Squashed 'third_party/immer/' content from commit ad3e3556d

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

14 files changed, 2272 insertions, 0 deletions

diff --git a/test/flex_vector/B3-BL0.cpp b/test/flex_vector/B3-BL0.cpp
new file mode 100644
index 000000000000..83af21d688fc
--- /dev/null
+++ b/test/flex_vector/B3-BL0.cpp
@@ -0,0 +1,21 @@
+//
+// 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
+//
+
+#include <immer/flex_vector.hpp>
+#include <immer/vector.hpp>
+
+template <typename T>
+using test_flex_vector_t =
+    immer::flex_vector<T, immer::default_memory_policy, 3u, 0u>;
+
+template <typename T>
+using test_vector_t = immer::vector<T, immer::default_memory_policy, 3u, 0u>;
+
+#define FLEX_VECTOR_T test_flex_vector_t
+#define VECTOR_T test_vector_t
+#include "generic.ipp"
diff --git a/test/flex_vector/B3-BL3.cpp b/test/flex_vector/B3-BL3.cpp
new file mode 100644
index 000000000000..e44f7e2bfaba
--- /dev/null
+++ b/test/flex_vector/B3-BL3.cpp
@@ -0,0 +1,21 @@
+//
+// 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
+//
+
+#include <immer/flex_vector.hpp>
+#include <immer/vector.hpp>
+
+template <typename T>
+using test_flex_vector_t =
+    immer::flex_vector<T, immer::default_memory_policy, 3u, 3u>;
+
+template <typename T>
+using test_vector_t = immer::vector<T, immer::default_memory_policy, 3u, 3u>;
+
+#define FLEX_VECTOR_T test_flex_vector_t
+#define VECTOR_T test_vector_t
+#include "generic.ipp"
diff --git a/test/flex_vector/default.cpp b/test/flex_vector/default.cpp
new file mode 100644
index 000000000000..7258cc07728d
--- /dev/null
+++ b/test/flex_vector/default.cpp
@@ -0,0 +1,14 @@
+//
+// 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
+//
+
+#include <immer/flex_vector.hpp>
+#include <immer/vector.hpp>
+
+#define FLEX_VECTOR_T ::immer::flex_vector
+#define VECTOR_T ::immer::vector
+#include "generic.ipp"
diff --git a/test/flex_vector/fuzzed-0.cpp b/test/flex_vector/fuzzed-0.cpp
new file mode 100644
index 000000000000..063ad133a09d
--- /dev/null
+++ b/test/flex_vector/fuzzed-0.cpp
@@ -0,0 +1,380 @@
+//
+// 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
+//
+
+#include "extra/fuzzer/fuzzer_input.hpp"
+#include <array>
+#include <catch.hpp>
+#include <immer/flex_vector.hpp>
+#include <iostream>
+
+#define IMMER_FUZZED_TRACE_ENABLE 0
+
+#if IMMER_FUZZED_TRACE_ENABLE
+#define IMMER_FUZZED_TRACE(...) std::cout << __VA_ARGS__ << std::endl;
+#else
+#define IMMER_FUZZED_TRACE(...)
+#endif
+
+namespace {
+
+template <std::size_t VarCount = 8, unsigned Bits = 3>
+int run_input(const std::uint8_t* data, std::size_t size)
+{
+    using vector_t =
+        immer::flex_vector<int, immer::default_memory_policy, Bits, Bits>;
+    using size_t = std::uint8_t;
+
+    auto vars = std::array<vector_t, VarCount>{};
+
+#if IMMER_FUZZED_TRACE_ENABLE
+    for (auto i = 0u; i < VarCount; ++i)
+        std::cout << "auto var" << i << " = vector_t{};" << std::endl;
+#endif
+
+    auto is_valid_var   = [&](auto idx) { return idx >= 0 && idx < VarCount; };
+    auto is_valid_index = [](auto& v) {
+        return [&](auto idx) { return idx >= 0 && idx < v.size(); };
+    };
+    auto is_valid_size = [](auto& v) {
+        return [&](auto idx) { return idx >= 0 && idx <= v.size(); };
+    };
+
+    return fuzzer_input{data, size}.run([&](auto& in) {
+        enum ops
+        {
+            op_push_back,
+            op_update,
+            op_take,
+            op_drop,
+            op_concat,
+        };
+        auto src = read<std::uint8_t>(in, is_valid_var);
+        auto dst = read<std::uint8_t>(in, is_valid_var);
+        switch (read<char>(in)) {
+        case op_push_back:
+            IMMER_FUZZED_TRACE("var" << +dst << " = var" << +src
+                                     << ".push_back(42);");
+            vars[dst] = vars[src].push_back(42);
+            break;
+        case op_update: {
+            auto idx = read<size_t>(in, is_valid_index(vars[src]));
+            IMMER_FUZZED_TRACE("var" << +dst << " = var" << +src << ".update("
+                                     << +idx
+                                     << ", [] (auto x) { return x + 1; });");
+            vars[dst] = vars[src].update(idx, [](auto x) { return x + 1; });
+            break;
+        }
+        case op_take: {
+            auto idx = read<size_t>(in, is_valid_size(vars[src]));
+            IMMER_FUZZED_TRACE("var" << +dst << " = var" << +src << ".take("
+                                     << +idx << ");");
+            vars[dst] = vars[src].take(idx);
+            break;
+        }
+        case op_drop: {
+            auto idx = read<size_t>(in, is_valid_size(vars[src]));
+            IMMER_FUZZED_TRACE("var" << +dst << " = var" << +src << ".take("
+                                     << +idx << ");");
+            vars[dst] = vars[src].drop(idx);
+            break;
+        }
+        case op_concat: {
+            auto src2 = read<std::uint8_t>(in, is_valid_var);
+            IMMER_FUZZED_TRACE("var" << +dst << " = var" << +src << " + var"
+                                     << +src2 << ";");
+            vars[dst] = vars[src] + vars[src2];
+            break;
+        }
+        default:
+            break;
+        };
+        return true;
+    });
+}
+
+} // anonymous namespace
+
+TEST_CASE("bug: concatenate too big vectors")
+{
+    // The problem here was that since we were using 32 bit sizes,
+    // concatenating big flex_vectors can overflow the sizes.  Let's
+    // just use std::size_t like normal people.
+    //
+    // Still, the problem could re-ocurr with longer inputs.  For this
+    // reason later fuzzing efforts do check that concatenation is
+    // valid for the given vector sizes.  Similar assertions are put
+    // in the code too.
+    SECTION("simplified example")
+    {
+        using vector_t =
+            immer::flex_vector<int, immer::default_memory_policy, 3, 3>;
+        auto var0 = vector_t{};
+        auto var1 = vector_t{};
+        auto var2 = vector_t{};
+        auto var4 = vector_t{};
+        var1      = var1.push_back(42);
+        var0      = var0.push_back(42);
+        var0      = var0.push_back(42);
+        var0      = var2.push_back(42);
+        var0      = var0.push_back(42);
+        var2      = var0;
+        var0      = var0.push_back(42);
+        var0      = var0.push_back(42);
+        var4      = var4.push_back(42);
+        var0      = var0.push_back(42);
+        var0      = var0.push_back(42);
+        var0      = var0 + var0;
+        var0      = var0.push_back(42);
+        var0      = var0.push_back(42);
+        var2      = var0.push_back(42);
+        var0      = var0 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var0      = var0.push_back(42);
+        var0      = var0.push_back(42);
+        var1      = var2 + var4;
+        var4      = var4 + var4;
+        var0      = var1.push_back(42);
+        var0      = var0.push_back(42);
+        var0      = var0 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4.push_back(42);
+    }
+
+#if __GNUC__ != 9
+    // Assertion `!p->relaxed()' failed
+    SECTION("")
+    {
+        constexpr std::uint8_t input[] = {
+            0x1,  0x1, 0x0, 0x0, 0x0,  0x0, 0x0, 0x0, 0x0, 0x39, 0x6a, 0x76,
+            0xb9, 0x2, 0x0, 0x0, 0x0,  0x0, 0x0, 0x0, 0x2, 0x1,  0x0,  0x0,
+            0x2a, 0x0, 0x0, 0x0, 0x0,  0x0, 0x4, 0x4, 0x0, 0x0,  0x0,  0x0,
+            0x0,  0x0, 0x0, 0x0, 0x0,  0x4, 0x0, 0x0, 0x0, 0x0,  0x0,  0x0,
+            0x0,  0x0, 0x2, 0x0, 0x0,  0x0, 0x4, 0x4, 0x4, 0x4,  0x4,  0x4,
+            0x4,  0x4, 0x4, 0x4, 0x4,  0x4, 0x4, 0x4, 0x0, 0x0,  0x0,  0x0,
+            0x0,  0x0, 0x2, 0x1, 0x4,  0x4, 0x4, 0x4, 0x4, 0x4,  0x1,  0x0,
+            0x0,  0x0, 0x0, 0x0, 0x0,  0x0, 0x4, 0x4, 0x4, 0x4,  0x4,  0x4,
+            0x4,  0x4, 0x4, 0x4, 0x4,  0x4, 0x4, 0x4, 0x4, 0x4,  0x4,  0x4,
+            0x4,  0x4, 0x4, 0x4, 0x4,  0x4, 0x4, 0x4, 0x4, 0x4,  0x4,  0x4,
+            0x4,  0x4, 0x4, 0x4, 0x4,  0x4, 0x4, 0x4, 0x4, 0x4,  0x4,  0x4,
+            0x4,  0x4, 0x4, 0x4, 0x4,  0x4, 0x4, 0x4, 0x4, 0x4,  0x4,  0x4,
+            0x4,  0x4, 0x4, 0x4, 0x4,  0x4, 0x4, 0x4, 0x4, 0x4,  0x4,  0x4,
+            0x4,  0x4, 0x4, 0x4, 0x4,  0x4, 0x4, 0x4, 0x4, 0x4,  0x4,  0x4,
+            0x4,  0x4, 0x4, 0x4, 0x4,  0x4, 0x4, 0x4, 0x4, 0x4,  0x4,  0x4,
+            0x4,  0x4, 0x4, 0x4, 0x4,  0x4, 0x4, 0x4, 0x4, 0x4,  0x4,  0x4,
+            0x4,  0x4, 0x4, 0x4, 0x4,  0x4, 0x4, 0x4, 0x4, 0x4,  0x4,  0x4,
+            0x4,  0x4, 0x0, 0x0, 0x2a, 0x4, 0x4, 0x4, 0x4, 0x4,  0x4,  0x4,
+            0x4,  0x4, 0x4, 0x4, 0x0,  0x0, 0x0,
+        };
+        CHECK(run_input(input, sizeof(input)) == 0);
+    }
+    SECTION("")
+    {
+        constexpr std::uint8_t input[] = {
+            0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0x1,  0x1,  0x0,
+            0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0x0,
+            0x0,  0x3,  0x0,  0x0,  0x0,  0x0,  0x0,  0x4,  0x0,  0x0,  0x0,
+            0x0,  0x0,  0x0,  0xe2, 0xe2, 0xe2, 0xe2, 0xe2, 0xe2, 0xe2, 0xe2,
+            0xe2, 0xe2, 0xe2, 0xe2, 0xe2, 0xe2, 0xe2, 0xe2, 0x4,  0x4,  0x4,
+            0x4,  0x4,  0xf8, 0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,
+            0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,
+            0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x21, 0x4,
+            0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,
+            0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,
+            0x4,  0x4,  0xb,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,
+            0x4,  0x4,  0x4,  0x4,  0x4,  0x17, 0x4,  0xe2, 0xe2, 0xe2, 0x2a,
+            0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,
+            0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,
+            0x4,  0x21, 0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,
+            0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,
+            0x4,  0x4,  0x4,  0x4,  0x4,  0x17, 0x4,  0xe2, 0xe2, 0xe2, 0x2a,
+            0xe2, 0xe2, 0xe2, 0xe2, 0xe2, 0x1f, 0xe2, 0x0,  0x0,  0x0,  0x0,
+            0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0xff, 0xe2,
+            0xe2, 0xe2, 0xe2, 0xe2, 0xe2, 0x0,  0x0,  0x0,  0x15, 0x15, 0x15,
