pythongh-123935: Fix typo in _get_slots in dataclasses.py

Author: sobolevn

Lib/dataclasses.py

@@ -1208,7 +1208,7 @@ def _get_slots(cls):
             slots = []
             if getattr(cls, '__weakrefoffset__', -1) != 0:
                 slots.append('__weakref__')
-            if getattr(cls, '__dictrefoffset__', -1) != 0:
+            if getattr(cls, '__dictoffset__', -1) != 0:
                 slots.append('__dict__')
             yield from slots
         case str(slot):

Lib/test/test_dataclasses/__init__.py

@@ -3664,6 +3664,23 @@ class A(WithDictSlot): ...
         self.assertEqual(A().__dict__, {})
         A()
 
+    @support.cpython_only
+    def test_dataclass_slot_dict_ctype(self):
+        # https://github.com/python/cpython/issues/123935
+        from test.support import import_helper
+        # Skips test if `_testcapi` is not present:
+        _testcapi = import_helper.import_module('_testcapi')
+
+        @dataclass(slots=True)
+        class A(_testcapi.HeapCTypeWithDict):
+            __dict__: dict = {}
+        self.assertEqual(A.__slots__, ())
+
+        @dataclass(slots=True)
+        class A(_testcapi.HeapCTypeWithWeakref):
+            __dict__: dict = {}
+        self.assertEqual(A.__slots__, ('__dict__',))
+
     @support.cpython_only
     def test_slots_with_wrong_init_subclass(self):
         # TODO: This test is for a kinda-buggy behavior.