fix: classify __slots__ as class variables

Mark ordinary and plugin-generated __slots__ symbols as class variables so inheritance does not produce spurious instance/class variable override errors.

Add regression coverage for ordinary classes, dataclasses, and attrs classes.

Author: XuehaiPan

mypy/plugins/attrs.py

@@ -960,7 +960,9 @@ def _add_slots(ctx: mypy.plugin.ClassDefContext, attributes: list[Attribute]) ->
         [ctx.api.named_type("builtins.str") for _ in attributes],
         fallback=ctx.api.named_type("builtins.tuple"),
     )
-    add_attribute_to_class(api=ctx.api, cls=ctx.cls, name="__slots__", typ=slots_type)
+    add_attribute_to_class(
+        api=ctx.api, cls=ctx.cls, name="__slots__", typ=slots_type, is_classvar=True
+    )
 
 
 def _add_match_args(ctx: mypy.plugin.ClassDefContext, attributes: list[Attribute]) -> None:

mypy/plugins/dataclasses.py

@@ -475,6 +475,7 @@ def add_slots(self, info: TypeInfo, attributes: list[DataclassAttribute]) -> Non
             self._cls,
             "__slots__",
             slots_type,
+            is_classvar=True,
             overwrite_existing=slots_defined_by_plugin,
         )
 

mypy/semanal.py

@@ -4648,6 +4648,7 @@ def make_name_lvalue_var(
             assert self.type is not None
             v.info = self.type
             v.is_initialized_in_class = True
+            v.is_classvar = name == "__slots__"
             v.allow_incompatible_override = name in ALLOW_INCOMPATIBLE_OVERRIDE
         if kind != LDEF:
             v._fullname = self.qualified_name(name)

test-data/unit/check-dataclasses.test

@@ -1514,6 +1514,18 @@ class Some:
         self.y = 1  # E: Trying to assign name "y" that is not in "__slots__" of type "__main__.Some"
 [builtins fixtures/dataclasses.pyi]
 
+[case testDataclassGeneratedSlotsClassVariableOverride]
+from dataclasses import dataclass
+from typing import ClassVar
+
+@dataclass(slots=True)
+class Base:
+    value: int
+
+class Child(Base):
+    __slots__: ClassVar[tuple[str, ...]] = ()
+[builtins fixtures/tuple.pyi]
+
 [case testDataclassWithSlotsDef]
 from dataclasses import dataclass
 

test-data/unit/check-plugin-attrs.test

@@ -1745,6 +1745,19 @@ class C:
         self.c = 2  # E: Trying to assign name "c" that is not in "__slots__" of type "__main__.C"
 [builtins fixtures/plugin_attrs.pyi]
 
+[case testAttrsGeneratedSlotsClassVariableOverride]
+from typing import ClassVar
+
+import attrs
+
+@attrs.define(slots=True)
+class Base:
+    value: int
+
+class Child(Base):
+    __slots__: ClassVar[tuple[str, ...]] = ()
+[builtins fixtures/plugin_attrs.pyi]
+
 [case testAttrsClassWithSlotsDerivedFromNonSlots]
 import attrs
 

test-data/unit/check-slots.test

@@ -85,6 +85,17 @@ A.b = 1
 [builtins fixtures/tuple.pyi]
 
 
+[case testSlotsClassVariableOverride]
+from typing import ClassVar
+
+class Base:
+    __slots__ = ()
+
+class Child(Base):
+    __slots__: ClassVar[tuple[str, ...]] = ()
+[builtins fixtures/tuple.pyi]
+
+
 [case testSlotsDefinitionMultipleVars1]
 class A:
     __slots__ = __fields__ = ("a", "b")