gh-123465: Allow Py_RELATIVE_OFFSET for __*offset__ members (GH-123474)

8 files changed, 422 insertions, 74 deletions

diff --git a/Doc/c-api/structures.rst b/Doc/c-api/structures.rst
index f9461ab..d333df3 100644
--- a/Doc/c-api/structures.rst
+++ b/Doc/c-api/structures.rst
@@ -485,7 +485,8 @@ Accessing attributes of extension types
    ``PyMemberDef`` may contain a definition for the special member
    ``"__vectorcalloffset__"``, corresponding to
    :c:member:`~PyTypeObject.tp_vectorcall_offset` in type objects.
-   These must be defined with ``Py_T_PYSSIZET`` and ``Py_READONLY``, for example::
+   This member must be defined with ``Py_T_PYSSIZET``, and either
+   ``Py_READONLY`` or ``Py_READONLY | Py_RELATIVE_OFFSET``. For example::
 
       static PyMemberDef spam_type_members[] = {
           {"__vectorcalloffset__", Py_T_PYSSIZET,
@@ -506,6 +507,12 @@ Accessing attributes of extension types
       ``PyMemberDef`` is always available.
       Previously, it required including ``"structmember.h"``.
 
+   .. versionchanged:: 3.14
+
+      :c:macro:`Py_RELATIVE_OFFSET` is now allowed for
+      ``"__vectorcalloffset__"``, ``"__dictoffset__"`` and
+      ``"__weaklistoffset__"``.
+
 .. c:function:: PyObject* PyMember_GetOne(const char *obj_addr, struct PyMemberDef *m)
 
    Get an attribute belonging to the object at address *obj_addr*.  The
diff --git a/Lib/test/test_call.py b/Lib/test/test_call.py
index 504f880..68e3b2a 100644
--- a/Lib/test/test_call.py
+++ b/Lib/test/test_call.py
@@ -851,8 +851,13 @@ class TestPEP590(unittest.TestCase):
     @requires_limited_api
     def test_vectorcall_limited_incoming(self):
         from _testcapi import pyobject_vectorcall
-        obj = _testlimitedcapi.LimitedVectorCallClass()
-        self.assertEqual(pyobject_vectorcall(obj, (), ()), "vectorcall called")
+        for cls in (_testlimitedcapi.LimitedVectorCallClass,
+                    _testlimitedcapi.LimitedRelativeVectorCallClass):
+            with self.subTest(cls=cls):
+                obj = cls()
+                self.assertEqual(
+                    pyobject_vectorcall(obj, (), ()),
+                    "vectorcall called")
 
     @requires_limited_api
     def test_vectorcall_limited_outgoing(self):
diff --git a/Lib/test/test_capi/test_misc.py b/Lib/test/test_capi/test_misc.py
index b103bf2..d50217b 100644
--- a/Lib/test/test_capi/test_misc.py
+++ b/Lib/test/test_capi/test_misc.py
@@ -541,14 +541,19 @@ class CAPITest(unittest.TestCase):
         self.assertEqual(new_type_refcnt, sys.getrefcount(A))
 
     def test_heaptype_with_dict(self):
-        inst = _testcapi.HeapCTypeWithDict()
-        inst.foo = 42
-        self.assertEqual(inst.foo, 42)
-        self.assertEqual(inst.dictobj, inst.__dict__)
-        self.assertEqual(inst.dictobj, {"foo": 42})
+        for cls in (
+            _testcapi.HeapCTypeWithDict,
+            _testlimitedcapi.HeapCTypeWithRelativeDict,
+        ):
+            with self.subTest(cls=cls):
+                inst = cls()
+                inst.foo = 42
+                self.assertEqual(inst.foo, 42)
+                self.assertEqual(inst.dictobj, inst.__dict__)
+                self.assertEqual(inst.dictobj, {"foo": 42})
 
