@@ -2616,8 +2616,11 @@ Notes on using *__slots__*:
 
 * :exc:`TypeError` will be raised if *__slots__* other than *__dict__* and
   *__weakref__* are defined for a class derived from a
-  :c:member:`"variable-length" built-in type <PyTypeObject.tp_itemsize>` such as
-  :class:`int`, :class:`bytes`, and :class:`type`, except :class:`tuple`.
+  :c:member:`"variable-length" built-in type <PyTypeObject.tp_itemsize>`,
+  except :class:`tuple`,
+  unless the C type is defined with :c:macro:`Py_TPFLAGS_ITEMS_AT_END`.
+  For example, they cannot be defined on subclasses of
+  :class:`int` or :class:`bytes`.
 
 * Any non-string :term:`iterable` may be assigned to *__slots__*.
 
@@ -2642,7 +2645,10 @@ Notes on using *__slots__*:
 
 .. versionchanged:: 3.15
    Allowed defining the *__dict__* and *__weakref__* *__slots__* for any class.
-   Allowed defining any *__slots__* for a class derived from :class:`tuple`.
+   Allowed defining any *__slots__* for a class derived from :class:`tuple`,
+   :class:`type` or other
+   :c:member:`"variable-length" built-in type <PyTypeObject.tp_itemsize>`
+   with the :c:macro:`Py_TPFLAGS_ITEMS_AT_END` flag.
 
 
 .. _class-customization:

@@ -403,6 +403,12 @@ Other language changes
   (Contributed by Serhiy Storchaka in :gh:`41779`.)
 
 
+* Allowed defining any *__slots__* for a class derived from :class:`type` or
+  other :c:member:`"variable-length" built-in type <PyTypeObject.tp_itemsize>`
+  with the :c:macro:`Py_TPFLAGS_ITEMS_AT_END` flag.
+  (Contributed by Serhiy Storchaka in :gh:`103740`.)
+
+
 New modules
 ===========
 

diff --git a/Lib/test/test_descr.py b/Lib/test/test_descr.py
@@ -1320,6 +1320,32 @@ class X(object):
         with self.assertRaisesRegex(AttributeError, "'X' object has no attribute 'a'"):
             X().a
 
+    def test_slots_before_items(self):
+        class C(type):
+            __slots__ = ['a']
+        x = C('A', (int,), {})
+        self.assertNotHasAttr(x, "a")
+        x.a = 1
+        x.b = 2
+        self.assertEqual(x.a, 1)
+        self.assertEqual(x.b, 2)
+        self.assertNotIn('a', x.__dict__)
+        self.assertIn('b', x.__dict__)
+        del x.a
+        self.assertNotHasAttr(x, "a")
+
+    @unittest.skipIf(_testcapi is None, 'need the _testcapi module')
+    def test_slots_before_items2(self):
+        class D(_testcapi.HeapCCollection):
+            __slots__ = ['a']
+        x = D(1, 2, 3)
+        self.assertNotHasAttr(x, "a")
+        x.a = 42
+        self.assertEqual(x.a, 42)
+        del x.a
+        self.assertNotHasAttr(x, "a")
+        self.assertEqual(list(x), [1, 2, 3])
+
     def test_slots_after_items(self):
         class C(tuple):
             __slots__ = ['a']
@@ -1396,14 +1422,6 @@ class W(_testcapi.HeapCCollection):
             a.foo = 42
         self.assertIs(weakref.ref(a)(), a)
 
-        with self.assertRaises(TypeError):
-            class X(_testcapi.HeapCCollection):
-                __slots__ = ['x']
-
-        with self.assertRaises(TypeError):
-            class X(_testcapi.HeapCCollection):
-                __slots__ = ['__dict__', 'x']
-
     @support.subTests(('base', 'arg'), [
         (tuple, (1, 2, 3)),
         (int, 9876543210**2),
@@ -1434,8 +1452,7 @@ class W(base):
         self.assertIs(weakref.ref(a)(), a)
         self.assertEqual(a, base(arg))
 
-    @support.subTests('base', [int, bytes] +
-                      ([_testcapi.HeapCCollection] if _testcapi else []))
+    @support.subTests('base', [int, bytes])
     def test_unsupported_slots(self, base):
         with self.assertRaises(TypeError):
             class X(base):

diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2025-11-16-21-14-48.gh-issue-103740.rXIj5h.rst b/Misc/NEWS.d/next/Core_and_Builtins/2025-11-16-21-14-48.gh-issue-103740.rXIj5h.rst
@@ -0,0 +1,5 @@
+Allowed defining the *__dict__* and *__weakref__* :ref:`__slots__ <slots>`
+for any class. Allowed defining any *__slots__* for a class derived from
+:class:`type` or other :c:member:`"variable-length" built-in type
+<PyTypeObject.tp_itemsize>` with the :c:macro:`Py_TPFLAGS_ITEMS_AT_END`
+flag.
@@ -4644,7 +4644,7 @@ type_new_descriptors(const type_new_ctx *ctx, PyTypeObject *type, PyObject *dict
         Py_ssize_t nslot = PyTuple_GET_SIZE(et->ht_slots);
         int after_items = (ctx->base->tp_itemsize != 0 &&
                            !(ctx->base->tp_flags & Py_TPFLAGS_ITEMS_AT_END));
-        if (ctx->base->tp_itemsize != 0 &&
+        if (after_items &&
             !(ctx->base->tp_flags & Py_TPFLAGS_TUPLE_SUBCLASS))
         {
             PyErr_Format(PyExc_TypeError,