VERY MESSY SNAPSHOT of WIP, this was the starting point for cl/454658864, which has more changes on top.

include/pybind11/cast.h

@@ -47,30 +47,60 @@ class type_caster_for_class_ : public type_caster_base<T> {};
 template <typename type, typename SFINAE = void>
 class type_caster : public type_caster_for_class_<type> {};
 
-inline std::unordered_map<std::type_index, std::type_index> &odr_guard_registry() {
+inline std::unordered_map<std::type_index, std::uint64_t> &odr_guard_registry() {
-    static std::unordered_map<std::type_index, std::type_index> reg;
+    static std::unordered_map<std::type_index, std::uint64_t> reg;
     return reg;
 }
 
+namespace {
+
+template <typename IntrinsicType>
+bool odr_guard_impl(const std::type_index & it_ti, const std::uint64_t& tc_id) {
+    auto match = odr_guard_registry().find(it_ti);
+    printf("\nLOOOK %s %llu\n", type_id<IntrinsicType>().c_str(), (long long) tc_id);
+    fflush(stdout);
+    if (match == odr_guard_registry().end()) {
+        odr_guard_registry().insert({it_ti, tc_id});
+    } else if (match->second != tc_id) {
+        throw std::system_error(std::make_error_code(std::errc::state_not_recoverable),
+                                "pybind11::detail::type_caster<" + type_id<IntrinsicType>()
+                                    + "> ODR VIOLATION DETECTED");
+    }
+    return true;
+}
+
 template <typename IntrinsicType>
 struct type_caster_odr_guard : type_caster<IntrinsicType> {
     type_caster_odr_guard() {
-        auto it_ti = std::type_index(typeid(IntrinsicType));
+        odr_guard_hook = !!odr_guard_hook;
-        auto tc_ti = std::type_index(typeid(type_caster<IntrinsicType>));
-        auto match = odr_guard_registry().find(it_ti);
-        if (match == odr_guard_registry().end()) {
-            odr_guard_registry().insert({it_ti, tc_ti});
-        } else if (match->second != tc_ti) {
-            throw std::system_error(std::make_error_code(std::errc::state_not_recoverable),
-                                    "pybind11::detail::type_caster<" + type_id<IntrinsicType>()
-                                        + "> ODR VIOLATION DETECTED");
-        }
     }
+
+    // type_caster_odr_guard(const type_caster_odr_guard &) = default;
+    // type_caster_odr_guard(type_caster_odr_guard &&) = default;
+
+    template <typename CType, typename... Arg>
+    static handle cast(CType &&src, return_value_policy policy, handle parent,
+                       Arg &&...arg) {
+        odr_guard_hook = !!odr_guard_hook;
+        return type_caster<IntrinsicType>::cast(std::forward<CType>(src), policy, parent,
+                                                std::forward<Arg>(arg)...);
+    }
+
+    static bool odr_guard_hook;
 };
 
+template <typename IntrinsicType>
+bool type_caster_odr_guard<IntrinsicType>::odr_guard_hook = [](){
+    return odr_guard_impl<IntrinsicType>(
+        std::type_index(typeid(IntrinsicType)),
+        type_caster<IntrinsicType>::universally_unique_identifier);
+}();
+
 template <typename type>
 using make_caster = type_caster_odr_guard<intrinsic_t<type>>;
 
+} // namespace
+
 template <typename T>
 struct type_uses_smart_holder_type_caster {
     static constexpr bool value
@@ -106,6 +136,7 @@ private:
 public:
     bool load(handle src, bool convert) { return subcaster.load(src, convert); }
     static constexpr auto name = caster_t::name;
+    static constexpr std::uint64_t universally_unique_identifier = 1655073597;
     static handle
     cast(const std::reference_wrapper<type> &src, return_value_policy policy, handle parent) {
         // It is definitely wrong to take ownership of this pointer, so mask that rvp
@@ -277,6 +308,7 @@ public:
     }
 