-        inst = _testcapi.HeapCTypeWithDict()
-        self.assertEqual({}, inst.__dict__)
+                inst = cls()
+                self.assertEqual({}, inst.__dict__)
 
     def test_heaptype_with_managed_dict(self):
         inst = _testcapi.HeapCTypeWithManagedDict()
@@ -585,10 +590,15 @@ class CAPITest(unittest.TestCase):
         self.assertEqual({}, inst.__dict__)
 
     def test_heaptype_with_weakref(self):
-        inst = _testcapi.HeapCTypeWithWeakref()
-        ref = weakref.ref(inst)
-        self.assertEqual(ref(), inst)
-        self.assertEqual(inst.weakreflist, ref)
+        for cls in (
+            _testcapi.HeapCTypeWithWeakref,
+            _testlimitedcapi.HeapCTypeWithRelativeWeakref,
+        ):
+            with self.subTest(cls=cls):
+                inst = cls()
+                ref = weakref.ref(inst)
+                self.assertEqual(ref(), inst)
+                self.assertEqual(inst.weakreflist, ref)
 
     def test_heaptype_with_managed_weakref(self):
         inst = _testcapi.HeapCTypeWithManagedWeakref()
@@ -730,45 +740,56 @@ class CAPITest(unittest.TestCase):
         self.assertIsInstance(sub, metaclass)
 
     def test_multiple_inheritance_ctypes_with_weakref_or_dict(self):
+        for weakref_cls in (_testcapi.HeapCTypeWithWeakref,
+                            _testlimitedcapi.HeapCTypeWithRelativeWeakref):
+            for dict_cls in (_testcapi.HeapCTypeWithDict,
+                             _testlimitedcapi.HeapCTypeWithRelativeDict):
+                with self.subTest(weakref_cls=weakref_cls, dict_cls=dict_cls):
 
-        with self.assertRaises(TypeError):
-            class Both1(_testcapi.HeapCTypeWithWeakref, _testcapi.HeapCTypeWithDict):
-                pass
-        with self.assertRaises(TypeError):
-            class Both2(_testcapi.HeapCTypeWithDict, _testcapi.HeapCTypeWithWeakref):
-                pass
+                    with self.assertRaises(TypeError):
+                        class Both1(weakref_cls, dict_cls):
+                            pass
+                    with self.assertRaises(TypeError):
+                        class Both2(dict_cls, weakref_cls):
+                            pass
 
     def test_multiple_inheritance_ctypes_with_weakref_or_dict_and_other_builtin(self):
+        for dict_cls in (_testcapi.HeapCTypeWithDict,
+                         _testlimitedcapi.HeapCTypeWithRelativeDict):
+            for weakref_cls in (_testcapi.HeapCTypeWithWeakref,
+                                _testlimitedcapi.HeapCTypeWithRelativeWeakref):
+                with self.subTest(dict_cls=dict_cls, weakref_cls=weakref_cls):
 
-        with self.assertRaises(TypeError):
-            class C1(_testcapi.HeapCTypeWithDict, list):
-                pass
+                    with self.assertRaises(TypeError):
+                        class C1(dict_cls, list):
+                            pass
 
-        with self.assertRaises(TypeError):
-            class C2(_testcapi.HeapCTypeWithWeakref, list):
-                pass
+                    with self.assertRaises(TypeError):
+                        class C2(weakref_cls, list):
+                            pass
 
-        class C3(_testcapi.HeapCTypeWithManagedDict, list):
-            pass
-        class C4(_testcapi.HeapCTypeWithManagedWeakref, list):
-            pass
+                    class C3(_testcapi.HeapCTypeWithManagedDict, list):
+                        pass
+                    class C4(_testcapi.HeapCTypeWithManagedWeakref, list):
+                        pass
 
-        inst = C3()
-        inst.append(0)
-        str(inst.__dict__)
+                    inst = C3()
+                    inst.append(0)
+                    str(inst.__dict__)
 
