if (x->refcount_data_.dec())
+            delete x;
+    }
+};
+
+} // namespace immer