     PYBIND11_TYPE_CASTER(T, const_name<std::is_integral<T>::value>("int", "float"));
+    static constexpr std::uint64_t universally_unique_identifier = 1655073597;
 };
 
 template <typename T>
@@ -292,6 +324,7 @@ public:
         return none().inc_ref();
     }
     PYBIND11_TYPE_CASTER(T, const_name("None"));
+    static constexpr std::uint64_t universally_unique_identifier = 1655073597;
 };
 
 template <>
@@ -339,6 +372,7 @@ public:
     using cast_op_type = void *&;
     explicit operator void *&() { return value; }
     static constexpr auto name = const_name("capsule");
+    static constexpr std::uint64_t universally_unique_identifier = 1655073597;
 
 private:
     void *value = nullptr;
@@ -395,6 +429,7 @@ public:
         return handle(src ? Py_True : Py_False).inc_ref();
     }
     PYBIND11_TYPE_CASTER(bool, const_name("bool"));
+    static constexpr std::uint64_t universally_unique_identifier = 1655073597;
 };
 
 // Helper class for UTF-{8,16,32} C++ stl strings:
@@ -487,6 +522,7 @@ struct string_caster {
     }
 
     PYBIND11_TYPE_CASTER(StringType, const_name(PYBIND11_STRING_NAME));
+    static constexpr std::uint64_t universally_unique_identifier = 1655073597;
 
 private:
     static handle decode_utfN(const char *buffer, ssize_t nbytes) {
@@ -659,6 +695,7 @@ public:
     }
 
     static constexpr auto name = const_name(PYBIND11_STRING_NAME);
+    static constexpr std::uint64_t universally_unique_identifier = 1655073597;
     template <typename _T>
     using cast_op_type = pybind11::detail::cast_op_type<_T>;
 };
@@ -703,6 +740,7 @@ public:
 
     static constexpr auto name
         = const_name("Tuple[") + concat(make_caster<Ts>::name...) + const_name("]");
+    static constexpr std::uint64_t universally_unique_identifier = 1655073597;
 
     template <typename T>
     using cast_op_type = type;
@@ -875,6 +913,7 @@ struct move_only_holder_caster {
         return type_caster_base<type>::cast_holder(ptr, std::addressof(src));
     }
     static constexpr auto name = type_caster_base<type>::name;
+    static constexpr std::uint64_t universally_unique_identifier = 1655073597;
 };
 
 #ifndef PYBIND11_USE_SMART_HOLDER_AS_DEFAULT
@@ -984,6 +1023,7 @@ struct pyobject_caster {
         return src.inc_ref();
     }
     PYBIND11_TYPE_CASTER(type, handle_type_name<type>::name);
+    static constexpr std::uint64_t universally_unique_identifier = 3434;
 };
 
 template <typename T>

include/pybind11/chrono.h

@@ -97,6 +97,7 @@ public:
     }
 
     PYBIND11_TYPE_CASTER(type, const_name("datetime.timedelta"));
+    static constexpr std::uint64_t universally_unique_identifier = 1655073597;
 };
 
 inline std::tm *localtime_thread_safe(const std::time_t *time, std::tm *buf) {
@@ -208,6 +209,7 @@ public:
                                           us.count());
     }
     PYBIND11_TYPE_CASTER(type, const_name("datetime.datetime"));
+    static constexpr std::uint64_t universally_unique_identifier = 1655073597;
 };
 
 // Other clocks that are not the system clock are not measured as datetime.datetime objects

include/pybind11/complex.h

@@ -69,6 +69,7 @@ public:
     }
 
     PYBIND11_TYPE_CASTER(std::complex<T>, const_name("complex"));
+    static constexpr std::uint64_t universally_unique_identifier = 1655073597;
 };
 PYBIND11_NAMESPACE_END(detail)
 PYBIND11_NAMESPACE_END(PYBIND11_NAMESPACE)

include/pybind11/detail/init.h

@@ -27,6 +27,7 @@ public:
     using cast_op_type = value_and_holder &;
     explicit operator value_and_holder &() { return *value; }
     static constexpr auto name = const_name<value_and_holder>();
+    static constexpr std::uint64_t universally_unique_identifier = 1655073597;
 
 private:
     value_and_holder *value = nullptr;

include/pybind11/detail/smart_holder_type_casters.h

@@ -616,6 +616,7 @@ template <typename T>
 struct smart_holder_type_caster : smart_holder_type_caster_load<T>,
                                   smart_holder_type_caster_class_hooks {
     static constexpr auto name = const_name<T>();
+    static constexpr std::uint64_t universally_unique_identifier = 1655073597;
 