-        inst = C4()
-        inst.append(0)
-        str(inst.__weakref__)
+                    inst = C4()
+                    inst.append(0)
+                    str(inst.__weakref__)
 
-        for cls in (_testcapi.HeapCTypeWithManagedDict, _testcapi.HeapCTypeWithManagedWeakref):
-            for cls2 in (_testcapi.HeapCTypeWithDict, _testcapi.HeapCTypeWithWeakref):
-                class S(cls, cls2):
-                    pass
-            class B1(C3, cls):
-                pass
-            class B2(C4, cls):
-                pass
+                    for cls in (_testcapi.HeapCTypeWithManagedDict,
+                                _testcapi.HeapCTypeWithManagedWeakref):
+                        for cls2 in (dict_cls, weakref_cls):
+                            class S(cls, cls2):
+                                pass
+                        class B1(C3, cls):
+                            pass
+                        class B2(C4, cls):
+                            pass
 
     def test_pytype_fromspec_with_repeated_slots(self):
         for variant in range(2):
@@ -1272,6 +1293,53 @@ class TestHeapTypeRelative(unittest.TestCase):
                 SystemError, r"PyMember_SetOne used with Py_RELATIVE_OFFSET"):
             instance.set_memb_relative(0)
 
+    def test_heaptype_relative_special_members_errors(self):
+        for member_name in "__vectorcalloffset__", "__dictoffset__", "__weaklistoffset__":
+            with self.subTest(member_name=member_name):
+                with self.assertRaisesRegex(
+                        SystemError,
+                        r"With Py_RELATIVE_OFFSET, basicsize must be negative."):
+                    _testlimitedcapi.make_heaptype_with_member(
+                        basicsize=sys.getsizeof(object()) + 100,
+                        add_relative_flag=True,
+                        member_name=member_name,
+                        member_offset=0,
+                        member_type=_testlimitedcapi.Py_T_PYSSIZET,
+                        member_flags=_testlimitedcapi.Py_READONLY,
+                        )
+                with self.assertRaisesRegex(
+                        SystemError,
+                        r"Member offset out of range \(0\.\.-basicsize\)"):
+                    _testlimitedcapi.make_heaptype_with_member(
+                        basicsize=-8,
+                        add_relative_flag=True,
+                        member_name=member_name,
+                        member_offset=-1,
+                        member_type=_testlimitedcapi.Py_T_PYSSIZET,
+                        member_flags=_testlimitedcapi.Py_READONLY,
+                        )
+                with self.assertRaisesRegex(
+                        SystemError,
+                        r"type of %s must be Py_T_PYSSIZET" % member_name):
+                    _testlimitedcapi.make_heaptype_with_member(
+                        basicsize=-100,
+                        add_relative_flag=True,
+                        member_name=member_name,
+                        member_offset=0,
+                        member_flags=_testlimitedcapi.Py_READONLY,
+                        )
+                with self.assertRaisesRegex(
+                        SystemError,
+                        r"flags for %s must be " % member_name):
+                    _testlimitedcapi.make_heaptype_with_member(
+                        basicsize=-100,
+                        add_relative_flag=True,
+                        member_name=member_name,
+                        member_offset=0,