+            0x15, 0x15, 0x15, 0x15, 0x15, 0x15, 0x15, 0x4,  0x4,  0x4,  0x4,
+            0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x15, 0x15, 0x15, 0x15,
+            0x0,  0x0,
+        };
+        CHECK(run_input(input, sizeof(input)) == 0);
+    }
+    SECTION("")
+    {
+        constexpr std::uint8_t input[] = {
+            0x0,  0x1,  0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0x21,
+            0x0,  0x0,  0x0,  0x0,  0xff, 0xff, 0xff, 0xff, 0xff, 0x0,  0x0,
+            0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0x8,  0x0,  0x0,  0x0,
+            0x0,  0x0,  0x0,  0x0,  0x0,  0x27, 0x0,  0x21, 0x0,  0x0,  0x0,
+            0x0,  0x0,  0x1,  0x0,  0x3a, 0x0,  0x0,  0x0,  0x0,  0x0,  0x0,
+            0x0,  0x0,  0x0,  0x0,  0x0,  0x40, 0x0,  0x0,  0x0,  0x0,  0x0,
+            0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0x1,  0x0,  0x0,  0x0,  0x0,
+            0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0x0,
+            0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0x4,  0x0,  0x0,
+            0x4,  0x0,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,
+            0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,
+            0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0xff, 0xff, 0xff,
+            0xff, 0xff, 0xff, 0xff, 0xff, 0x4,  0x4,  0x4,  0x4,  0x4,  0x4,
+            0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,
+            0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,
+            0x4,  0x4,  0x4,  0x4,  0x4,  0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+            0xff, 0xff, 0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,
+            0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,
+            0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,
+            0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,
+            0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,
+            0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0xe4, 0xe4, 0x0,  0x0,
+            0x0,  0x0,  0xe4, 0x0,  0xe4, 0x0,  0x0,  0x0,  0x0,  0x0,  0x8,
+            0x0,
+        };
+        CHECK(run_input(input, sizeof(input)) == 0);
+    }
+
+    // buffer overflow when looking inside the sizes array for the
+    // index of a position
+    SECTION("")
+    {
+        constexpr std::uint8_t input[] = {
+            0x0,  0x0,  0x0,  0x0, 0x0,  0x0,  0x0, 0x0, 0x1,  0x0, 0x0, 0xff,
+            0xff, 0xff, 0xff, 0x0, 0x0,  0x0,  0x0, 0x1, 0x0,  0x0, 0x0, 0x0,
+            0x0,  0x0,  0x0,  0x0, 0x0,  0x0,  0x0, 0x4, 0x4,  0x4, 0x4, 0x4,
+            0x4,  0x4,  0x4,  0x4, 0x4,  0x4,  0x4, 0x4, 0x4,  0x4, 0x4, 0x4,
+            0x4,  0x4,  0x4,  0x4, 0x4,  0x4,  0x0, 0x0, 0x4,  0x4, 0x4, 0x4,
+            0x4,  0x4,  0x4,  0x4, 0x4,  0x0,  0x0, 0x0, 0x4,  0x4, 0x4, 0x4,
+            0x4,  0x4,  0x4,  0x4, 0x4,  0x4,  0x4, 0x4, 0x4,  0x4, 0x4, 0x4,
+            0x4,  0x4,  0x4,  0x4, 0x4,  0x4,  0x4, 0x4, 0x4,  0x4, 0x0, 0x4,
+            0x4,  0x4,  0x4,  0x4, 0x3,  0xff, 0x0, 0x0, 0x4,  0x4, 0x4, 0x4,
+            0x4,  0x4,  0x4,  0x4, 0x4,  0x4,  0x4, 0x4, 0x4,  0x4, 0x4, 0x4,
+            0x4,  0x4,  0x4,  0x4, 0x0,  0x0,  0x0, 0x0, 0x0,  0x0, 0x4, 0x4,
+            0x4,  0x4,  0x4,  0x4, 0x4,  0x4,  0x4, 0x4, 0x4,  0x4, 0x4, 0x4,
+            0x4,  0x4,  0x4,  0x4, 0x4,  0x4,  0x4, 0x4, 0x4,  0x4, 0x4, 0x4,
+            0x4,  0x4,  0x4,  0x4, 0x4,  0x4,  0x4, 0x4, 0x0,  0x0, 0x4, 0x4,
+            0x4,  0x4,  0x4,  0x4, 0x4,  0x4,  0x4, 0x4, 0x4,  0x4, 0x4, 0x4,
+            0x4,  0x4,  0x4,  0x4, 0x4,  0x4,  0x4, 0x4, 0x4,  0x4, 0x4, 0x4,
+            0x4,  0x4,  0x4,  0x4, 0x4,  0x4,  0x4, 0x4, 0x1e, 0x0, 0x4, 0x4,
+            0x4,  0x4,  0x4,  0x3, 0xff, 0x0,  0x0, 0x4, 0x4,  0x4, 0x4, 0x4,
+            0x4,  0x4,  0x4,  0x4, 0x4,  0x4,  0x4, 0x4, 0x4,  0x4, 0x4, 0x4,
+            0x4,  0x4,  0x4,  0x4, 0x4,  0x4,  0x4, 0x4, 0x4,  0x0, 0x0, 0x0,
+            0x0,  0x0,  0x0,  0x0, 0xdb, 0x0,  0x0, 0x0, 0x0,  0x0, 0x0, 0x0,
+            0x0,  0x0,  0x0,
+        };
+        CHECK(run_input(input, sizeof(input)) == 0);
+    }
+    SECTION("")
+    {
+        constexpr std::uint8_t input[] = {
+            0x0,  0x0, 0x0, 0x0, 0x0, 0x0, 0x0,  0x0,  0x0, 0x0, 0x0, 0x0, 0x0,
+            0x0,  0x0, 0x0, 0x0, 0x0, 0x0, 0x0,  0x0,  0x0, 0x0, 0x0, 0x0, 0x0,
+            0x0,  0x0, 0x0, 0x0, 0x0, 0x0, 0x9,  0x0,  0x0, 0x0, 0x0, 0x0, 0x1,
+            0x0,  0x0, 0x0, 0x0, 0x0, 0x0, 0x0,  0x0,  0x0, 0x0, 0x0, 0x4, 0x4,
+            0x4,  0x4, 0x4, 0x4, 0x4, 0x4, 0x4,  0x4,  0x4, 0x4, 0x4, 0x4, 0x4,
+            0x4,  0x4, 0x4, 0x4, 0x4, 0x3, 0xfa, 0x4,  0x4, 0x4, 0x4, 0x4, 0x4,
+            0x4,  0x4, 0x4, 0x0, 0x0, 0x0, 0x9,  0x0,  0x0, 0x0, 0x0, 0x0, 0x1,
+            0x0,  0x0, 0x0, 0x0, 0x0, 0x0, 0x0,  0x0,  0x0, 0x0, 0x0, 0x4, 0x4,
+            0x4,  0x4, 0x4, 0x4, 0x4, 0x4, 0x4,  0x4,  0x4, 0x4, 0x4, 0x4, 0x4,
+            0x4,  0x4, 0x4, 0x4, 0x4, 0x3, 0xfa, 0x4,  0x4, 0x4, 0x4, 0x4, 0x4,
+            0x4,  0x4, 0x4, 0x4, 0x4, 0x4, 0x4,  0x4,  0x4, 0x4, 0x4, 0x4, 0x4,
+            0x4,  0x4, 0x4, 0x4, 0x4, 0x4, 0x4,  0x4,  0x4, 0x4, 0x4, 0x4, 0x4,
+            0x4,  0x4, 0x4, 0x4, 0x4, 0x4, 0x4,  0x4,  0x4, 0x4, 0x4, 0x0, 0x0,
+            0x0,  0x0, 0x0, 0x0, 0x0, 0x0, 0x4,  0x4,  0x4, 0x4, 0x4, 0x0, 0x0,
+            0x0,  0x0, 0x0, 0x0, 0x0, 0x0, 0x0,  0x0,  0x0, 0x0, 0x0, 0x0, 0x0,
+            0x0,  0x0, 0x0, 0x0, 0x0, 0x0, 0x0,  0x0,  0x0, 0x0, 0x0, 0x0, 0x0,
+            0x4,  0x4, 0x4, 0x4, 0x4, 0x4, 0x4,  0x4,  0x4, 0x4, 0x4, 0x4, 0x4,
+            0x4,  0x4, 0x4, 0x4, 0x4, 0x4, 0x4,  0x4,  0x4, 0x4, 0x4, 0x4, 0x4,
+            0x4,  0x4, 0x4, 0x4, 0x4, 0x4, 0x4,  0x4,  0x4, 0x4, 0x4, 0x4, 0x3,
+            0xfa, 0x4, 0x4, 0x4, 0x0, 0x0, 0x0,  0x55, 0x0,
+        };
+        CHECK(run_input(input, sizeof(input)) == 0);
+    }
+
+    // fail when deref some null node
+    SECTION("")
+    {
+        constexpr std::uint8_t input[] = {
+            0x0,  0x0,  0x0, 0x0,  0x0,  0x0, 0x0, 0x4,  0x0,  0x0,  0x4, 0x0,
+            0x0,  0x0,  0x0, 0x0,  0x4,  0x4, 0x4, 0x4,  0x4,  0x4,  0x0, 0x0,
+            0x4,  0x4,  0x4, 0x4,  0x4,  0x4, 0x4, 0x4,  0x4,  0x4,  0x4, 0x4,
+            0x4,  0x4,  0x4, 0x4,  0x4,  0x4, 0x4, 0x4,  0x4,  0x4,  0x4, 0x4,
+            0x4,  0x4,  0x4, 0x4,  0x4,  0x4, 0x4, 0x4,  0x0,  0x0,  0x4, 0x0,
+            0x4,  0x4,  0x4, 0xe4, 0x4,  0x4, 0x4, 0x4,  0x4,  0x4,  0x4, 0x4,
+            0x4,  0x6,  0x4, 0x4,  0x4,  0x4, 0x4, 0x4,  0x4,  0x4,  0x4, 0x4,
+            0x4,  0x4,  0x4, 0x4,  0xe5, 0x0, 0x4, 0x4,  0x4,  0x4,  0x4, 0x4,
+            0x0,  0x0,  0x4, 0x4,  0x4,  0x4, 0x4, 0x4,  0x4,  0x4,  0x4, 0x4,
+            0x4,  0x4,  0x4, 0x4,  0x4,  0x4, 0x4, 0x4,  0x4,  0x4,  0x4, 0x4,
+            0x4,  0x4,  0x4, 0x4,  0xff, 0x3, 0x4, 0x4,  0x4,  0x0,  0x0, 0x0,
+            0x0,  0x4,  0x4, 0x4,  0x4,  0x4, 0x4, 0x4,  0x4,  0x4,  0x4, 0x4,
+            0x4,  0x4,  0x4, 0x4,  0x4,  0x0, 0x0, 0x4,  0x0,  0x4,  0x4, 0x4,
+            0xe4, 0x4,  0x4, 0x4,  0x4,  0x4, 0x4, 0x4,  0x4,  0x4,  0x6, 0x4,
+            0x4,  0x4,  0x4, 0x4,  0x4,  0x4, 0x4, 0x4,  0x4,  0x4,  0x4, 0x4,
+            0x4,  0xe5, 0x0, 0x4,  0x4,  0x4, 0x4, 0x4,  0x4,  0x0,  0x0, 0x4,
+            0x4,  0x4,  0x4, 0x4,  0x4,  0x4, 0x0, 0x0,  0x4,  0x4,  0x4, 0x4,
+            0x4,  0x4,  0x4, 0x4,  0x4,  0x4, 0x4, 0x4,  0x0,  0x0,  0x0, 0x0,
+            0x4,  0x4,  0x4, 0x4,  0xe1, 0x0, 0x0, 0x80, 0x0,  0x0,  0x1, 0x6,
+            0x0,  0x0,  0x0, 0x0,  0x0,  0x4, 0x0, 0x75, 0x75, 0x45, 0x0, 0x0,
+            0x0,  0x0,  0x0, 0x0,  0x0,  0x0, 0x0, 0x0,  0x0,  0x1,  0x0, 0x0,
+            0x0,  0x75, 0x4, 0x0,
+        };
+        CHECK(run_input(input, sizeof(input)) == 0);
+    }
+    SECTION("")
+    {
+        constexpr std::uint8_t input[] = {
+            0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0x1,  0x0,  0x0,  0x0,
+            0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0x85, 0xff, 0xff, 0xff, 0xff,
+            0xff, 0xff, 0xff, 0xff, 0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0x0,
+            0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0x0,
+            0x0,  0x0,  0x0,  0xff, 0xff, 0xff, 0xff, 0xff, 0x4,  0x4,  0x4,
+            0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,
+            0x4,  0x4,  0x5,  0x4,  0x28, 0x4,  0x4,  0x4,  0x0,  0x4,  0x4,
+            0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x24, 0x4,  0x4,  0x4,  0x4,
+            0x4,  0x4,  0x4,  0x4,  0x0,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,
+            0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,
+            0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,
+            0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,
+            0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,
+            0x4,  0x4,  0x4,  0x4,  0x4,  0xf3, 0x4,  0x4,  0x4,  0x4,  0x4,
+            0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,
+            0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,
+            0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,
+            0x4,  0x4,  0x4,  0xf3, 0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x4,
+            0x4,  0x4,  0x4,  0x4,  0x4,  0x4,  0x3,  0x4,  0x4,  0x4,  0xff,
+            0xff, 0xff, 0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0x0,
+            0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0x0,
+            0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0xad, 0x0,
+            0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0x0,  0x0,
+        };
+        CHECK(run_input(input, sizeof(input)) == 0);
+    }
+#endif
+}
diff --git a/test/flex_vector/fuzzed-1.cpp b/test/flex_vector/fuzzed-1.cpp
new file mode 100644
index 000000000000..d924dc3a8310
--- /dev/null
+++ b/test/flex_vector/fuzzed-1.cpp
@@ -0,0 +1,368 @@
+//
+// 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
+//
+
+#include "extra/fuzzer/fuzzer_input.hpp"
+#include <array>
+#include <catch.hpp>
+#include <immer/flex_vector.hpp>
+#include <iostream>
+
+#define IMMER_FUZZED_TRACE_ENABLE 0
+
+#if IMMER_FUZZED_TRACE_ENABLE
+#define IMMER_FUZZED_TRACE(...) std::cout << __VA_ARGS__ << std::endl;
+#else
+#define IMMER_FUZZED_TRACE(...)