     // static handle cast(T, ...)
     // is redundant (leads to ambiguous overloads).
@@ -777,6 +778,7 @@ template <typename T>
 struct smart_holder_type_caster<std::shared_ptr<T>> : smart_holder_type_caster_load<T>,
                                                       smart_holder_type_caster_class_hooks {
     static constexpr auto name = const_name<T>();
+    static constexpr std::uint64_t universally_unique_identifier = 1655073597;
 
     static handle cast(const std::shared_ptr<T> &src, return_value_policy policy, handle parent) {
         switch (policy) {
@@ -841,6 +843,7 @@ template <typename T>
 struct smart_holder_type_caster<std::shared_ptr<T const>> : smart_holder_type_caster_load<T>,
                                                             smart_holder_type_caster_class_hooks {
     static constexpr auto name = const_name<T>();
+    static constexpr std::uint64_t universally_unique_identifier = 1655073597;
 
     static handle
     cast(const std::shared_ptr<T const> &src, return_value_policy policy, handle parent) {
@@ -861,6 +864,7 @@ template <typename T, typename D>
 struct smart_holder_type_caster<std::unique_ptr<T, D>> : smart_holder_type_caster_load<T>,
                                                          smart_holder_type_caster_class_hooks {
     static constexpr auto name = const_name<T>();
+    static constexpr std::uint64_t universally_unique_identifier = 1655073597;
 
     static handle cast(std::unique_ptr<T, D> &&src, return_value_policy policy, handle parent) {
         if (policy != return_value_policy::automatic
@@ -944,6 +948,7 @@ template <typename T, typename D>
 struct smart_holder_type_caster<std::unique_ptr<T const, D>>
     : smart_holder_type_caster_load<T>, smart_holder_type_caster_class_hooks {
     static constexpr auto name = const_name<T>();
+    static constexpr std::uint64_t universally_unique_identifier = 1655073597;
 
     static handle
     cast(std::unique_ptr<T const, D> &&src, return_value_policy policy, handle parent) {

include/pybind11/detail/type_caster_base.h

@@ -911,6 +911,7 @@ class type_caster_base : public type_caster_generic {
 
 public:
     static constexpr auto name = const_name<type>();
+    static constexpr std::uint64_t universally_unique_identifier = 1655073597;
 
     type_caster_base() : type_caster_base(typeid(type)) {}
     explicit type_caster_base(const std::type_info &info) : type_caster_generic(info) {}

include/pybind11/eigen.h

@@ -392,6 +392,7 @@ public:
     }
 
     static constexpr auto name = props::descriptor;
+    static constexpr std::uint64_t universally_unique_identifier = 1655073597;
 
     // NOLINTNEXTLINE(google-explicit-constructor)
     operator Type *() { return &value; }
@@ -436,6 +437,7 @@ public:
     }
 
     static constexpr auto name = props::descriptor;
+    static constexpr std::uint64_t universally_unique_identifier = 1655073597;
 
     // Explicitly delete these: support python -> C++ conversion on these (i.e. these can be return
     // types but not bound arguments).  We still provide them (with an explicitly delete) so that
@@ -623,6 +625,7 @@ public:
     }
 
     static constexpr auto name = props::descriptor;
+    static constexpr std::uint64_t universally_unique_identifier = 1655073597;
 
     // Explicitly delete these: support python -> C++ conversion on these (i.e. these can be return
     // types but not bound arguments).  We still provide them (with an explicitly delete) so that
@@ -699,6 +702,7 @@ struct type_caster<Type, enable_if_t<is_eigen_sparse<Type>::value>> {
                          const_name<(Type::IsRowMajor) != 0>("scipy.sparse.csr_matrix[",
                                                              "scipy.sparse.csc_matrix[")
                              + npy_format_descriptor<Scalar>::name + const_name("]"));
+    static constexpr std::uint64_t universally_unique_identifier = 1655073597;
 };
 
 PYBIND11_NAMESPACE_END(detail)

include/pybind11/functional.h

@@ -124,6 +124,7 @@ public:
                          const_name("Callable[[") + concat(make_caster<Args>::name...)
                              + const_name("], ") + make_caster<retval_type>::name
                              + const_name("]"));
+    static constexpr std::uint64_t universally_unique_identifier = 1655073597;
 };
 
