Added error for class attribute access with slot

mypy/checkmember.py

@@ -912,6 +912,10 @@ def analyze_class_attribute_access(
         if isinstance(node.node, TypeInfo):
             mx.msg.fail(message_registry.CANNOT_ASSIGN_TO_TYPE, mx.context)
 
+    # Refuse class attribute access if slot defined
+    if info.slots and name in info.slots:
+        mx.msg.fail(message_registry.CLASS_VAR_CONFLICTS_SLOTS.format(name), mx.context)
+
     # If a final attribute was declared on `self` in `__init__`, then it
     # can't be accessed on the class object.
     if node.implicit and isinstance(node.node, Var) and node.node.is_final:

mypy/message_registry.py

@@ -137,6 +137,7 @@ def with_additional_msg(self, info: str) -> ErrorMessage:
 MODULE_LEVEL_GETATTRIBUTE: Final = ErrorMessage(
     "__getattribute__ is not valid at the module level"
 )
+CLASS_VAR_CONFLICTS_SLOTS: Final = '"{}" in __slots__ conflicts with class variable access'
 NAME_NOT_IN_SLOTS: Final = ErrorMessage(
     'Trying to assign name "{}" that is not in "__slots__" of type "{}"'
 )

test-data/unit/check-slots.test

@@ -517,3 +517,13 @@ class A:
         self.b = 2
         self.missing = 3
 [builtins fixtures/tuple.pyi]
+
+[case testSlotsWithClassVar]
+from typing import ClassVar
+class X:
+    __slots__ = ('a',)
+    a: int
+x = X()
+X.a  # E: "a" in __slots__ conflicts with class variable access
+x.a
+[builtins fixtures/tuple.pyi]