+#endif
+
+namespace {
+
+template <std::size_t VarCount = 2, unsigned Bits = 2>
+int run_input(const std::uint8_t* data, std::size_t size)
+{
+    using vector_t =
+        immer::flex_vector<int, immer::default_memory_policy, Bits, Bits>;
+    using size_t = std::uint8_t;
+
+    auto vars = std::array<vector_t, VarCount>{};
+
+#if IMMER_FUZZED_TRACE_ENABLE
+    std::cout << "/// new test run" << std::endl;
+    for (auto i = 0u; i < VarCount; ++i)
+        std::cout << "auto var" << i << " = vector_t{};" << std::endl;
+#endif
+
+    auto is_valid_var   = [&](auto idx) { return idx >= 0 && idx < VarCount; };
+    auto is_valid_index = [](auto& v) {
+        return [&](auto idx) { return idx >= 0 && idx < v.size(); };
+    };
+    auto is_valid_size = [](auto& v) {
+        return [&](auto idx) { return idx >= 0 && idx <= v.size(); };
+    };
+    auto can_concat = [](auto&& v1, auto&& v2) {
+        using size_type = decltype(v1.size());
+        return v2.size() < (std::numeric_limits<size_type>::max() - v1.size());
+    };
+    auto can_insert = [](auto&& v1) {
+        using size_type = decltype(v1.size());
+        return v1.size() < std::numeric_limits<size_type>::max();
+    };
+
+    return fuzzer_input{data, size}.run([&](auto& in) {
+        enum ops
+        {
+            op_push_back,
+            op_update,
+            op_take,
+            op_drop,
+            op_concat,
+            op_push_back_move,
+            op_update_move,
+        };
+        auto src = read<std::uint8_t>(in, is_valid_var);
+        auto dst = read<std::uint8_t>(in, is_valid_var);
+        switch (read<char>(in)) {
+        case op_push_back:
+            if (can_insert(vars[src])) {
+                IMMER_FUZZED_TRACE("var" << +dst << " = var" << +src
+                                         << ".push_back(42);");
+                vars[dst] = vars[src].push_back(42);
+            }
+            break;
+        case op_update: {
+            auto idx = read<size_t>(in, is_valid_index(vars[src]));
+            IMMER_FUZZED_TRACE("var" << +dst << " = var" << +src << ".update("
+                                     << +idx
+                                     << ", [] (auto x) { return x + 1; });");
+            vars[dst] = vars[src].update(idx, [](auto x) { return x + 1; });
+            break;
+        }
+        case op_take: {
+            auto idx = read<size_t>(in, is_valid_size(vars[src]));
+            IMMER_FUZZED_TRACE("var" << +dst << " = var" << +src << ".take("
+                                     << +idx << ");");
+            vars[dst] = vars[src].take(idx);
+            break;
+        }
+        case op_drop: {
+            auto idx = read<size_t>(in, is_valid_size(vars[src]));
+            IMMER_FUZZED_TRACE("var" << +dst << " = var" << +src << ".take("
+                                     << +idx << ");");
+            vars[dst] = vars[src].drop(idx);
+            break;
+        }
+        case op_concat: {
+            auto src2 = read<std::uint8_t>(in, is_valid_var);
+            if (can_concat(vars[src], vars[src2])) {
+                IMMER_FUZZED_TRACE("var" << +dst << " = var" << +src << " + var"
+                                         << +src2 << ";");
+                vars[dst] = vars[src] + vars[src2];
+            }
+            break;
+        }
+        case op_push_back_move: {
+            if (can_insert(vars[src])) {
+                IMMER_FUZZED_TRACE("var" << +dst << " = std::move(var" << +src
+                                         << ").push_back(21);");
+                vars[dst] = std::move(vars[src]).push_back(21);
+            }
+            break;
+        }
+        case op_update_move: {
+            auto idx = read<size_t>(in, is_valid_index(vars[src]));
+            IMMER_FUZZED_TRACE("var" << +dst << " = std::move(var" << +src
+                                     << ").update(" << +idx
+                                     << ", [] (auto x) { return x + 1; });");
+            vars[dst] =
+                std::move(vars[src]).update(idx, [](auto x) { return x + 1; });
+            break;
+        }
+        default:
+            break;
+        };
+        return true;
+    });
+}
+
+} // anonymous namespace
+
+TEST_CASE("bug: memory leak because of move update")
+{
+    // There was a problem caused with shared "sizes buffer" in
+    // relaxed nodes.  In particular, the ensure_mutable_relaxed(...)
+    // function was not decremeting the old sizes buffer. That is why
+    // the last transient push_back (which uses mutable operations)
+    // causes some of the relaxed buffers that are created during the
+    // previous concatenations, and that start to be shared from the
+    // update() onwards, to later be leaked.
+    SECTION("simplified")
+    {
+        using vector_t =
+            immer::flex_vector<int, immer::default_memory_policy, 2, 2>;
+        auto var0 = vector_t{};
+        auto var1 = vector_t{};
+        var0      = var0.push_back(42);
+        var0      = var0.push_back(42);
+        var0      = var0.push_back(42);
+        var0      = var0 + var0;
+        var1      = var0.push_back(42);
+        var0      = var0 + var1;
+        var1      = var0.push_back(42);
+        var0      = var0 + var0;
+        var0      = var1 + var0;
+        var0      = var1.update(5, [](auto x) { return x + 1; });
+        var0      = std::move(var0).push_back(21);
+    }
+
+#if __GNUC__ != 9
+    SECTION("")
+    {
+        constexpr std::uint8_t input[] = {
+            0xff, 0x0,  0xff, 0x0, 0x0,  0x0,  0x0,  0x0,  0x0,  0x0, 0x0,
+            0x40, 0x0,  0x0,  0x4, 0x0,  0x6d, 0x6d, 0x0,  0x1,  0x0, 0x4,
+            0x6d, 0x6d, 0x6d, 0x0, 0x0,  0x4,  0x1,  0x6d, 0x6d, 0x0, 0x1,
+            0x0,  0x0,  0x0,  0x4, 0x28, 0x0,  0xfc, 0x1,  0x0,  0x4, 0x0,
+            0x0,  0x0,  0xfc, 0x1, 0x0,  0x1,  0x5,  0x0,  0x0,  0x1, 0x5,
+            0x0,  0x0,  0x5,  0x0, 0x0,  0xff, 0xff, 0xff, 0x27,
+        };
+        CHECK(run_input(input, sizeof(input)) == 0);
+    }
+#endif
+}
+
+TEST_CASE("non-bug: crash")
+{
+    // This is an interesting finding that is left here for
+    // documentation.  This test actually should not run... the
+    // problem is that when we build too large vectors via
+    // concatenation, we can sometimes "overflow the shift".  This is
+    // a degenerate case that we won't fix, but this helped adding
+    // appropriate assertions to the code.
+    //
+    // To prevent this in further fuzzing, the can_concat check has
+    // been made stricter.