+                        member_type=_testlimitedcapi.Py_T_PYSSIZET,
+                        member_flags=0,
+                        )
+
     def test_pyobject_getitemdata_error(self):
         """Test PyObject_GetItemData fails on unsupported types"""
         with self.assertRaises(TypeError):
diff --git a/Misc/NEWS.d/next/C_API/2024-08-29-15-05-19.gh-issue-123465.eqwNWq.rst b/Misc/NEWS.d/next/C_API/2024-08-29-15-05-19.gh-issue-123465.eqwNWq.rst
new file mode 100644
index 0000000..1935adf
--- /dev/null
+++ b/Misc/NEWS.d/next/C_API/2024-08-29-15-05-19.gh-issue-123465.eqwNWq.rst
@@ -0,0 +1,4 @@
+:c:macro:`Py_RELATIVE_OFFSET` is now allowed in :c:type:`PyMemberDef` for
+the special offset member ``"__vectorcalloffset__"``, as well as the
+discouraged special offset members ``"__dictoffset__"`` and
+``"__weaklistoffset__"``
diff --git a/Modules/_testlimitedcapi/clinic/heaptype_relative.c.h b/Modules/_testlimitedcapi/clinic/heaptype_relative.c.h
new file mode 100644
index 0000000..994f831
--- /dev/null
+++ b/Modules/_testlimitedcapi/clinic/heaptype_relative.c.h
@@ -0,0 +1,44 @@
+/*[clinic input]
+preserve
+[clinic start generated code]*/
+
+PyDoc_STRVAR(make_heaptype_with_member__doc__,
+"make_heaptype_with_member($module, /, extra_base_size=0, basicsize=0,\n"
+"                          member_offset=0, add_relative_flag=False, *,\n"
+"                          member_name=\'memb\', member_flags=0,\n"
+"                          member_type=-1)\n"
+"--\n"
+"\n");
+
+#define MAKE_HEAPTYPE_WITH_MEMBER_METHODDEF    \
+    {"make_heaptype_with_member", (PyCFunction)(void(*)(void))make_heaptype_with_member, METH_VARARGS|METH_KEYWORDS, make_heaptype_with_member__doc__},
+
+static PyObject *
+make_heaptype_with_member_impl(PyObject *module, int extra_base_size,
+                               int basicsize, int member_offset,
+                               int add_relative_flag,
+                               const char *member_name, int member_flags,
+                               int member_type);
+
+static PyObject *
+make_heaptype_with_member(PyObject *module, PyObject *args, PyObject *kwargs)
+{
+    PyObject *return_value = NULL;
+    static char *_keywords[] = {"extra_base_size", "basicsize", "member_offset", "add_relative_flag", "member_name", "member_flags", "member_type", NULL};
+    int extra_base_size = 0;
+    int basicsize = 0;
+    int member_offset = 0;
+    int add_relative_flag = 0;
+    const char *member_name = "memb";
+    int member_flags = 0;
+    int member_type = Py_T_BYTE;
+
+    if (!PyArg_ParseTupleAndKeywords(args, kwargs, "|iiip$sii:make_heaptype_with_member", _keywords,
+        &extra_base_size, &basicsize, &member_offset, &add_relative_flag, &member_name, &member_flags, &member_type))
+        goto exit;
+    return_value = make_heaptype_with_member_impl(module, extra_base_size, basicsize, member_offset, add_relative_flag, member_name, member_flags, member_type);
+
+exit:
+    return return_value;
+}
+/*[clinic end generated code: output=01933185947faecc input=a9049054013a1b77]*/
diff --git a/Modules/_testlimitedcapi/heaptype_relative.c b/Modules/_testlimitedcapi/heaptype_relative.c
index c253151..45d65ee 100644
--- a/Modules/_testlimitedcapi/heaptype_relative.c
+++ b/Modules/_testlimitedcapi/heaptype_relative.c
@@ -8,6 +8,8 @@
 #include <stddef.h>               // max_align_t
 #include <string.h>               // memset
 
+#include "clinic/heaptype_relative.c.h"
+
 static PyType_Slot empty_slots[] = {
     {0, NULL},
 };
@@ -247,6 +249,81 @@ heaptype_with_member_set_memb_relative(PyObject *self, PyObject *value)
     Py_RETURN_NONE;
 }
 
