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diff --git a/third_party/immer/immer/heap/gc_heap.hpp b/third_party/immer/immer/heap/gc_heap.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/config.hpp>
+#include <immer/heap/tags.hpp>
+
+#if IMMER_HAS_LIBGC
+#include <gc/gc.h>
+#else
+#error "Using garbage collection requires libgc"
+#endif
+
+#include <cstdlib>
+#include <memory>
+
+namespace immer {
+
+#ifdef __APPLE__
+#define IMMER_GC_REQUIRE_INIT 1
+#else
+#define IMMER_GC_REQUIRE_INIT 0
+#endif
+
+#if IMMER_GC_REQUIRE_INIT
+
+namespace detail {
+
+template <int Dummy = 0>
+struct gc_initializer
+{
+    gc_initializer() { GC_init(); }
+    static gc_initializer init;
+};
+
+template <int D>
+gc_initializer<D> gc_initializer<D>::init{};
+
+inline void gc_initializer_guard()
+{
+    static gc_initializer<> init_ = gc_initializer<>::init;
+    (void) init_;
+}
+
+} // namespace detail
+
+#define IMMER_GC_INIT_GUARD_ ::immer::detail::gc_initializer_guard()
+
+#else
+
+#define IMMER_GC_INIT_GUARD_
+
+#endif // IMMER_GC_REQUIRE_INIT
+
+/*!
+ * Heap that uses a tracing garbage collector.
+ *
+ * @rst
+ *
+ * This heap uses the `Boehm's conservative garbage collector`_ under
+ * the hood.  This is a tracing garbage collector that automatically
+ * reclaims unused memory.  Thus, it is not needed to call
+ * ``deallocate()`` in order to release memory.
+ *
+ * .. admonition:: Dependencies
+ *    :class: tip
+ *
+ *    In order to use this header file, you need to make sure that
+ *    Boehm's ``libgc`` is your include path and link to its binary
+ *    library.
+ *
+ * .. caution:: Memory that is allocated with the standard ``malloc``
+ *    and ``free`` is not visible to ``libgc`` when it is looking for
+ *    references.  This means that if, let's say, you store a
+ *    :cpp:class:`immer::vector` using a ``gc_heap`` inside a
+ *    ``std::vector`` that uses a standard allocator, the memory of
+ *    the former might be released automatically at unexpected times
+ *    causing crashes.
+ *
+ * .. caution:: When using a ``gc_heap`` in combination with immutable
+ *    containers, the destructors of the contained objects will never
+ *    be called.  It is ok to store containers inside containers as
+ *    long as all of them use a ``gc_heap`` consistently, but storing
+ *    other kinds of objects with relevant destructors
+ *    (e.g. containers with reference counting or other kinds of
+ *    *resource handles*) might cause memory leaks and other problems.
+ *
+ * .. _boehm's conservative garbage collector: https://github.com/ivmai/bdwgc
+ *
+ * @endrst
+ */
+class gc_heap
+{
+public:
+    static void* allocate(std::size_t n)
+    {
+        IMMER_GC_INIT_GUARD_;
+        auto p = GC_malloc(n);
+        if (IMMER_UNLIKELY(!p))
+            throw std::bad_alloc{};
+        return p;
+    }
+
+    static void* allocate(std::size_t n, norefs_tag)
+    {
+        IMMER_GC_INIT_GUARD_;
+        auto p = GC_malloc_atomic(n);
+        if (IMMER_UNLIKELY(!p))
+            throw std::bad_alloc{};
+        return p;
+    }
+
+    static void deallocate(std::size_t, void* data) { GC_free(data); }
+
+    static void deallocate(std::size_t, void* data, norefs_tag)
+    {
+        GC_free(data);
+    }
+};
+
+} // namespace immer