+    return;
+
+    SECTION("simplified")
+    {
+        using vector_t =
+            immer::flex_vector<int, immer::default_memory_policy, 2, 2>;
+        auto var4 = vector_t{};
+        var4      = var4.push_back(42);
+        var4      = var4.push_back(42);
+        var4      = var4.push_back(42);
+        var4      = var4.push_back(42);
+        var4      = var4.push_back(42);
+        auto var0 = var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var0 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4 + var4;
+        var4      = var4.update(4, [](auto x) { return x + 1; });
+    }
+#if __GNUC__ != 9
+    SECTION("")
+    {
+        constexpr std::uint8_t input[] = {
+            0x00, 0x00, 0x00, 0x23, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+            0x00, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+            0x00, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x00,
+            0x00, 0x00, 0x00, 0x04, 0x04, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x04, 0x00, 0x00, 0x00,
+            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+            0x00, 0x00, 0x00, 0x00, 0x00, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04,
+            0x04, 0x00, 0x04, 0x04, 0x04, 0x00, 0x00, 0x00, 0x00, 0x04, 0x04,
+            0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04,
+            0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x00, 0x2a, 0x00,
+            0x00, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04,
+            0x04, 0x04, 0x04, 0x04, 0xfc, 0xf9, 0x04, 0x04, 0x04, 0x04, 0x04,
+            0x04, 0x04, 0x04, 0x04, 0x05, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04,
+            0x00, 0x00, 0x00, 0x00, 0x00, 0x23, 0x00, 0x00, 0x00, 0x00, 0x00,
+            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+            0x00, 0x00, 0x00, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04,
+            0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04,
+            0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x05, 0x04, 0x04, 0x04,
+            0x04, 0x04, 0x04, 0x00, 0x00, 0x00, 0x00, 0x00, 0x23, 0x00, 0x00,
+            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x04, 0x04, 0x04, 0x04, 0x04,
+            0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04,
+            0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x05, 0x04,
+            0x04, 0x04, 0x04, 0x04, 0x04, 0x00, 0x00, 0x00, 0x00, 0x00, 0x23,
+            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x04, 0x04, 0x04,
+            0x04, 0x04, 0x04, 0x04, 0x00, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04,
+            0x04, 0x04, 0x04, 0x04, 0x04, 0xd5, 0x04, 0x04, 0x04, 0x04, 0x04,
+            0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x05, 0x04, 0x04,
+            0x04, 0x04, 0x04, 0x04, 0x00, 0x00, 0x00, 0x00, 0x00, 0x23, 0x00,
+            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x04, 0x04, 0x04, 0x04,
+            0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x00, 0x00,
+            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+            0x00, 0x00, 0x00, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04,
+            0x04, 0x04, 0x04, 0x00, 0x01, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04,
+            0x04, 0x04, 0x04, 0x04, 0x04, 0x05, 0x04, 0x04, 0x04, 0x04, 0x04,
+            0x04, 0x00, 0x00, 0x00, 0x00, 0x00, 0x23, 0x00, 0x00, 0x00, 0x00,
+            0x00, 0x00, 0x00, 0x00, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x00,
+            0x01, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04,
+            0x04, 0x05, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x00, 0x00, 0x00,
+            0x00, 0x00, 0x23, 0x00, 0x00, 0x00, 0x00, 0x00, 0x04, 0x00, 0x00,
+            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x3a,
+            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+            0x00, 0x00, 0x21, 0x04, 0x00, 0x00, 0x00, 0x00, 0x00, 0x04, 0x04,
+            0x04, 0x04, 0x04, 0x00, 0x04, 0x04, 0x00, 0x00, 0x04, 0x00, 0x00,
+            0x00, 0x00, 0x00, 0x00, 0x04, 0x00, 0x04, 0x00, 0x00, 0x00, 0x00,
+            0x00, 0x00, 0x00, 0x00, 0x00, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+            0xff, 0xff, 0x13, 0x13, 0x13, 0x13, 0x13, 0x00, 0x00, 0x00, 0x00,
+            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x00,
+            0x00, 0x00, 0x29, 0x00, 0x00, 0x00, 0x00, 0xff, 0xff, 0x00, 0x00,
+            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+            0x00, 0x00, 0x00, 0x00, 0x03, 0x03, 0x03, 0x03, 0x03, 0x03, 0x03,
+            0x03, 0x03, 0x03, 0x03, 0x03, 0x03, 0x03, 0x03, 0x03, 0x03, 0x03,
+            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x21, 0x00, 0x10, 0x00,
+            0x00, 0x00, 0x00, 0x00, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04,
+            0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04,
+            0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x05, 0x04, 0x04, 0x04, 0x04,
+            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+            0x00, 0x00, 0x00, 0x00, 0x00, 0x3a, 0x00, 0x02, 0x00, 0x00, 0x00,
+            0x04, 0x00, 0x00, 0x04, 0x00, 0x00, 0x00, 0x23, 0x00, 0x00, 0x00,
+            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x04, 0x04, 0x04, 0x04, 0x04,
+            0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04,
+            0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x05, 0x04,
+            0x04, 0x04, 0x04, 0x04, 0x04, 0x00, 0x00, 0x00, 0x00, 0x00, 0x23,
+            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0xff, 0xff,
+            0xff, 0xff, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x04, 0x04, 0x04,
+            0x04, 0x04, 0x04, 0x04, 0x00, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04,
+            0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04,
+            0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x05, 0x04, 0x04,
+            0x04, 0x04, 0x04, 0x04, 0x00, 0x00, 0x00, 0x00, 0x00, 0x23, 0x00,
+            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x04, 0x04, 0x04, 0x04,
+            0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x00, 0x01,
+            0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04,
+            0x05, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x00, 0x00, 0x00, 0x00,
+            0x00, 0x23, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x04,
+            0x04, 0x04, 0x04, 0x04, 0x04, 0x00, 0x01, 0x04, 0x04, 0x04, 0x04,
+            0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x05, 0x04, 0x04, 0x04,
+            0x04, 0x04, 0x04, 0x00, 0x00, 0x00, 0x00, 0x00, 0x23, 0x00, 0x00,
+            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+            0x00, 0x00, 0x00, 0x00, 0x3a, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x21, 0x04, 0x00, 0x00,
+            0x00, 0x00, 0x00, 0x00, 0x04, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00,
+            0x00, 0x00, 0x00, 0x23, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+            0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x04, 0x04, 0x04,
+            0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04, 0x04,
+            0x00,
+        };
+        CHECK(run_input<8>(input, sizeof(input)) == 0);
+    }
+#endif
+}
diff --git a/test/flex_vector/fuzzed-2.cpp b/test/flex_vector/fuzzed-2.cpp
new file mode 100644
index 000000000000..ba17c1a620ec
--- /dev/null
+++ b/test/flex_vector/fuzzed-2.cpp
@@ -0,0 +1,188 @@
+//
+// 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
+//
+
+#include "extra/fuzzer/fuzzer_input.hpp"
+#include <array>
+#include <catch.hpp>
+#include <immer/flex_vector.hpp>
+#include <iostream>
+
+#define IMMER_FUZZED_TRACE_ENABLE 0
+
+#if IMMER_FUZZED_TRACE_ENABLE
+#define IMMER_FUZZED_TRACE(...) std::cout << __VA_ARGS__ << std::endl;
+#else
+#define IMMER_FUZZED_TRACE(...)
+#endif
+
+namespace {
+
+template <std::size_t VarCount = 2, unsigned Bits = 2>
+int run_input(const std::uint8_t* data, std::size_t size)
+{
+    using vector_t =
+        immer::flex_vector<int, immer::default_memory_policy, Bits, Bits>;
+    using size_t = std::uint8_t;
+
+    auto vars = std::array<vector_t, VarCount>{};
+
+#if IMMER_FUZZED_TRACE_ENABLE
+    std::cout << "/// new test run" << std::endl;
+    for (auto i = 0u; i < VarCount; ++i)
+        std::cout << "auto var" << i << " = vector_t{};" << std::endl;
+#endif
+
+    auto is_valid_var   = [&](auto idx) { return idx >= 0 && idx < VarCount; };
+    auto is_valid_index = [](auto& v) {
+        return [&](auto idx) { return idx >= 0 && idx < v.size(); };
+    };
+    auto is_valid_size = [](auto& v) {
+        return [&](auto idx) { return idx >= 0 && idx <= v.size(); };
+    };
+    auto can_concat = [](auto&& v1, auto&& v2) {
+        using size_type = decltype(v1.size());
+        return v2.size() < (std::numeric_limits<size_type>::max() - v1.size());
+    };
+    auto can_insert = [](auto&& v1) {
+        using size_type = decltype(v1.size());
+        return v1.size() < std::numeric_limits<size_type>::max();
+    };
+
+    return fuzzer_input{data, size}.run([&](auto& in) {
+        enum ops
+        {
+            op_push_back,
+            op_update,
+            op_take,
+            op_drop,
+            op_concat,
+            op_push_back_move,
+            op_update_move,
+            op_take_move,
+            op_drop_move,
+        };
+        auto src = read<std::uint8_t>(in, is_valid_var);
+        auto dst = read<std::uint8_t>(in, is_valid_var);
+        switch (read<char>(in)) {
+        case op_push_back:
+            if (can_insert(vars[src])) {
+                IMMER_FUZZED_TRACE("var" << +dst << " = var" << +src
+                                         << ".push_back(42);");
+                vars[dst] = vars[src].push_back(42);
+            }
+            break;
+        case op_update: {
+            auto idx = read<size_t>(in, is_valid_index(vars[src]));
+            IMMER_FUZZED_TRACE("var" << +dst << " = var" << +src << ".update("
+                                     << +idx
+                                     << ", [] (auto x) { return x + 1; });");
+            vars[dst] = vars[src].update(idx, [](auto x) { return x + 1; });
+            break;
+        }
+        case op_take: {
+            auto idx = read<size_t>(in, is_valid_size(vars[src]));
+            IMMER_FUZZED_TRACE("var" << +dst << " = var" << +src << ".take("
+                                     << +idx << ");");
+            vars[dst] = vars[src].take(idx);
+            break;
+        }
+        case op_drop: {
+            auto idx = read<size_t>(in, is_valid_size(vars[src]));
+            IMMER_FUZZED_TRACE("var" << +dst << " = var" << +src << ".take("
+                                     << +idx << ");");
+            vars[dst] = vars[src].drop(idx);
+            break;
+        }
+        case op_concat: {
+            auto src2 = read<std::uint8_t>(in, is_valid_var);
+            if (can_concat(vars[src], vars[src2])) {
+                IMMER_FUZZED_TRACE("var" << +dst << " = var" << +src << " + var"
+                                         << +src2 << ";");
+                vars[dst] = vars[src] + vars[src2];
+            }
+            break;
+        }
+        case op_push_back_move: {
+            if (can_insert(vars[src])) {
+                IMMER_FUZZED_TRACE("var" << +dst << " = std::move(var" << +src
+                                         << ").push_back(21);");
+                vars[dst] = std::move(vars[src]).push_back(21);
+            }
+            break;
+        }
+        case op_update_move: {
+            auto idx = read<size_t>(in, is_valid_index(vars[src]));
+            IMMER_FUZZED_TRACE("var" << +dst << " = std::move(var" << +src
+                                     << ").update(" << +idx
+                                     << ", [] (auto x) { return x + 1; });");
+            vars[dst] =
+                std::move(vars[src]).update(idx, [](auto x) { return x + 1; });
+            break;
+        }
+        case op_take_move: {
+            auto idx = read<size_t>(in, is_valid_size(vars[src]));
+            IMMER_FUZZED_TRACE("var" << +dst << " = std::move(var" << +src
+                                     << ").take(" << +idx << ");");
+            vars[dst] = std::move(vars[src]).take(idx);
+            break;
+        }
+        case op_drop_move: {
+            auto idx = read<size_t>(in, is_valid_size(vars[src]));
+            IMMER_FUZZED_TRACE("var" << +dst << " = std::move(var" << +src
+                                     << ").drop(" << +idx << ");");
+            vars[dst] = std::move(vars[src]).drop(idx);
+            break;
+        }
+        default:
+            break;
+        };
+        return true;
+    });
+}
+
+} // anonymous namespace
+
+TEST_CASE("bug: use after free on move-take")
+{
+    SECTION("")
+    {
+        using vector_t =
+            immer::flex_vector<int, immer::default_memory_policy, 2, 2>;
+        auto var0 = vector_t{};
+        auto var1 = vector_t{};
+        var0      = var0.push_back(42);
+        var0      = var0.push_back(42);
+        var0      = var0.push_back(42);
+        var0      = var0.push_back(42);
+        var0      = var0 + var0;
+        var0      = var0 + var0;
+        var0      = var0 + var0;
+        var0      = var0 + var0;
+        var0      = var0.push_back(42);
+        var0      = var0.push_back(42);
+        var0      = var0.push_back(42);
+        var0      = var0.push_back(42);
+        var0      = var0.push_back(42);
+        var1      = var0.push_back(42);
+        var0      = std::move(var0).take(68);
+    }
+
+#if __GNUC__ != 9
+    SECTION("")
+    {
+        constexpr std::uint8_t input[] = {
+            0x0, 0x0,  0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x1,  0x0,  0x0, 0x0, 0x0,
+            0x0, 0x40, 0x0, 0x0, 0x0, 0x0, 0x4, 0x4, 0x0,  0x0,  0x0, 0x4, 0x0,
+            0x0, 0x0,  0x4, 0x0, 0x0, 0x0, 0x4, 0x0, 0x0,  0x4,  0x0, 0x0, 0x0,
+            0x4, 0x0,  0x0, 0x0, 0x4, 0x0, 0x0, 0x0, 0x4,  0x0,  0x0, 0x0, 0x4,
+            0x0, 0x0,  0x0, 0x1, 0x0, 0x0, 0x0, 0x7, 0x41, 0xb5, 0x0, 0x0,
+        };
+        CHECK(run_input(input, sizeof(input)) == 0);
+    }
+#endif
+}
diff --git a/test/flex_vector/fuzzed-3.cpp b/test/flex_vector/fuzzed-3.cpp
new file mode 100644
index 000000000000..5ef1eaffb755
--- /dev/null
+++ b/test/flex_vector/fuzzed-3.cpp
@@ -0,0 +1,222 @@
+//
+// 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
+//
+
+#include "extra/fuzzer/fuzzer_input.hpp"
+#include <array>
+#include <catch.hpp>
+#include <immer/flex_vector.hpp>
+#include <iostream>
+
+#define IMMER_FUZZED_TRACE_ENABLE 0
+
+#if IMMER_FUZZED_TRACE_ENABLE
+#define IMMER_FUZZED_TRACE(...) std::cout << __VA_ARGS__ << std::endl;
+#else
+#define IMMER_FUZZED_TRACE(...)