+typedef struct {
+    int padding;  // just so the offset isn't 0
+    PyObject *dict;
+} HeapCTypeWithDictStruct;
+
+static void
+heapctypewithrelativedict_dealloc(PyObject* self)
+{
+    PyTypeObject *tp = Py_TYPE(self);
+    HeapCTypeWithDictStruct *data = PyObject_GetTypeData(self, tp);
+    Py_XDECREF(data->dict);
+    PyObject_Free(self);
+    Py_DECREF(tp);
+}
+
+static PyType_Spec HeapCTypeWithRelativeDict_spec = {
+    .name = "_testcapi.HeapCTypeWithRelativeDict",
+    .basicsize = -(int)sizeof(HeapCTypeWithDictStruct),
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
+    .slots = (PyType_Slot[]) {
+        {Py_tp_dealloc, heapctypewithrelativedict_dealloc},
+        {Py_tp_getset, (PyGetSetDef[]) {
+            {"__dict__", PyObject_GenericGetDict, PyObject_GenericSetDict},
+            {NULL} /* Sentinel */
+        }},
+        {Py_tp_members, (PyMemberDef[]) {
+            {"dictobj", _Py_T_OBJECT,
+             offsetof(HeapCTypeWithDictStruct, dict),
+             Py_RELATIVE_OFFSET},
+            {"__dictoffset__", Py_T_PYSSIZET,
+             offsetof(HeapCTypeWithDictStruct, dict),
+             Py_READONLY | Py_RELATIVE_OFFSET},
+            {NULL} /* Sentinel */
+        }},
+        {0, 0},
+    }
+};
+
+typedef struct {
+    char padding;  // just so the offset isn't 0
+    PyObject *weakreflist;
+} HeapCTypeWithWeakrefStruct;
+
+static void
+heapctypewithrelativeweakref_dealloc(PyObject* self)
+{
+    PyTypeObject *tp = Py_TYPE(self);
+    HeapCTypeWithWeakrefStruct *data = PyObject_GetTypeData(self, tp);
+    if (data->weakreflist != NULL) {
+        PyObject_ClearWeakRefs(self);
+    }
+    Py_XDECREF(data->weakreflist);
+    PyObject_Free(self);
+    Py_DECREF(tp);
+}
+
+static PyType_Spec HeapCTypeWithRelativeWeakref_spec = {
+    .name = "_testcapi.HeapCTypeWithRelativeWeakref",
+    .basicsize = -(int)sizeof(HeapCTypeWithWeakrefStruct),
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
+    .slots = (PyType_Slot[]) {
+        {Py_tp_dealloc, heapctypewithrelativeweakref_dealloc},
+        {Py_tp_members, (PyMemberDef[]) {
+            {"weakreflist", _Py_T_OBJECT,
+             offsetof(HeapCTypeWithWeakrefStruct, weakreflist),
+             Py_RELATIVE_OFFSET},
+            {"__weaklistoffset__", Py_T_PYSSIZET,
+             offsetof(HeapCTypeWithWeakrefStruct, weakreflist),
+             Py_READONLY | Py_RELATIVE_OFFSET},
+            {NULL} /* Sentinel */
+        }},
+        {0, 0},
+    }
+};
+
 static PyMethodDef heaptype_with_member_methods[] = {
     {"get_memb", heaptype_with_member_get_memb, METH_NOARGS},
     {"set_memb", heaptype_with_member_set_memb, METH_O},
@@ -256,19 +333,31 @@ static PyMethodDef heaptype_with_member_methods[] = {
     {NULL},
 };
 
+/*[clinic input]
+make_heaptype_with_member
+
+    extra_base_size: int = 0
+    basicsize: int = 0
+    member_offset: int = 0
+    add_relative_flag: bool = False
+    *
+    member_name: str = "memb"
+    member_flags: int = 0
+    member_type: int(c_default="Py_T_BYTE") = -1
+
+[clinic start generated code]*/
+
 static PyObject *
-make_heaptype_with_member(PyObject *module, PyObject *args)
+make_heaptype_with_member_impl(PyObject *module, int extra_base_size,
+                               int basicsize, int member_offset,
+                               int add_relative_flag,
+                               const char *member_name, int member_flags,
+                               int member_type)
+/*[clinic end generated code: output=7005db9a07396997 input=007e29cdbe1d3390]*/
 {
     PyObject *base = NULL;
     PyObject *result = NULL;
 
