fix: prevent override when there exists a setter

Author: shenyih0ng

mypy/checker.py

@@ -2285,10 +2285,11 @@ def check_method_override_for_base_with_name(
                     isinstance(original_node, Var)
                     and original_node.is_classvar
                     and defn.name == original_node.name
-                    and isinstance(defn, Decorator)
+                    and (isinstance(defn, (Decorator, OverloadedFuncDef)))
                 ):
                     self.fail(
-                        message_registry.CANNOT_OVERRIDE_CLASS_VAR.format(base.name), defn.func
+                        message_registry.CANNOT_OVERRIDE_CLASS_VAR.format(base.name),
+                        defn.func if isinstance(defn, Decorator) else defn.items[0].func,
                     )
 
                 if (

test-data/unit/check-classvar.test

@@ -277,10 +277,12 @@ class A:
 class B(A):
     @property
     def x(self) -> int: ...
+
+    @x.setter
+    def x(self, value: int) -> None: ...
 [builtins fixtures/property.pyi]
 [out]
 main:7: error: Cannot override class variable (previously declared on base class "A") with instance variable
-main:7: error: Cannot override writeable attribute with read-only property
 
 [case testAcrossModules]
 import m