+#endif
+
+namespace {
+
+template <std::size_t VarCount = 2, unsigned Bits = 2>
+int run_input(const std::uint8_t* data, std::size_t size)
+{
+    using vector_t =
+        immer::flex_vector<int, immer::default_memory_policy, Bits, Bits>;
+    using size_t = std::uint8_t;
+
+    auto vars = std::array<vector_t, VarCount>{};
+
+#if IMMER_FUZZED_TRACE_ENABLE
+    std::cout << "/// new test run" << std::endl;
+    for (auto i = 0u; i < VarCount; ++i)
+        std::cout << "auto var" << i << " = vector_t{};" << std::endl;
+#endif
+
+    auto is_valid_var = [&](auto idx) { return idx >= 0 && idx < VarCount; };
+    auto is_valid_var_neq = [](auto other) {
+        return [=](auto idx) {
+            return idx >= 0 && idx < VarCount && idx != other;
+        };
+    };
+    auto is_valid_index = [](auto& v) {
+        return [&](auto idx) { return idx >= 0 && idx < v.size(); };
+    };
+    auto is_valid_size = [](auto& v) {
+        return [&](auto idx) { return idx >= 0 && idx <= v.size(); };
+    };
+    auto can_concat = [](auto&& v1, auto&& v2) {
+        using size_type = decltype(v1.size());
+        return v2.size() < (std::numeric_limits<size_type>::max() - v1.size());
+    };
+    auto can_insert = [](auto&& v1) {
+        using size_type = decltype(v1.size());
+        return v1.size() < std::numeric_limits<size_type>::max();
+    };
+
+    return fuzzer_input{data, size}.run([&](auto& in) {
+        enum ops
+        {
+            op_push_back,
+            op_update,
+            op_take,
+            op_drop,
+            op_concat,
+            op_push_back_move,
+            op_update_move,
+            op_take_move,
+            op_drop_move,
+            op_concat_move_l,
+            op_concat_move_r,
+            op_concat_move_lr,
+        };
+        auto src = read<std::uint8_t>(in, is_valid_var);
+        auto dst = read<std::uint8_t>(in, is_valid_var);
+        switch (read<char>(in)) {
+        case op_push_back:
+            if (can_insert(vars[src])) {
+                IMMER_FUZZED_TRACE("var" << +dst << " = var" << +src
+                                         << ".push_back(42);");
+                vars[dst] = vars[src].push_back(42);
+            }
+            break;
+        case op_update: {
+            auto idx = read<size_t>(in, is_valid_index(vars[src]));
+            IMMER_FUZZED_TRACE("var" << +dst << " = var" << +src << ".update("
+                                     << +idx
+                                     << ", [] (auto x) { return x + 1; });");
+            vars[dst] = vars[src].update(idx, [](auto x) { return x + 1; });
+            break;
+        }
+        case op_take: {
+            auto idx = read<size_t>(in, is_valid_size(vars[src]));
+            IMMER_FUZZED_TRACE("var" << +dst << " = var" << +src << ".take("
+                                     << +idx << ");");
+            vars[dst] = vars[src].take(idx);
+            break;
+        }
+        case op_drop: {
+            auto idx = read<size_t>(in, is_valid_size(vars[src]));
+            IMMER_FUZZED_TRACE("var" << +dst << " = var" << +src << ".take("
+                                     << +idx << ");");
+            vars[dst] = vars[src].drop(idx);
+            break;
+        }
+        case op_concat: {
+            auto src2 = read<std::uint8_t>(in, is_valid_var);
+            if (can_concat(vars[src], vars[src2])) {
+                IMMER_FUZZED_TRACE("var" << +dst << " = var" << +src << " + var"
+                                         << +src2 << ";");
+                vars[dst] = vars[src] + vars[src2];
+            }
+            break;
+        }
+        case op_push_back_move: {
+            if (can_insert(vars[src])) {
+                IMMER_FUZZED_TRACE("var" << +dst << " = std::move(var" << +src
+                                         << ").push_back(21);");
+                vars[dst] = std::move(vars[src]).push_back(21);
+            }
+            break;
+        }
+        case op_update_move: {
+            auto idx = read<size_t>(in, is_valid_index(vars[src]));
+            IMMER_FUZZED_TRACE("var" << +dst << " = std::move(var" << +src
+                                     << ").update(" << +idx
+                                     << ", [] (auto x) { return x + 1; });");
+            vars[dst] =
+                std::move(vars[src]).update(idx, [](auto x) { return x + 1; });
+            break;
+        }
+        case op_take_move: {
+            auto idx = read<size_t>(in, is_valid_size(vars[src]));
+            IMMER_FUZZED_TRACE("var" << +dst << " = std::move(var" << +src
+                                     << ").take(" << +idx << ");");
+            vars[dst] = std::move(vars[src]).take(idx);
+            break;
+        }
+        case op_drop_move: {
+            auto idx = read<size_t>(in, is_valid_size(vars[src]));
+            IMMER_FUZZED_TRACE("var" << +dst << " = std::move(var" << +src
+                                     << ").drop(" << +idx << ");");
+            vars[dst] = std::move(vars[src]).drop(idx);
+            break;
+        }
+        case op_concat_move_l: {
+            auto src2 = read<std::uint8_t>(in, is_valid_var_neq(src));
+            if (can_concat(vars[src], vars[src2])) {
+                IMMER_FUZZED_TRACE("var" << +dst << " = std::move(var" << +src
+                                         << ") + var" << +src2 << ";");
+                vars[dst] = std::move(vars[src]) + vars[src2];
+            }
+            break;
+        }
+        case op_concat_move_r: {
+            auto src2 = read<std::uint8_t>(in, is_valid_var_neq(src));
+            if (can_concat(vars[src], vars[src2])) {
+                IMMER_FUZZED_TRACE("var" << +dst << " = var" << +src
+                                         << " + std::move(var" << +src2
+                                         << ");");
+                vars[dst] = vars[src] + std::move(vars[src2]);
+            }
+            break;
+        }
+        case op_concat_move_lr: {
+            auto src2 = read<std::uint8_t>(in, is_valid_var_neq(src));
+            if (can_concat(vars[src], vars[src2])) {
+                IMMER_FUZZED_TRACE("var" << +dst << " = std::move(var" << +src
+                                         << ") + std::move(var" << +src2
+                                         << ");");
+                vars[dst] = std::move(vars[src]) + std::move(vars[src2]);
+            }
+            break;
+        }
+        default:
+            break;
+        };
+        return true;
+    });
+}
+
+} // namespace
+
+TEST_CASE("bug: concat with moving the right side")
+{
+    // This was a stupid bug on the concatenation of small vectors
+    // when moving one of the sides.
+    SECTION("simplified")
+    {
+        using vector_t =
+            immer::flex_vector<int, immer::default_memory_policy, 2, 2>;
+        auto var0 = vector_t{};
+        auto var1 = vector_t{};
+        var0      = var0.push_back(42);
+        var0      = var0.push_back(42);
+        var0      = var0.push_back(42);
+        var1      = var0.push_back(42);
+        var0      = var0 + var0;
+        var0      = var0.push_back(42);
+        var0      = var0 + var1;
+        var0      = var0 + var0;
+        var0      = var0 + std::move(var1);
+    }
+
+#if __GNUC__ != 9
+    SECTION("vm")
+    {
+        constexpr std::uint8_t input[] = {
+            0x0,  0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x0,
+            0x0,  0x0,  0x0,  0x1,  0x0,  0x40, 0x28, 0x0,  0x4,  0x3f,
+            0x20, 0x0,  0x0,  0x4,  0x3f, 0x8,  0x0,  0x0,  0x0,  0x0,
+            0x1,  0x0,  0x0,  0x0,  0x4,  0x3f, 0x0,  0x0,  0x40, 0x0,
+            0x0,  0x0,  0x0,  0x4,  0x3f, 0x1,  0x0,  0x0,  0x4,  0x3f,
+            0x0,  0x0,  0xff, 0x0,  0xa,  0x1,  0x0,  0xff, 0x0,  0x0,
+        };
+        CHECK(run_input(input, sizeof(input)) == 0);
+    }
+#endif
+}
diff --git a/test/flex_vector/fuzzed-4.cpp b/test/flex_vector/fuzzed-4.cpp
new file mode 100644
index 000000000000..d9ecdd7fba7f
--- /dev/null
+++ b/test/flex_vector/fuzzed-4.cpp
@@ -0,0 +1,364 @@
+//
+// 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
+//
+
+#include "extra/fuzzer/fuzzer_input.hpp"
+#include <array>
+#include <catch.hpp>
+#include <immer/flex_vector.hpp>
+#include <immer/flex_vector_transient.hpp>
+#include <immer/heap/gc_heap.hpp>
+#include <immer/refcount/no_refcount_policy.hpp>
+#include <iostream>
+
+#define IMMER_FUZZED_TRACE_ENABLE 0
+
+#if IMMER_FUZZED_TRACE_ENABLE
+#define IMMER_FUZZED_TRACE(...) std::cout << __VA_ARGS__ << std::endl;
+#else
+#define IMMER_FUZZED_TRACE(...)
