Fix slots issue with deferred base (python#20573)

Fixes python#17121

Generated by codex + hand simplification commit

Author: hauntsaninja

mypy/plugins/dataclasses.py

@@ -457,10 +457,9 @@ def add_slots(
             )
             return
 
-        generated_slots = {attr.name for attr in attributes}
-        if (info.slots is not None and info.slots != generated_slots) or info.names.get(
-            "__slots__"
-        ):
+        existing_slots = info.names.get("__slots__")
+        slots_defined_by_plugin = existing_slots is not None and existing_slots.plugin_generated
+        if existing_slots is not None and not slots_defined_by_plugin:
             # This means we have a slots conflict.
             # Class explicitly specifies a different `__slots__` field.
             # And `@dataclass(slots=True)` is used.
@@ -478,14 +477,21 @@ def add_slots(
             # does not have concrete `__slots__` defined. Ignoring.
             return
 
+        generated_slots = {attr.name for attr in attributes}
         info.slots = generated_slots
 
         # Now, insert `.__slots__` attribute to class namespace:
         slots_type = TupleType(
             [self._api.named_type("builtins.str") for _ in generated_slots],
             self._api.named_type("builtins.tuple"),
         )
-        add_attribute_to_class(self._api, self._cls, "__slots__", slots_type)
+        add_attribute_to_class(
+            self._api,
+            self._cls,
+            "__slots__",
+            slots_type,
+            overwrite_existing=slots_defined_by_plugin,
+        )
 
     def reset_init_only_vars(self, info: TypeInfo, attributes: list[DataclassAttribute]) -> None:
         """Remove init-only vars from the class and reset init var declarations."""

test-data/unit/check-dataclasses.test

@@ -1606,6 +1606,20 @@ class PublishedMessages:
     left: int
 [builtins fixtures/dataclasses.pyi]
 
+[case testDataclassSlotsWithDeferredBase]
+# flags: --python-version 3.10
+from dataclasses import dataclass
+
+@dataclass
+class A(B): ...
+
+@dataclass
+class B: ...
+
+@dataclass(slots=True)
+class C: ...
+[builtins fixtures/dataclasses.pyi]
+
 [case testDataclassesAnyInherit]
 from dataclasses import dataclass
 from typing import Any