-    int extra_base_size, basicsize, offset, add_flag;
-
-    int r = PyArg_ParseTuple(args, "iiip", &extra_base_size, &basicsize, &offset, &add_flag);
-    if (!r) {
-        goto finally;
-    }
-
     PyType_Spec base_spec = {
         .name = "_testcapi.Base",
         .basicsize = sizeof(PyObject) + extra_base_size,
@@ -281,7 +370,8 @@ make_heaptype_with_member(PyObject *module, PyObject *args)
     }
 
     PyMemberDef members[] = {
-        {"memb", Py_T_BYTE, offset, add_flag ? Py_RELATIVE_OFFSET : 0},
+        {member_name, member_type, member_offset,
+            member_flags | (add_relative_flag ? Py_RELATIVE_OFFSET : 0)},
         {0},
     };
     PyType_Slot slots[] = {
@@ -325,7 +415,7 @@ static PyMethodDef TestMethods[] = {
     {"make_sized_heaptypes", make_sized_heaptypes, METH_VARARGS},
     {"subclass_var_heaptype", subclass_var_heaptype, METH_VARARGS},
     {"subclass_heaptype", subclass_heaptype, METH_VARARGS},
-    {"make_heaptype_with_member", make_heaptype_with_member, METH_VARARGS},
+    MAKE_HEAPTYPE_WITH_MEMBER_METHODDEF
     {"test_alignof_max_align_t", test_alignof_max_align_t, METH_NOARGS},
     {NULL},
 };
@@ -341,5 +431,42 @@ _PyTestLimitedCAPI_Init_HeaptypeRelative(PyObject *m)
         return -1;
     }
 
+#define ADD_FROM_SPEC(SPEC) do {                                \
+        PyObject *tp = PyType_FromSpec(SPEC);                   \
+        if (!tp) {                                              \
+            return -1;                                          \
+        }                                                       \
+        if (PyModule_AddType(m, (PyTypeObject *)tp) < 0) {      \
+            return -1;                                          \
+        }                                                       \
+    } while (0)
+
+    PyObject *tp;
+
+    tp = PyType_FromSpec(&HeapCTypeWithRelativeDict_spec);
+    if (!tp) {
+        return -1;
+    }
+    if (PyModule_AddType(m, (PyTypeObject *)tp) < 0) {
+        return -1;
+    }
+    Py_DECREF(tp);
+
+    tp = PyType_FromSpec(&HeapCTypeWithRelativeWeakref_spec);
+    if (!tp) {
+        return -1;
+    }
+    if (PyModule_AddType(m, (PyTypeObject *)tp) < 0) {
+        return -1;
+    }
+    Py_DECREF(tp);
+
+    if (PyModule_AddIntMacro(m, Py_T_PYSSIZET) < 0) {
+        return -1;
+    }
+    if (PyModule_AddIntMacro(m, Py_READONLY) < 0) {
+        return -1;
+    }
+
     return 0;
 }
diff --git a/Modules/_testlimitedcapi/vectorcall_limited.c b/Modules/_testlimitedcapi/vectorcall_limited.c
index 5ef97ca..4a7af96 100644
--- a/Modules/_testlimitedcapi/vectorcall_limited.c
+++ b/Modules/_testlimitedcapi/vectorcall_limited.c
@@ -6,6 +6,8 @@
 #  define Py_LIMITED_API 0x030c0000
 #endif
 
+#include <stddef.h>                         // offsetof
+
 #include "parts.h"
 #include "clinic/vectorcall_limited.c.h"
 
@@ -175,6 +177,41 @@ static PyType_Spec LimitedVectorCallClass_spec = {
     .slots = LimitedVectorallClass_slots,
 };
 