+#endif
+
+using gc_memory = immer::memory_policy<immer::heap_policy<immer::gc_heap>,
+                                       immer::no_refcount_policy,
+                                       immer::gc_transience_policy,
+                                       false>;
+
+namespace {
+
+template <std::size_t VarCount = 4, unsigned Bits = 2>
+int run_input(const std::uint8_t* data, std::size_t size)
+{
+    using vector_t    = immer::flex_vector<int, gc_memory, Bits, Bits>;
+    using transient_t = typename vector_t::transient_type;
+    using size_t      = std::uint8_t;
+
+    auto vs = std::array<vector_t, VarCount>{};
+    auto ts = std::array<transient_t, VarCount>{};
+
+#if IMMER_FUZZED_TRACE_ENABLE
+    std::cout << "/// new test run" << std::endl;
+    for (auto i = 0; i < VarCount; ++i)
+        std::cout << "auto v" << i << " = vector_t{};" << std::endl;
+    for (auto i = 0; i < VarCount; ++i)
+        std::cout << "auto t" << i << " = transient_t{};" << std::endl;
+#endif
+
+    auto is_valid_var = [&](auto idx) { return idx >= 0 && idx < VarCount; };
+    auto is_valid_var_neq = [](auto other) {
+        return [=](auto idx) {
+            return idx >= 0 && idx < VarCount && idx != other;
+        };
+    };
+    auto is_valid_index = [](auto& v) {
+        return [&](auto idx) { return idx >= 0 && idx < v.size(); };
+    };
+    auto is_valid_size = [](auto& v) {
+        return [&](auto idx) { return idx >= 0 && idx <= v.size(); };
+    };
+    auto can_concat = [](auto&& v1, auto&& v2) {
+        using size_type = decltype(v1.size());
+        auto max        = std::numeric_limits<size_type>::max() >> (Bits * 4);
+        return v1.size() < max && v2.size() < max;
+    };
+
+    return fuzzer_input{data, size}.run([&](auto& in) {
+        enum ops
+        {
+            op_transient,
+            op_persistent,
+            op_push_back,
+            op_update,
+            op_take,
+            op_drop,
+            op_concat,
+            op_push_back_mut,
+            op_update_mut,
+            op_take_mut,
+            op_drop_mut,
+            op_prepend_mut,
+            op_prepend_mut_move,
+            op_append_mut,
+            op_append_mut_move,
+        };
+        auto dst = read<std::uint8_t>(in, is_valid_var);
+        switch (read<char>(in)) {
+        case op_transient: {
+            auto src = read<std::uint8_t>(in, is_valid_var);
+            IMMER_FUZZED_TRACE("t" << +dst << " = v" << +src
+                                   << ".transient();");
+            ts[dst] = vs[src].transient();
+            break;
+        }
+        case op_persistent: {
+            auto src = read<std::uint8_t>(in, is_valid_var);
+            IMMER_FUZZED_TRACE("v" << +dst << " = t" << +src
+                                   << ".persistent();");
+            vs[dst] = ts[src].persistent();
+            break;
+        }
+        case op_push_back: {
+            auto src = read<std::uint8_t>(in, is_valid_var);
+            IMMER_FUZZED_TRACE("v" << +dst << " = v" << +src
+                                   << ".push_back(42);");
+            vs[dst] = vs[src].push_back(42);
+            break;
+        }
+        case op_update: {
+            auto src = read<std::uint8_t>(in, is_valid_var);
+            auto idx = read<size_t>(in, is_valid_index(vs[src]));
+            IMMER_FUZZED_TRACE("v" << +dst << " = v" << +src << ".update("
+                                   << +idx
+                                   << ", [] (auto x) { return x + 1; });");
+            vs[dst] = vs[src].update(idx, [](auto x) { return x + 1; });
+            break;
+        }
+        case op_take: {
+            auto src = read<std::uint8_t>(in, is_valid_var);
+            auto idx = read<size_t>(in, is_valid_size(vs[src]));
+            IMMER_FUZZED_TRACE("v" << +dst << " = v" << +src << ".take(" << +idx
+                                   << ");");
+            vs[dst] = vs[src].take(idx);
+            break;
+        }
+        case op_drop: {
+            auto src = read<std::uint8_t>(in, is_valid_var);
+            auto idx = read<size_t>(in, is_valid_size(vs[src]));
+            IMMER_FUZZED_TRACE("v" << +dst << " = v" << +src << ".take(" << +idx
+                                   << ");");
+            vs[dst] = vs[src].drop(idx);
+            break;
+        }
+        case op_concat: {
+            auto src  = read<std::uint8_t>(in, is_valid_var);
+            auto src2 = read<std::uint8_t>(in, is_valid_var);
+            if (can_concat(vs[src], vs[src2])) {
+                IMMER_FUZZED_TRACE("v" << +dst << " = v" << +src << " + v"
+                                       << +src2 << ";");
+                vs[dst] = vs[src] + vs[src2];
+            }
+            break;
+        }
+        case op_push_back_mut: {
+            IMMER_FUZZED_TRACE("t" << +dst << ".push_back(13);");
+            ts[dst].push_back(13);
+            break;
+        }
+        case op_update_mut: {
+            auto idx = read<size_t>(in, is_valid_index(ts[dst]));
+            IMMER_FUZZED_TRACE("t" << +dst << ".update(" << +idx
+                                   << ", [] (auto x) { return x + 1; });");
+            ts[dst].update(idx, [](auto x) { return x + 1; });
+            break;
+        }
+        case op_take_mut: {
+            auto idx = read<size_t>(in, is_valid_size(ts[dst]));
+            IMMER_FUZZED_TRACE("t" << +dst << ").take(" << +idx << ");");
+            ts[dst].take(idx);
+            break;
+        }
+        case op_prepend_mut: {
+            auto src = read<std::uint8_t>(in, is_valid_var_neq(dst));
+            if (can_concat(ts[dst], ts[src])) {
+                IMMER_FUZZED_TRACE("t" << +dst << ".prepend(t" << +src << ");");
+                ts[dst].prepend(ts[src]);
+            }
+            break;
+        }
+        case op_prepend_mut_move: {
+            auto src = read<std::uint8_t>(in, is_valid_var_neq(dst));
+            if (can_concat(ts[dst], ts[src])) {
+                IMMER_FUZZED_TRACE("t" << +dst << ".prepend(std::move(t" << +src
+                                       << "));"
+                                       << " t" << +src << " = {};");
+                ts[dst].prepend(std::move(ts[src]));
+                ts[src] = {};
+            }
+            break;
+        }
+        case op_append_mut: {
+            auto src = read<std::uint8_t>(in, is_valid_var_neq(dst));
+            if (can_concat(ts[dst], ts[src])) {
+                IMMER_FUZZED_TRACE("t" << +dst << ".append(t" << +src << ");");
+                ts[dst].append(ts[src]);
+            }
+            break;
+        }
+        case op_append_mut_move: {
+            auto src = read<std::uint8_t>(in, is_valid_var_neq(dst));
+            if (can_concat(ts[dst], ts[src])) {
+                IMMER_FUZZED_TRACE("t" << +dst << ".append(std::move(t" << +src
+                                       << "));"
+                                       << " t" << +src << " = {};");
+                ts[dst].append(std::move(ts[src]));
+                ts[src] = {};
+            }
+            break;
+        }
+        default:
+            break;
+        };
+        return true;
+    });
+}
+
+} // namespace
+
+TEST_CASE("bug: concatenating transients")
+{
+    // When concatenating two transients vectors the nodes from the
+    // argument become aliased in the result.  As such, we need to
+    // reset the identitiy of the argument.
+    SECTION("simplified")
+    {
+        using vector_t = immer::flex_vector<int, gc_memory, 2, 2>;
+        auto t0        = vector_t{}.transient();
+        t0.push_back(42);
+        t0.push_back(42);
+        t0.push_back(42);
+        t0.push_back(42);
+        t0.push_back(42);
+        t0.push_back(42);
+        auto t1 = t0;
+        t1.append(t0);
+        t1.append(t0);
+        t0.append(t1);
+        t1.append(t0);
+    }
+
+#if __GNUC__ != 9
+    SECTION("")
+    {
+        constexpr std::uint8_t input[] = {
+            0x2, 0x2, 0x2, 0x2, 0x29, 0x32, 0x0, 0x0,  0x2,  0x2,  0x2,
+            0x2, 0x2, 0x2, 0x2, 0x2,  0x2,  0x2, 0x2,  0x2,  0x6,  0x2,
+            0x2, 0x2, 0x2, 0x2, 0x6,  0x2,  0x2, 0x2,  0x0,  0x38, 0x2,
+            0x0, 0x0, 0x2, 0x2, 0xd,  0x0,  0x0, 0x3b, 0xff, 0x3a, 0x2,
+            0xd, 0xd, 0x2, 0x0, 0x0,  0x10, 0xe, 0x0,  0xd,  0x0,  0x0,
+            0x2, 0x2, 0xd, 0x0, 0x1,  0x5,
+        };
+        CHECK(run_input(input, sizeof(input)) == 0);
+    }
+#endif
+}
+
+TEST_CASE("bug: concatenating moved transients")
+{
+    // A moved from concatenated transient is totally smashed, we can
+    // not do anything with it but reasign...
+    SECTION("simplified")
+    {
+        using vector_t    = immer::flex_vector<int, gc_memory, 2, 2>;
+        using transient_t = typename vector_t::transient_type;
+        auto v0           = vector_t{};
+        auto t0           = transient_t{};
+        auto t2           = transient_t{};
+        v0                = v0.push_back(42);
+        t2                = v0.transient();
+        v0                = v0 + v0;
+        v0                = v0 + v0;
+        v0                = v0 + v0;
+        v0                = v0 + v0;
+        t0                = v0.transient();
+        t0                = v0.transient();
+        t0.append(std::move(t2));
+        t2 = {};
+        t2.append(std::move(t0));
+    }
+
+#if __GNUC__ != 9
+    SECTION("")
+    {
+        constexpr std::uint8_t input[] = {
+            0x0,  0x2, 0x0,  0x2,  0x0,  0x0,  0x0,  0x6,  0x0,  0x0,  0x0,
+            0x6,  0x0, 0x0,  0x0,  0x9d, 0x0,  0x6,  0x0,  0x0,  0x0,  0x6,
+            0x0,  0x0, 0x0,  0x9d, 0x28, 0x0,  0x0,  0x0,  0x0,  0x0,  0xf7,
+            0xc5, 0x0, 0xa,  0xa,  0x0,  0xfa, 0xe7, 0xff, 0xe7, 0xff, 0x0,
+            0xe,  0x2, 0x9,  0x0,  0x28, 0x2,  0xe,  0x0,  0x0,  0x2,  0xd,
+            0x0,  0x0, 0x28, 0x0,  0xd,  0x2,  0x5,  0x0,  0x2,
+        };
+        CHECK(run_input(input, sizeof(input)) == 0);
+    }
+#endif
+
+    SECTION("simplified")
+    {
+        using vector_t    = immer::flex_vector<int, gc_memory, 2, 2>;
+        using transient_t = typename vector_t::transient_type;
+        auto v0           = vector_t{};
+        auto t0           = transient_t{};
+        auto t2           = transient_t{};
+        v0                = v0.push_back(42);
+        v0                = v0.push_back(42);
+        v0                = v0 + v0;
+        t0                = v0.transient();
+        t2.prepend(std::move(t0));
+        t0 = {};
+        t0 = v0.transient();
+        t0.push_back(13);
+        t2.append(std::move(t0));
+        t0 = {};
+        t0 = v0.transient();
+        t0.push_back(13);
+        t2.prepend(std::move(t0));
+        t0 = {};
+    }
+
+#if __GNUC__ != 9
+    SECTION("")
+    {
+        return;
+        constexpr std::uint8_t input[] = {
+            0x0,  0x2, 0x0,  0x0,  0x2, 0xb7, 0x1, 0x36, 0x40, 0x0,  0x0,
