[3.12] gh-105866: fix dataclass with slots=True, weakref_slot=True (GH-105870) (GH-116978)

(cherry picked from commit a22d05f04c074dbb4f71e7837f54c0bb693db75d)

Co-authored-by: Aviel Boag <avboag@gmail.com>
Co-authored-by: Kirill Podoprigora <kirill.bast9@mail.ru>
Co-authored-by: Carl Meyer <carl@oddbird.net>

2 files changed, 14 insertions, 1 deletions

diff --git a/Lib/dataclasses.py b/Lib/dataclasses.py
index 3eacba8..12b2dfd 100644
--- a/Lib/dataclasses.py
+++ b/Lib/dataclasses.py
@@ -1168,8 +1168,10 @@ def _dataclass_setstate(self, state):
 
 def _get_slots(cls):
     match cls.__dict__.get('__slots__'):
+        # A class which does not define __slots__ at all is equivalent
+        # to a class defining __slots__ = ('__dict__', '__weakref__')
         case None:
-            return
+            yield from ('__dict__', '__weakref__')
         case str(slot):
             yield slot
         # Slots may be any iterable, but we cannot handle an iterator
diff --git a/Lib/test/test_dataclasses/__init__.py b/Lib/test/test_dataclasses/__init__.py
index 639631c..4e05506 100644
--- a/Lib/test/test_dataclasses/__init__.py
+++ b/Lib/test/test_dataclasses/__init__.py
@@ -3403,6 +3403,17 @@ class TestSlots(unittest.TestCase):
         self.assertIs(a.__weakref__, a_ref)
 
 
+    def test_dataclass_derived_weakref_slot(self):
+        class A:
+            pass
+
+        @dataclass(slots=True, weakref_slot=True)
+        class B(A):
+            pass
+
+        B()
+
+
 class TestDescriptors(unittest.TestCase):
     def test_set_name(self):
         # See bpo-33141.