+typedef struct {
+    vectorcallfunc vfunc;
+} LimitedRelativeVectorCallStruct;
+
+static PyObject *
+LimitedRelativeVectorCallClass_new(PyTypeObject *tp, PyTypeObject *a, PyTypeObject *kw)
+{
+    PyObject *self = ((allocfunc)PyType_GetSlot(tp, Py_tp_alloc))(tp, 0);
+    if (!self) {
+        return NULL;
+    }
+    LimitedRelativeVectorCallStruct *data = PyObject_GetTypeData(self, tp);
+    data->vfunc = LimitedVectorCallClass_vectorcall;
+    return self;
+}
+
+
+static PyType_Spec LimitedRelativeVectorCallClass_spec = {
+    .name = "_testlimitedcapi.LimitedRelativeVectorCallClass",
+    .basicsize = -(int)sizeof(LimitedRelativeVectorCallStruct),
+    .flags = Py_TPFLAGS_DEFAULT
+        | Py_TPFLAGS_HAVE_VECTORCALL,
+    .slots = (PyType_Slot[]) {
+        {Py_tp_new, LimitedRelativeVectorCallClass_new},
+        {Py_tp_call, LimitedVectorCallClass_tpcall},
+        {Py_tp_members, (PyMemberDef[]){
+            {"__vectorcalloffset__", Py_T_PYSSIZET,
+             offsetof(LimitedRelativeVectorCallStruct, vfunc),
+             Py_READONLY | Py_RELATIVE_OFFSET},
+            {NULL}
+        }},
+        {0}
+    },
+};
+
 static PyMethodDef TestMethods[] = {
     _TESTLIMITEDCAPI_CALL_VECTORCALL_METHODDEF
     _TESTLIMITEDCAPI_CALL_VECTORCALL_METHOD_METHODDEF
@@ -197,5 +234,16 @@ _PyTestLimitedCAPI_Init_VectorcallLimited(PyObject *m)
         return -1;
     }
     Py_DECREF(LimitedVectorCallClass);
+
+    PyObject *LimitedRelativeVectorCallClass = PyType_FromModuleAndSpec(
+        m, &LimitedRelativeVectorCallClass_spec, NULL);
+    if (!LimitedRelativeVectorCallClass) {
+        return -1;
+    }
+    if (PyModule_AddType(m, (PyTypeObject *)LimitedRelativeVectorCallClass) < 0) {
+        return -1;
+    }
+    Py_DECREF(LimitedRelativeVectorCallClass);
+
     return 0;
 }
diff --git a/Objects/typeobject.c b/Objects/typeobject.c
index 78f6931..9dc0ebd 100644
--- a/Objects/typeobject.c
+++ b/Objects/typeobject.c
@@ -4642,6 +4642,41 @@ check_basicsize_includes_size_and_offsets(PyTypeObject* type)
     return 1;
 }
 
+/* Set *dest to the offset specified by a special "__*offset__" member.
+ * Return 0 on success, -1 on failure.
+ */
+static inline int
+special_offset_from_member(
+            const PyMemberDef *memb /* may be NULL */,
+            Py_ssize_t type_data_offset,
+            Py_ssize_t *dest /* not NULL */)
+{
+    if (memb == NULL) {
+        *dest = 0;
+        return 0;
+    }
+    if (memb->type != Py_T_PYSSIZET) {
+        PyErr_Format(
+            PyExc_SystemError,
+            "type of %s must be Py_T_PYSSIZET",
+            memb->name);
+        return -1;
+    }
+    if (memb->flags == Py_READONLY) {
+        *dest = memb->offset;
+        return 0;
+    }
+    else if (memb->flags == (Py_READONLY | Py_RELATIVE_OFFSET)) {
+        *dest = memb->offset + type_data_offset;
+        return 0;
+    }
+    PyErr_Format(
+        PyExc_SystemError,
+        "flags for %s must be Py_READONLY or (Py_READONLY | Py_RELATIVE_OFFSET)",
+        memb->name);
+    return -1;
+}
+
 static PyObject *
 _PyType_FromMetaclass_impl(
     PyTypeObject *metaclass, PyObject *module,
@@ -4667,10 +4702,11 @@ _PyType_FromMetaclass_impl(
 