 PYBIND11_NAMESPACE_END(detail)

include/pybind11/numpy.h

@@ -1214,6 +1214,7 @@ struct pyobject_caster<array_t<T, ExtraFlags>> {
         return src.inc_ref();
     }
     PYBIND11_TYPE_CASTER(type, handle_type_name<type>::name);
+    static constexpr std::uint64_t universally_unique_identifier = 1655073597;
 };
 
 template <typename T>

include/pybind11/stl.h

@@ -87,6 +87,7 @@ struct set_caster {
     }
 
     PYBIND11_TYPE_CASTER(type, const_name("Set[") + key_conv::name + const_name("]"));
+    static constexpr std::uint64_t universally_unique_identifier = 1655073597;
 };
 
 template <typename Type, typename Key, typename Value>
@@ -136,6 +137,7 @@ struct map_caster {
     PYBIND11_TYPE_CASTER(Type,
                          const_name("Dict[") + key_conv::name + const_name(", ") + value_conv::name
                              + const_name("]"));
+    static constexpr std::uint64_t universally_unique_identifier = 1655073597;
 };
 
 template <typename Type, typename Value>
@@ -188,6 +190,7 @@ public:
     }
 
     PYBIND11_TYPE_CASTER(Type, const_name("List[") + value_conv::name + const_name("]"));
+    static constexpr std::uint64_t universally_unique_identifier = 1655073597;
 };
 
 template <typename Type, typename Alloc>
@@ -257,6 +260,7 @@ public:
                                                      const_name("[") + const_name<Size>()
                                                          + const_name("]"))
                              + const_name("]"));
+    static constexpr std::uint64_t universally_unique_identifier = 1655073597;
 };
 
 template <typename Type, size_t Size>
@@ -315,6 +319,7 @@ struct optional_caster {
     }
 
     PYBIND11_TYPE_CASTER(Type, const_name("Optional[") + value_conv::name + const_name("]"));
+    static constexpr std::uint64_t universally_unique_identifier = 1655073597;
 };
 
 #if defined(PYBIND11_HAS_OPTIONAL)
@@ -401,6 +406,7 @@ struct variant_caster<V<Ts...>> {
     PYBIND11_TYPE_CASTER(Type,
                          const_name("Union[") + detail::concat(make_caster<Ts>::name...)
                              + const_name("]"));
+    static constexpr std::uint64_t universally_unique_identifier = 1655073597;
 };
 
 #if defined(PYBIND11_HAS_VARIANT)

include/pybind11/stl/filesystem.h

@@ -99,6 +99,7 @@ public:
     }
 
     PYBIND11_TYPE_CASTER(T, const_name("os.PathLike"));
+    static constexpr std::uint64_t universally_unique_identifier = 1655073597;
 };
 
 #endif // PYBIND11_HAS_FILESYSTEM || defined(PYBIND11_HAS_EXPERIMENTAL_FILESYSTEM)

tests/pybind11_tests.h

@@ -61,6 +61,7 @@ template <>
 class type_caster<RValueCaster> {
 public:
     PYBIND11_TYPE_CASTER(RValueCaster, const_name("RValueCaster"));
+    static constexpr std::uint64_t universally_unique_identifier = 1655073597;
     static handle cast(RValueCaster &&, return_value_policy, handle) {
         return py::str("rvalue").release();
     }

tests/test_async.cpp

@@ -9,29 +9,32 @@
 
 #include "pybind11_tests.h"
 
+#define USE_MRC_AAA
+#ifdef USE_MRC_AAA
 namespace mrc_ns { // minimal real caster
 
-struct minimal_real_caster;
+template <typename ValType>
-
 struct type_mrc {
     int value = -9999;
 };
 
+template <typename CType>
 struct minimal_real_caster {
-    static constexpr auto name = py::detail::const_name<type_mrc>();
+    static constexpr auto name = py::detail::const_name<CType>();
+    static constexpr std::uint64_t universally_unique_identifier = 1000000;
 
     static py::handle
-    cast(type_mrc const &src, py::return_value_policy /*policy*/, py::handle /*parent*/) {
+    cast(CType const &src, py::return_value_policy /*policy*/, py::handle /*parent*/) {
         return py::int_(src.value + 1010).release();
     }
 
     // Maximizing simplicity. This will go terribly wrong for other arg types.
     template <typename>
-    using cast_op_type = const type_mrc &;
+    using cast_op_type = const CType &;
 
