diff --git a/tests/test_odr_guard_1.cpp b/tests/test_odr_guard_1.cpp
new file mode 100644
index 000000000..035984b68
--- /dev/null
+++ b/tests/test_odr_guard_1.cpp
@@ -0,0 +1,60 @@
+#include "pybind11_tests.h"
+
+#define USE_MRC_AAA
+#ifdef USE_MRC_AAA
+namespace mrc_ns { // minimal real caster
+
+struct type_mrc {
+    int value = -9999;
+};
+
+template <typename Ignored = void>
+struct minimal_real_caster {
+    static constexpr auto name = py::detail::const_name<type_mrc>();
+    static std::int32_t odr_guard; // WANTED: ASAN detect_odr_violation
+
+    static py::handle
+    cast(type_mrc const &src, py::return_value_policy /*policy*/, py::handle /*parent*/) {
+        odr_guard++;                                 // Just to make sure it is used.
+        return py::int_(src.value + 1010).release(); // Actual ODR violation.
+    }
+
+    // Maximizing simplicity. This will go terribly wrong for other arg types.
+    template <typename>
+    using cast_op_type = const type_mrc &;
+
+    // NOLINTNEXTLINE(google-explicit-constructor)
+    operator type_mrc const &() {
+        static type_mrc obj;
+        obj.value = 11; // Actual ODR violation.
+        return obj;
+    }
+
+    bool load(py::handle src, bool /*convert*/) {
+        // Only accepts str, but the value is ignored.
+        return py::isinstance<py::str>(src);
+    }
+};
+
+template <typename Ignored>
+std::int32_t minimal_real_caster<Ignored>::odr_guard = 0;
+
+} // namespace mrc_ns
+
+namespace pybind11 {
+namespace detail {
+template <>
+struct type_caster<mrc_ns::type_mrc> : mrc_ns::minimal_real_caster<> {};
+} // namespace detail
+} // namespace pybind11
+#endif
+
+TEST_SUBMODULE(odr_guard_1, m) {
+#ifdef USE_MRC_AAA
+    m.def("sizeof_mrc_odr_guard",
+          []() { return sizeof(mrc_ns::minimal_real_caster<>::odr_guard); });
+    m.def("type_mrc_to_python", []() { return mrc_ns::type_mrc{101}; });
+    m.def("type_mrc_from_python", [](const mrc_ns::type_mrc &obj) { return obj.value + 100; });
+    m.def("mrc_odr_guard", []() { return mrc_ns::minimal_real_caster<>::odr_guard; });
+#endif
+}
diff --git a/tests/test_odr_guard_1.py b/tests/test_odr_guard_1.py
new file mode 100644
index 000000000..fefe6cde4
--- /dev/null
+++ b/tests/test_odr_guard_1.py
@@ -0,0 +1,34 @@
+import pytest
+
+import pybind11_tests.odr_guard_1 as m
+
+
+def test_sizeof_mrc_odr_guard():
+    if hasattr(m, "sizeof_mrc_odr_guard"):
+        assert m.sizeof_mrc_odr_guard() == 4
+    else:
+        pytest.skip("sizeof_mrc_odr_guard")
+
+
+def test_type_mrc_to_python():
+    if hasattr(m, "type_mrc_to_python"):
+        assert m.type_mrc_to_python() == 1111
+    else:
+        pytest.skip("type_mrc_to_python")
+
+
+def test_type_mrc_from_python():
+    if hasattr(m, "type_mrc_from_python"):
+        assert m.type_mrc_from_python("ignored") == 111
+    else:
+        pytest.skip("type_mrc_from_python")
+
+
+def test_mrc_odr_guard():
+    if hasattr(m, "mrc_odr_guard"):
+        i = m.mrc_odr_guard()
+        m.type_mrc_to_python()
+        j = m.mrc_odr_guard()
+        assert j == i + 1
+    else:
+        pytest.skip("mrc_odr_guard")
diff --git a/tests/test_odr_guard_2.cpp b/tests/test_odr_guard_2.cpp
new file mode 100644
index 000000000..820b7cc6c
--- /dev/null
+++ b/tests/test_odr_guard_2.cpp
@@ -0,0 +1,60 @@
+#include "pybind11_tests.h"
+
+#define USE_MRC_BBB
+#ifdef USE_MRC_BBB
+namespace mrc_ns { // minimal real caster
+
+struct type_mrc {
+    int value = -9999;
+};
+
+template <typename Ignored = void>
+struct minimal_real_caster {
+    static constexpr auto name = py::detail::const_name<type_mrc>();
+    static std::int64_t odr_guard; // WANTED: ASAN detect_odr_violation
+
+    static py::handle
+    cast(type_mrc const &src, py::return_value_policy /*policy*/, py::handle /*parent*/) {
+        odr_guard++;                                 // Just to make sure it is used.
+        return py::int_(src.value + 2020).release(); // Actual ODR violation.
+    }
+
+    // Maximizing simplicity. This will go terribly wrong for other arg types.
+    template <typename>
+    using cast_op_type = const type_mrc &;
+
+    // NOLINTNEXTLINE(google-explicit-constructor)
+    operator type_mrc const &() {
+        static type_mrc obj;
+        obj.value = 22; // Actual ODR violation.
+        return obj;
+    }
+
+    bool load(py::handle src, bool /*convert*/) {
+        // Only accepts str, but the value is ignored.
+        return py::isinstance<py::str>(src);
+    }
+};
+
+template <typename Ignored>
+std::int64_t minimal_real_caster<Ignored>::odr_guard = 0;
+
+} // namespace mrc_ns
+
+namespace pybind11 {
+namespace detail {
+template <>
+struct type_caster<mrc_ns::type_mrc> : mrc_ns::minimal_real_caster<> {};
+} // namespace detail
+} // namespace pybind11
+#endif
+
+TEST_SUBMODULE(odr_guard_2, m) {
+#ifdef USE_MRC_BBB
+    m.def("sizeof_mrc_odr_guard",
+          []() { return sizeof(mrc_ns::minimal_real_caster<>::odr_guard); });
+    m.def("type_mrc_to_python", []() { return mrc_ns::type_mrc{202}; });
+    m.def("type_mrc_from_python", [](const mrc_ns::type_mrc &obj) { return obj.value + 200; });
+    m.def("mrc_odr_guard", []() { return mrc_ns::minimal_real_caster<>::odr_guard; });
+#endif
+}
diff --git a/tests/test_odr_guard_2.py b/tests/test_odr_guard_2.py
new file mode 100644
index 000000000..4784a48f8
--- /dev/null
+++ b/tests/test_odr_guard_2.py
@@ -0,0 +1,34 @@
+import pytest
+
+import pybind11_tests.odr_guard_2 as m
+
+
+def test_sizeof_mrc_odr_guard():
+    if hasattr(m, "sizeof_mrc_odr_guard"):
+        assert m.sizeof_mrc_odr_guard() == 8
+    else:
+        pytest.skip("sizeof_mrc_odr_guard")
+
+
+def test_type_mrc_to_python():
+    if hasattr(m, "type_mrc_to_python"):
+        assert m.type_mrc_to_python() == 2222
+    else:
+        pytest.skip("type_mrc_to_python")
+
+
+def test_type_mrc_from_python():
+    if hasattr(m, "type_mrc_from_python"):
+        assert m.type_mrc_from_python("ignored") == 222
+    else:
+        pytest.skip("type_mrc_from_python")
+
+
+def test_mrc_odr_guard():
+    if hasattr(m, "mrc_odr_guard"):
+        i = m.mrc_odr_guard()
+        m.type_mrc_to_python()
+        j = m.mrc_odr_guard()
+        assert j == i + 1
+    else:
+        pytest.skip("mrc_odr_guard")