+            0x0,  0x0, 0xb6, 0x0,  0x2, 0x0,  0x0, 0x6,  0xe,  0x0,  0x0,
+            0xfe, 0x0, 0x0,  0xff, 0x0, 0x2,  0xc, 0xff, 0xfc, 0x29, 0x0,
+            0x0,  0x0, 0x0,  0x0,  0x7, 0x2,  0xe, 0xff, 0xfc, 0x29, 0x0,
+            0x0,  0x0, 0x0,  0x0,  0x7, 0x3,  0x0, 0x0,  0x2,  0xc,  0x2,
+            0xc,  0x0, 0xd,  0x0,  0x0, 0x0,  0x0, 0x25, 0x6,
+        };
+        CHECK(run_input(input, sizeof(input)) == 0);
+    }
+#endif
+}
+
+TEST_CASE("bug: aegsdas")
+{
+    SECTION("simplified")
+    {
+        using vector_t    = immer::flex_vector<int, gc_memory, 2, 2>;
+        using transient_t = typename vector_t::transient_type;
+        auto v2           = vector_t{};
+        auto t0           = transient_t{};
+        auto t1           = transient_t{};
+        v2                = v2.push_back(42);
+        v2                = v2.push_back(42);
+        v2                = v2.push_back(42);
+        v2                = v2.push_back(42);
+        v2                = v2.push_back(42);
+        t0                = v2.transient();
+        t1.prepend(t0);
+        t1.prepend(t0);
+        t1.prepend(t0);
+        t0.prepend(std::move(t1));
+        t1 = {};
+    }
+
+#if __GNUC__ != 9
+    SECTION("")
+    {
+        constexpr std::uint8_t input[] = {
+            0xff, 0xff, 0x2, 0x2,  0x2, 0x2,  0x2,  0x2,  0x2,  0x2,  0x2,
+            0x82, 0x2,  0x2, 0x2,  0x2, 0x2,  0x2,  0x0,  0x0,  0x3d, 0x0,
+            0x0,  0x0,  0x2, 0x84, 0x0, 0x3b, 0x1,  0xb,  0x0,  0xa,  0x1,
+            0xb,  0x0,  0x0, 0x3,  0x2, 0x0,  0x3b, 0x1,  0xb,  0x1,  0x0,
+            0x0,  0xc,  0xb, 0x1,  0x8, 0xff, 0xff, 0xfc, 0xfd, 0x0,  0x3b,
+            0x3,  0x2,  0x0, 0x3b, 0x1, 0x9,  0x1,  0x3b,
+        };
+        CHECK(run_input(input, sizeof(input)) == 0);
+    }
+#endif
+}
diff --git a/test/flex_vector/gc.cpp b/test/flex_vector/gc.cpp
new file mode 100644
index 000000000000..57b97f5ed7d7
--- /dev/null
+++ b/test/flex_vector/gc.cpp
@@ -0,0 +1,27 @@
+//
+// 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
+//
+
+#include <immer/flex_vector.hpp>
+#include <immer/heap/gc_heap.hpp>
+#include <immer/refcount/no_refcount_policy.hpp>
+#include <immer/vector.hpp>
+
+using gc_memory = immer::memory_policy<immer::heap_policy<immer::gc_heap>,
+                                       immer::no_refcount_policy,
+                                       immer::gc_transience_policy,
+                                       false>;
+
+template <typename T>
+using test_flex_vector_t = immer::flex_vector<T, gc_memory, 3u>;
+
+template <typename T>
+using test_vector_t = immer::vector<T, gc_memory, 3u>;
+
+#define FLEX_VECTOR_T test_flex_vector_t
+#define VECTOR_T test_vector_t
+#include "generic.ipp"
diff --git a/test/flex_vector/generic.ipp b/test/flex_vector/generic.ipp
new file mode 100644
index 000000000000..6dbd73f07cb6
--- /dev/null
+++ b/test/flex_vector/generic.ipp
@@ -0,0 +1,599 @@
+//
+// 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
+//
+
+#include "test/dada.hpp"
+#include "test/transient_tester.hpp"
+#include "test/util.hpp"
+
+#include <immer/algorithm.hpp>
+
+#include <boost/range/adaptors.hpp>
+#include <boost/range/irange.hpp>
+#include <catch.hpp>
+
+#include <algorithm>
+#include <array>
+#include <numeric>
+#include <vector>
+
+#ifndef FLEX_VECTOR_T
+#error "define the vector template to use in FLEX_VECTOR_T"
+#endif
+
+#ifndef VECTOR_T
+#error "define the vector template to use in VECTOR_T"
+#endif
+
+template <typename V = FLEX_VECTOR_T<unsigned>>
+auto make_test_flex_vector(unsigned min, unsigned max)
+{
+    auto v = V{};
+    for (auto i = min; i < max; ++i)
+        v = v.push_back({i});
+    return v;
+}
+
+template <typename V = FLEX_VECTOR_T<unsigned>>
+auto make_test_flex_vector_front(unsigned min, unsigned max)
+{
+    auto v = V{};
+    for (auto i = max; i > min;)
+        v = v.push_front({--i});
+    return v;
+}
+
+template <std::size_t N>
+auto make_many_test_flex_vector()
+{
+    using vektor_t = FLEX_VECTOR_T<unsigned>;
+    auto many      = std::array<vektor_t, N>{};
+    std::generate_n(many.begin(), N, [v = vektor_t{}, i = 0u]() mutable {
+        auto r = v;
+        v      = v.push_back(i++);
+        return r;
+    });
+    return many;
+}
+
+template <std::size_t N>
+auto make_many_test_flex_vector_front()
+{
+    using vektor_t = FLEX_VECTOR_T<unsigned>;
+    auto many      = std::array<vektor_t, N>{};
+    std::generate_n(many.begin(), N, [i = 0u]() mutable {
+        return make_test_flex_vector_front(0, i++);
+    });
+    return many;
+}
+
+template <std::size_t N>
+auto make_many_test_flex_vector_front_remainder()
+{
+    using vektor_t = FLEX_VECTOR_T<unsigned>;
+    auto many      = std::array<vektor_t, N>{};
+    std::generate_n(many.begin(), N, [v = vektor_t{}, i = N - 1]() mutable {
+        auto r = v;
+        v      = v.push_front(--i);
+        return r;
+    });
+    return many;
+}
+
+TEST_CASE("set relaxed")
+{
+    auto v = make_test_flex_vector_front(0, 666u);
+    for (decltype(v.size()) i = 0; i < v.size(); ++i) {
+        v = v.set(i, i + 1);
+        CHECK(v[i] == i + 1);
+    }
+}
+
+TEST_CASE("push_front")
+{
+    const auto n = 666u;
+    auto v       = FLEX_VECTOR_T<unsigned>{};
+
+    for (auto i = 0u; i < n; ++i) {
+        v = v.push_front(i);
+        CHECK(v.size() == i + 1);
+        for (decltype(v.size()) j = 0; j < v.size(); ++j)
+            CHECK(v[v.size() - j - 1] == j);
+    }
+}
+
+TEST_CASE("concat")
+{
+#if IMMER_SLOW_TESTS
+    constexpr auto n = 666u;
+#else
+    constexpr auto n = 101u;
+#endif
+
+    auto all_lhs = make_many_test_flex_vector<n>();
+    auto all_rhs = make_many_test_flex_vector_front_remainder<n>();
+
+    SECTION("regular plus regular")
+    {
+        for (auto i : test_irange(0u, n)) {
+            auto c = all_lhs[i] + all_lhs[i];
+            CHECK_VECTOR_EQUALS(
+                c, boost::join(boost::irange(0u, i), boost::irange(0u, i)));
+        }
+    }
+
+    SECTION("regular plus relaxed")
+    {
+        for (auto i : test_irange(0u, n)) {
+            auto c = all_lhs[i] + all_rhs[n - i - 1];
+            CHECK_VECTOR_EQUALS(c, boost::irange(0u, n - 1));
+        }
+    }
+}
+
+auto make_flex_vector_concat(std::size_t min, std::size_t max)
+{
+    using vektor_t = FLEX_VECTOR_T<unsigned>;
+
+    if (max == min)
+        return vektor_t{};
+    else if (max == min + 1)
+        return vektor_t{}.push_back(min);
+    else {
+        auto mid = min + (max - min) / 2;
+        return make_flex_vector_concat(min, mid) +
+               make_flex_vector_concat(mid, max);
+    }
+}
+
+TEST_CASE("concat recursive")
+{
+    const auto n = 666u;
+    auto v       = make_flex_vector_concat(0, n);
+    CHECK_VECTOR_EQUALS(v, boost::irange(0u, n));
+}
+
+TEST_CASE("insert")
+{
+    SECTION("normal")
+    {
+        const auto n = 666u;
+        auto v       = make_test_flex_vector(0, n);
+        v            = v.insert(42, 100);
+        CHECK_VECTOR_EQUALS(v,
+                            boost::join(boost::irange(0u, 42u),
+                                        boost::join(boost::irange(100u, 101u),
+                                                    boost::irange(42u, n))));
+    }
+
+    SECTION("move")
+    {
+        const auto n = 666u;
+        auto v       = make_test_flex_vector(0, n);
+        v            = std::move(v).insert(42, 100);
+        CHECK_VECTOR_EQUALS(v,
+                            boost::join(boost::irange(0u, 42u),
+                                        boost::join(boost::irange(100u, 101u),
+                                                    boost::irange(42u, n))));
+    }
+
+    SECTION("vec")
+    {
+        const auto n = 666u;
+        auto v       = make_test_flex_vector(0, n);
+        v            = std::move(v).insert(42, {100, 101, 102});
+        CHECK_VECTOR_EQUALS(v,
+                            boost::join(boost::irange(0u, 42u),
+                                        boost::join(boost::irange(100u, 103u),
+                                                    boost::irange(42u, n))));
+    }
+
+    SECTION("vec move")
+    {
+        const auto n = 666u;
+        auto v       = make_test_flex_vector(0, n);
+        v            = std::move(v).insert(42, {100, 101, 102});
+        CHECK_VECTOR_EQUALS(v,
+                            boost::join(boost::irange(0u, 42u),
+                                        boost::join(boost::irange(100u, 103u),
+                                                    boost::irange(42u, n))));
+    }
+}
+
+TEST_CASE("erase")
+{
+    const auto n = 666u;
+    auto v       = make_test_flex_vector(0, n);
+    auto vv      = v.erase(0);
+    CHECK_VECTOR_EQUALS(vv, boost::irange(1u, n));
+    CHECK_VECTOR_EQUALS(v.erase(v.size() - 1), boost::irange(0u, n - 1));
+    CHECK_VECTOR_EQUALS(v.erase(v.size() - 1), boost::irange(0u, n - 1));
+    CHECK_VECTOR_EQUALS(
+        v.erase(42),
+        boost::join(boost::irange(0u, 42u), boost::irange(43u, n)));
+    CHECK_VECTOR_EQUALS(v.erase(v.size() - 1, v.size()),
+                        boost::irange(0u, n - 1));
+    CHECK_VECTOR_EQUALS(v.erase(0, 0), boost::irange(0u, n));
+    CHECK_VECTOR_EQUALS(
+        v.erase(42, 50),
+        boost::join(boost::irange(0u, 42u), boost::irange(50u, n)));
+}
+
+TEST_CASE("accumulate relaxed")
+{
+    auto expected_n = [](auto n) { return n * (n - 1) / 2; };
+    auto expected_i = [&](auto i, auto n) {
+        return expected_n(n) - expected_n(i);
+    };
+
+    SECTION("sum")
+    {
+        const auto n = 666u;
+        auto v       = make_test_flex_vector_front(0, n);
+
+        auto sum      = immer::accumulate(v, 0u);
+        auto expected = v.size() * (v.size() - 1) / 2;
+        CHECK(sum == expected);
+    }
+
+    SECTION("sum complex")
+    {
+        const auto n = 20u;
+
+        auto v = FLEX_VECTOR_T<unsigned>{};
+        for (auto i = 0u; i < n; ++i)
+            v = v.push_front(i) + v;
+
+        auto sum      = immer::accumulate(v, 0u);
+        auto expected = (1 << n) - n - 1;
+        CHECK(sum == expected);
+    }
+
+    SECTION("sum range")
+    {
+        using namespace std;
+        const auto n = 666u;
+        auto v       = make_test_flex_vector_front(0, n);
+        {
+            auto sum = immer::accumulate(begin(v) + 100, begin(v) + 300, 0u);
+            CHECK(sum == expected_i(100, 300));
+        }
+        {
+            auto sum = immer::accumulate(begin(v) + 31, begin(v) + 300, 0u);
+            CHECK(sum == expected_i(31, 300));
+        }
+        {
+            auto sum = immer::accumulate(begin(v), begin(v) + 33, 0u);
+            CHECK(sum == expected_i(0, 33));
+        }
+        {
+            auto sum = immer::accumulate(begin(v) + 100, begin(v) + 660, 0u);
+            CHECK(sum == expected_i(100, 660));
+        }
+        {
+            auto sum = immer::accumulate(begin(v) + 100, begin(v) + 105, 0u);
+            CHECK(sum == expected_i(100, 105));
+        }
+        {
+            auto sum = immer::accumulate(begin(v) + 660, begin(v) + 664, 0u);
+            CHECK(sum == expected_i(660, 664));
+        }
+    }
+}
+
+TEST_CASE("equals")
+{
+    const auto n = 666u;
+    auto v       = make_test_flex_vector_front(0, n);
+
+    CHECK(v == v);
+    CHECK(v == v.set(42, 42));
+    CHECK(v != v.set(42, 24));
+    CHECK(v == v.set(42, 24).set(42, 42));
+    CHECK(v.set(42, 24) == v.set(42, 24));
+    CHECK(v != v.push_back(7));
+    CHECK(v.push_back(7) == v.push_back(7));
+    CHECK(v.push_back(5) != v.push_back(7));
+    CHECK(v != v.set(v.size() - 2, 24));
+    CHECK(v == v.set(v.size() - 2, 24).set(v.size() - 2, v[v.size() - 2]));
+    CHECK(v == v.insert(42, 12).erase(42));
+    CHECK(v == v.insert(0, 12).erase(0));
+}
+
+TEST_CASE("equals bugs")
+{
+    {
+        const auto n = 666u;
+        auto v       = make_test_flex_vector(0, n);
+        CHECK(v == v.insert(42, 12).erase(42));
+        CHECK(v == v.insert(0, 12).erase(0));
+    }
+    {
+        const auto n = 30u;
+        auto v       = make_test_flex_vector(0, n);
+        CHECK(v == v.insert(10, 12).erase(10));
+        CHECK(v == v.insert(0, 12).erase(0));
+    }
+    {
+        const auto n = 666u;
+        auto v       = make_test_flex_vector(0, n);
+        for (auto i : test_irange(0u, n))
+            CHECK(v == v.insert(i, 42).erase(i));
+    }
+    {
+        const auto n = 666u;
+        auto v       = make_test_flex_vector_front(0, n);
+        for (auto i : test_irange(0u, n))
+            CHECK(v == v.insert(i, 42).erase(i));
+    }
+    {
+        const auto n = 666u;
+        auto v       = FLEX_VECTOR_T<unsigned>{};
+        using size_t = decltype(v.size());
+        for (auto i : test_irange(0u, n)) {
+            while (v.size() < i)
+                v = std::move(v).push_back(i);
+            auto vv = v;
+            for (auto j : test_irange(size_t{}, v.size())) {
+                auto vz = vv.insert(j, 42).erase(j);
+                CHECK(v == vz);
+                CHECK(vv == vz);
+                vv = vz;
+            }
+        }
+    }
+}
+
+TEST_CASE("take relaxed")
+{
+    const auto n = 666u;
+    auto v       = make_test_flex_vector_front(0, n);
+
+    for (auto i : test_irange(0u, n)) {
+        auto vv = v.take(i);
+        CHECK_VECTOR_EQUALS_RANGE(vv, v.begin(), v.begin() + i);
+    }
+}
+
+TEST_CASE("drop")
+{
+    const auto n = 666u;
+
+    SECTION("regular")
+    {
+        auto v = make_test_flex_vector(0, n);
+
+        for (auto i : test_irange(0u, n)) {
+            auto vv = v.drop(i);
+            CHECK_VECTOR_EQUALS_RANGE(vv, v.begin() + i, v.end());
+        }
+    }
+
+    SECTION("relaxed")
+    {
+        auto v = make_test_flex_vector_front(0, n);
+
+        for (auto i : test_irange(0u, n)) {
+            auto vv = v.drop(i);
+            CHECK_VECTOR_EQUALS_RANGE(vv, v.begin() + i, v.end());
+        }
+    }
+}
+
+#if IMMER_SLOW_TESTS
+TEST_CASE("reconcat")
+{
+    constexpr auto n = 666u;
+    auto v           = make_test_flex_vector_front(0, n);
+    auto all_lhs     = make_many_test_flex_vector_front<n + 1>();
+    auto all_rhs     = make_many_test_flex_vector_front_remainder<n + 1>();
+
+    for (auto i = 0u; i < n; ++i) {
+        auto vv = all_lhs[i] + all_rhs[n - i];
+        CHECK_VECTOR_EQUALS(vv, v);
+        CHECK_SLOW(vv == v);
+    }
+}
+
+TEST_CASE("reconcat drop")
+{
+    constexpr auto n = 666u;
+    auto v           = make_test_flex_vector_front(0, n);
+    auto all_lhs     = make_many_test_flex_vector_front<n + 1>();
+
+    for (auto i = 0u; i < n; ++i) {
+        auto vv = all_lhs[i] + v.drop(i);
+        CHECK_VECTOR_EQUALS(vv, v);
+        CHECK_SLOW(vv == v);
+    }
+}
+
+TEST_CASE("reconcat take")
+{
+    constexpr auto n = 666u;
+    auto v           = make_test_flex_vector_front(0, n);
+    auto all_rhs     = make_many_test_flex_vector_front_remainder<n + 1>();
+
+    for (auto i = 0u; i < n; ++i) {
+        auto vv = v.take(i) + all_rhs[n - i];
+        CHECK_VECTOR_EQUALS(vv, v);
+        CHECK_SLOW(vv == v);
+    }
+}
+#endif
+
+TEST_CASE("reconcat take drop")
+{
+    const auto n = 666u;
+    auto v       = make_test_flex_vector_front(0, n);
+
+    for (auto i : test_irange(0u, n)) {
+        auto vv = v.take(i) + v.drop(i);
+        CHECK_VECTOR_EQUALS(vv, v);
+        CHECK_SLOW(vv == v);
+    }
+}
+
+TEST_CASE("reconcat take drop feedback")
+{
+    const auto n = 666u;
+    auto v       = make_test_flex_vector_front(0, n);
+    auto vv      = v;
+    for (auto i : test_irange(0u, n)) {
+        vv = vv.take(i) + vv.drop(i);
+        CHECK_VECTOR_EQUALS(vv, v);
+        CHECK_SLOW(vv == v);
+    }
+}
+
+TEST_CASE("iterator relaxed")
+{
+    const auto n = 666u;
+    auto v       = make_test_flex_vector_front(0, n);
+
+    SECTION("works with range loop")
+    {
+        auto i = 0u;
+        for (const auto& x : v)
+            CHECK(x == i++);
+        CHECK(i == v.size());
+    }
+
+    SECTION("works with standard algorithms")
+    {
+        auto s = std::vector<unsigned>(n);
+        std::iota(s.begin(), s.end(), 0u);
+        std::equal(v.begin(), v.end(), s.begin(), s.end());
+    }
+
+    SECTION("can go back from end")
+    {
+        auto expected = n - 1;
+        CHECK(expected == *--v.end());
+    }
+
+    SECTION("works with reversed range adaptor")
+    {
+        auto r = v | boost::adaptors::reversed;
+        auto i = n;
+        for (const auto& x : r)
+            CHECK(x == --i);
+    }
+
+    SECTION("works with strided range adaptor")
+    {
+        auto r = v | boost::adaptors::strided(5);
+        auto i = 0u;
+        for (const auto& x : r)
+            CHECK(x == 5 * i++);
+    }
+
+    SECTION("works reversed")
+    {
+        auto i = n;
+        for (auto iter = v.rbegin(), last = v.rend(); iter != last; ++iter)
+            CHECK(*iter == --i);
+    }
+
+    SECTION("advance and distance")
+    {
+        auto i1 = v.begin();
+        auto i2 = i1 + 100;
+        CHECK(100u == *i2);
+        CHECK(100 == i2 - i1);
+        CHECK(50u == *(i2 - 50));
+        CHECK(-30 == (i2 - 30) - i2);
+    }
+}
+
+TEST_CASE("adopt regular vector contents")
+{
+    const auto n = 666u;
+    auto v       = VECTOR_T<unsigned>{};
+    for (auto i = 0u; i < n; ++i) {
+        v       = v.push_back(i);
+        auto fv = FLEX_VECTOR_T<unsigned>{v};
+        CHECK_VECTOR_EQUALS_AUX(v, fv, [](auto&& v) { return &v; });
+    }
+}
+
+TEST_CASE("exception safety relaxed")
+{
+    using dadaist_vector_t =
+        typename dadaist_wrapper<FLEX_VECTOR_T<unsigned>>::type;
+    constexpr auto n = 666u;
+
+    SECTION("push back")
+    {
+        auto half = n / 2;
+        auto v    = make_test_flex_vector_front<dadaist_vector_t>(0, half);
+        auto d    = dadaism{};
+        for (auto i = half; v.size() < static_cast<decltype(v.size())>(n);) {
+            auto s = d.next();
+            try {
+                v = v.push_back({i});
+                ++i;
+            } catch (dada_error) {}
+            CHECK_VECTOR_EQUALS(v, boost::irange(0u, i));
+        }
+        CHECK(d.happenings > 0);
+        IMMER_TRACE_E(d.happenings);
+    }
+
+    SECTION("update")
+    {
+        auto v = make_test_flex_vector_front<dadaist_vector_t>(0, n);
+        auto d = dadaism{};
+        for (auto i = 0u; i < n;) {
+            auto s = d.next();
+            try {
+                v = v.update(i, [](auto x) { return dada(), x + 1; });
+                ++i;
+            } catch (dada_error) {}
+            CHECK_VECTOR_EQUALS(
+                v, boost::join(boost::irange(1u, 1u + i), boost::irange(i, n)));
+        }
+        CHECK(d.happenings > 0);
+        IMMER_TRACE_E(d.happenings);
+    }
+
+    SECTION("take")
+    {
+        auto v = make_test_flex_vector_front<dadaist_vector_t>(0, n);
+        auto d = dadaism{};
+        for (auto i = 0u; i < n;) {
+            auto s = d.next();
+            auto r = dadaist_vector_t{};
+            try {
+                r = v.take(i);
+                CHECK_VECTOR_EQUALS(r, boost::irange(0u, i++));
+            } catch (dada_error) {
+                CHECK_VECTOR_EQUALS(r, boost::irange(0u, 0u));
+            }
+        }
+        CHECK(d.happenings > 0);
+        IMMER_TRACE_E(d.happenings);
+    }
+
+    SECTION("concat")
+    {
+        auto v = make_test_flex_vector<dadaist_vector_t>(0, n);
+        auto d = dadaism{};
+        for (auto i = 0u; i < n;) {
+            auto lhs = v.take(i);
+            auto rhs = v.drop(i);
+            auto s   = d.next();
+            try {
+                v = lhs + rhs;
+                ++i;
+            } catch (dada_error) {}
+            CHECK_VECTOR_EQUALS(v, boost::irange(0u, n));
+        }
+        CHECK(d.happenings > 0);
+        IMMER_TRACE_E(d.happenings);
+    }
+}
diff --git a/test/flex_vector/issue-45.cpp b/test/flex_vector/issue-45.cpp
new file mode 100644
index 000000000000..76bd77b2e240
--- /dev/null
+++ b/test/flex_vector/issue-45.cpp
@@ -0,0 +1,35 @@
+//
+// 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
+//
+
+// Thanks Guiguiprim for reporting this issue
+// https://github.com/arximboldi/immer/issues/46
+
+#include <immer/flex_vector.hpp>
+#include <immer/vector.hpp>
+#include <immer/vector_transient.hpp>
+
+#include <catch.hpp>
+
+#if IMMER_CXX_STANDARD >= 17
+
+#include <variant>
+
+TEST_CASE("error when erasing an element from a "
+          "immer::flex_vector<std::variant/optional/any>")
+{
+    using Vector = immer::flex_vector<std::variant<int, double>>;
+    // using Vector = immer::flex_vector<std::optional<int>>;
+    // using Vector = immer::flex_vector<std::any>;
+
+    Vector v{1, 2, 3, 4};
+    Vector v2 = v.erase(2);
+
+    CHECK(v2.size() == 3);
+}
+
+#endif
diff --git a/test/flex_vector/issue-47.cpp b/test/flex_vector/issue-47.cpp
new file mode 100644
index 000000000000..7757fe4ee4e5
--- /dev/null
+++ b/test/flex_vector/issue-47.cpp
@@ -0,0 +1,5 @@
+#include <immer/flex_vector.hpp>
+
+immer::flex_vector<int> v{1};
+
+int main() { return 0; }
diff --git a/test/flex_vector/regular-B3-BL3.cpp b/test/flex_vector/regular-B3-BL3.cpp
new file mode 100644
index 000000000000..fb5d13d1aa8e
--- /dev/null
+++ b/test/flex_vector/regular-B3-BL3.cpp
@@ -0,0 +1,16 @@
+//
+// 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
+//
+
+#include <immer/flex_vector.hpp>
+
+template <typename T>
+using test_vector_t =
+    immer::flex_vector<T, immer::default_memory_policy, 3u, 3u>;
+
+#define VECTOR_T test_vector_t
+#include "../vector/generic.ipp"
diff --git a/test/flex_vector/regular-default.cpp b/test/flex_vector/regular-default.cpp
new file mode 100644
index 000000000000..8f28c3d8099f
--- /dev/null
+++ b/test/flex_vector/regular-default.cpp
@@ -0,0 +1,12 @@
+//
+// 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
+//
+
+#include <immer/flex_vector.hpp>
+
+#define VECTOR_T ::immer::flex_vector
+#include "../vector/generic.ipp"