     // NOLINTNEXTLINE(google-explicit-constructor)
-    operator type_mrc const &() {
+    operator CType const &() {
-        static type_mrc obj;
+        static CType obj;
         obj.value = 11;
         return obj;
     }
@@ -46,10 +49,11 @@ struct minimal_real_caster {
 
 namespace pybind11 {
 namespace detail {
-template <>
+template <typename ValType>
-struct type_caster<mrc_ns::type_mrc> : mrc_ns::minimal_real_caster {};
+struct type_caster<mrc_ns::type_mrc<ValType>> : mrc_ns::minimal_real_caster<mrc_ns::type_mrc<ValType>> {};
 } // namespace detail
 } // namespace pybind11
+#endif
 
 TEST_SUBMODULE(async_module, m) {
     struct DoesNotSupportAsync {};
@@ -64,6 +68,8 @@ TEST_SUBMODULE(async_module, m) {
             f.attr("set_result")(5);
             return f.attr("__await__")();
         });
-    m.def("type_mrc_to_python", []() { return mrc_ns::type_mrc{101}; });
+#ifdef USE_MRC_AAA
-    m.def("type_mrc_from_python", [](const mrc_ns::type_mrc &obj) { return obj.value + 100; });
+    m.def("type_mrc_to_python", []() { return mrc_ns::type_mrc<int>{101}; });
+    m.def("type_mrc_from_python", [](const mrc_ns::type_mrc<int> &obj) { return obj.value + 100; });
+#endif
 }

tests/test_async.py

@@ -24,6 +24,15 @@ def test_await_missing(event_loop):
         event_loop.run_until_complete(get_await_result(m.DoesNotSupportAsync()))
 
 
-def test_mrc():
+def test_type_mrc_to_python():
-    assert m.type_mrc_to_python() == 1111
+    if hasattr(m, "type_mrc_to_python"):
-    assert m.type_mrc_from_python("ignored") == 111
+        assert m.type_mrc_to_python() == 1111
+    else:
+        pytype.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:
+        pytype.skip("type_mrc_from_python")

tests/test_buffers.cpp

@@ -12,29 +12,32 @@
 #include "constructor_stats.h"
 #include "pybind11_tests.h"
 
+#define USE_MRC_BBB
+#ifdef USE_MRC_BBB
 namespace mrc_ns { // minimal real caster
 
-struct minimal_real_caster;
+template <typename ValType>
-
 struct type_mrc {
-    int value = -9999;
+    ValType value = -9999;
 };
 
+template <typename CType>
 struct minimal_real_caster {
-    static constexpr auto name = py::detail::const_name<type_mrc>();
+    static constexpr auto name = py::detail::const_name<CType>();
+    static constexpr std::uint64_t universally_unique_identifier = 2000000;
 
     static py::handle
-    cast(type_mrc const &src, py::return_value_policy /*policy*/, py::handle /*parent*/) {
+    cast(CType const &src, py::return_value_policy /*policy*/, py::handle /*parent*/) {
         return py::int_(src.value + 2020).release();
     }
 
     // Maximizing simplicity. This will go terribly wrong for other arg types.
     template <typename>
-    using cast_op_type = const type_mrc &;
+    using cast_op_type = const CType &;
 
     // NOLINTNEXTLINE(google-explicit-constructor)
-    operator type_mrc const &() {
+    operator CType const &() {
-        static type_mrc obj;
+        static CType obj;
         obj.value = 22;
         return obj;
     }
@@ -49,10 +52,11 @@ struct minimal_real_caster {
 
 namespace pybind11 {
 namespace detail {
-template <>
+template <typename ValType>
-struct type_caster<mrc_ns::type_mrc> : mrc_ns::minimal_real_caster {};
+struct type_caster<mrc_ns::type_mrc<ValType>> : mrc_ns::minimal_real_caster<mrc_ns::type_mrc<ValType>> {};
 } // namespace detail
 } // namespace pybind11
+#endif
 
 TEST_SUBMODULE(buffers, m) {
     // test_from_python / test_to_python:
@@ -264,6 +268,8 @@ TEST_SUBMODULE(buffers, m) {
 
     m.def("get_buffer_info", [](const py::buffer &buffer) { return buffer.request(); });
 
-    m.def("type_mrc_to_python", []() { return mrc_ns::type_mrc{202}; });
+#ifdef USE_MRC_BBB
-    m.def("type_mrc_from_python", [](const mrc_ns::type_mrc &obj) { return obj.value + 200; });
+    m.def("type_mrc_to_python", []() { return mrc_ns::type_mrc<int>{202}; });
+    m.def("type_mrc_from_python", [](const mrc_ns::type_mrc<int> &obj) { return obj.value + 200; });
+#endif
 }

tests/test_buffers.py

@@ -163,6 +163,15 @@ def test_ctypes_from_buffer():
         assert not cinfo.readonly
 
 
-def test_mrc():
+def test_type_mrc_to_python():
-    assert m.type_mrc_to_python() == 1111
+    if hasattr(m, "type_mrc_to_python"):
-    assert m.type_mrc_from_python("ignored") == 111
+        assert m.type_mrc_to_python() == 2222
+    else:
+        pytype.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:
+        pytype.skip("type_mrc_from_python")

tests/test_builtin_casters.cpp

@@ -28,6 +28,7 @@ template <>
 class type_caster<ConstRefCasted> {
 public:
     static constexpr auto name = const_name<ConstRefCasted>();
+    static constexpr std::uint64_t universally_unique_identifier = 1655073597;
 
     // Input is unimportant, a new value will always be constructed based on the
     // cast operator.

tests/test_copy_move.cpp

@@ -106,6 +106,7 @@ PYBIND11_NAMESPACE_BEGIN(detail)
 template <>
 struct type_caster<MoveOnlyInt> {
     PYBIND11_TYPE_CASTER(MoveOnlyInt, const_name("MoveOnlyInt"));
+    static constexpr std::uint64_t universally_unique_identifier = 1655073597;
     bool load(handle src, bool) {
         value = MoveOnlyInt(src.cast<int>());
         return true;
@@ -118,6 +119,7 @@ struct type_caster<MoveOnlyInt> {
 template <>
 struct type_caster<MoveOrCopyInt> {
     PYBIND11_TYPE_CASTER(MoveOrCopyInt, const_name("MoveOrCopyInt"));
+    static constexpr std::uint64_t universally_unique_identifier = 1655073597;
     bool load(handle src, bool) {
         value = MoveOrCopyInt(src.cast<int>());
         return true;
@@ -134,6 +136,7 @@ protected:
 
 public:
     static constexpr auto name = const_name("CopyOnlyInt");
+    static constexpr std::uint64_t universally_unique_identifier = 1655073597;
     bool load(handle src, bool) {
         value = CopyOnlyInt(src.cast<int>());
         return true;

tests/test_custom_type_casters.cpp

@@ -32,6 +32,7 @@ public:
 #else
     PYBIND11_TYPE_CASTER(ArgInspector1, const_name("ArgInspector1"));
 #endif
+    static constexpr std::uint64_t universally_unique_identifier = 1655073597;
 
     bool load(handle src, bool convert) {
         value.arg = "loading ArgInspector1 argument " + std::string(convert ? "WITH" : "WITHOUT")
@@ -49,6 +50,7 @@ template <>
 struct type_caster<ArgInspector2> {
 public:
     PYBIND11_TYPE_CASTER(ArgInspector2, const_name("ArgInspector2"));
+    static constexpr std::uint64_t universally_unique_identifier = 1655073597;
 
     bool load(handle src, bool convert) {
         value.arg = "loading ArgInspector2 argument " + std::string(convert ? "WITH" : "WITHOUT")
@@ -66,6 +68,7 @@ template <>
 struct type_caster<ArgAlwaysConverts> {
 public:
     PYBIND11_TYPE_CASTER(ArgAlwaysConverts, const_name("ArgAlwaysConverts"));
+    static constexpr std::uint64_t universally_unique_identifier = 1655073597;
 
     bool load(handle, bool convert) { return convert; }
 
@@ -97,6 +100,7 @@ namespace detail {
 template <>
 struct type_caster<DestructionTester> {
     PYBIND11_TYPE_CASTER(DestructionTester, const_name("DestructionTester"));
+    static constexpr std::uint64_t universally_unique_identifier = 1655073597;
     bool load(handle, bool) { return true; }
 
     static handle cast(const DestructionTester &, return_value_policy, handle) {
@@ -119,6 +123,7 @@ namespace py_ = ::pybind11;
 // don't have any symbol collision when using macro mixin.
 struct my_caster {
     PYBIND11_TYPE_CASTER(MyType, py_::detail::const_name("MyType"));
+    static constexpr std::uint64_t universally_unique_identifier = 1655073597;
     bool load(py_::handle, bool) { return true; }
 
     static py_::handle cast(const MyType &, py_::return_value_policy, py_::handle) {