     const PyType_Slot *slot;
     Py_ssize_t nmembers = 0;
-    Py_ssize_t weaklistoffset, dictoffset, vectorcalloffset;
+    const PyMemberDef *weaklistoffset_member = NULL;
+    const PyMemberDef *dictoffset_member = NULL;
+    const PyMemberDef *vectorcalloffset_member = NULL;
     char *res_start;
 
-    nmembers = weaklistoffset = dictoffset = vectorcalloffset = 0;
     for (slot = spec->slots; slot->slot; slot++) {
         if (slot->slot < 0
             || (size_t)slot->slot >= Py_ARRAY_LENGTH(pyslot_offsets)) {
@@ -4687,24 +4723,6 @@ _PyType_FromMetaclass_impl(
             }
             for (const PyMemberDef *memb = slot->pfunc; memb->name != NULL; memb++) {
                 nmembers++;
-                if (strcmp(memb->name, "__weaklistoffset__") == 0) {
-                    // The PyMemberDef must be a Py_ssize_t and readonly
-                    assert(memb->type == Py_T_PYSSIZET);
-                    assert(memb->flags == Py_READONLY);
-                    weaklistoffset = memb->offset;
-                }
-                if (strcmp(memb->name, "__dictoffset__") == 0) {
-                    // The PyMemberDef must be a Py_ssize_t and readonly
-                    assert(memb->type == Py_T_PYSSIZET);
-                    assert(memb->flags == Py_READONLY);
-                    dictoffset = memb->offset;
-                }
-                if (strcmp(memb->name, "__vectorcalloffset__") == 0) {
-                    // The PyMemberDef must be a Py_ssize_t and readonly
-                    assert(memb->type == Py_T_PYSSIZET);
-                    assert(memb->flags == Py_READONLY);
-                    vectorcalloffset = memb->offset;
-                }
                 if (memb->flags & Py_RELATIVE_OFFSET) {
                     if (spec->basicsize > 0) {
                         PyErr_SetString(
@@ -4719,6 +4737,15 @@ _PyType_FromMetaclass_impl(
                         goto finally;
                     }
                 }
+                if (strcmp(memb->name, "__weaklistoffset__") == 0) {
+                    weaklistoffset_member = memb;
+                }
+                if (strcmp(memb->name, "__dictoffset__") == 0) {
+                    dictoffset_member = memb;
+                }
+                if (strcmp(memb->name, "__vectorcalloffset__") == 0) {
+                    vectorcalloffset_member = memb;
+                }
             }
             break;
         case Py_tp_doc:
@@ -4882,6 +4909,24 @@ _PyType_FromMetaclass_impl(
 
     Py_ssize_t itemsize = spec->itemsize;
 
+    /* Compute special offsets */
+
+    Py_ssize_t weaklistoffset = 0;
+    if (special_offset_from_member(weaklistoffset_member, type_data_offset,
+                                  &weaklistoffset) < 0) {
+        goto finally;
+    }
+    Py_ssize_t dictoffset = 0;
+    if (special_offset_from_member(dictoffset_member, type_data_offset,
+                                  &dictoffset) < 0) {
+        goto finally;
+    }
+    Py_ssize_t vectorcalloffset = 0;
+    if (special_offset_from_member(vectorcalloffset_member, type_data_offset,
+                                  &vectorcalloffset) < 0) {
+        goto finally;
+    }
+
     /* Allocate the new type
      *
      * Between here and PyType_Ready, we should limit: