fixup! Support overriding dunder attributes in Enum subclass

Author: flaeppe

mypy/checker.py

@@ -1898,8 +1898,10 @@ def check_enum(self, defn: ClassDef) -> None:
             for sym in defn.info.names.values():
                 if (isinstance(sym.node, Var) and sym.node.has_explicit_value and
                         sym.node.name == '__members__'):
+                    # `__members__` will always be overwritten by `Enum` and is considered
+                    # read-only so we disallow assigning a value to it
                     self.fail(
-                        'Cannot declare read-only attribute "__members__"', sym.node
+                        message_registry.CANNOT_WRITE_ENUM_READONLY_MEMBERS, sym.node
                     )
         for base in defn.info.mro[1:-1]:  # we don't need self and `object`
             if base.is_enum and base.fullname not in ENUM_BASES:

mypy/message_registry.py

@@ -204,6 +204,11 @@ def format(self, *args: object, **kwargs: object) -> "ErrorMessage":
 )
 CANNOT_MAKE_DELETABLE_FINAL: Final = ErrorMessage("Deletable attribute cannot be final")
 
+# Enum
+CANNOT_WRITE_ENUM_READONLY_MEMBERS: Final = ErrorMessage(
+    'Cannot declare read-only attribute "__members__"'
+)
+
 # ClassVar
 CANNOT_OVERRIDE_INSTANCE_VAR: Final = ErrorMessage(
     'Cannot override instance variable (previously declared on base class "{}") with class '

test-data/unit/check-enum.test

@@ -1891,7 +1891,7 @@ def f2(c: C, a: Any) -> None:
 
 [case testCanOverrideDunderAttributes]
 import typing
-from enum import Enum
+from enum import Enum, Flag
 
 class BaseEnum(Enum):
     __dunder__ = 1
@@ -1902,7 +1902,20 @@ class Override(BaseEnum):
     __labels__ = {1: "1"}
 
 Override.__dunder__ = 3
+BaseEnum.__dunder__ = 3
 Override.__labels__ = {2: "2"}
+
+class FlagBase(Flag):
+    __dunder__ = 1
+    __labels__: typing.Dict[int, str]
+
+class FlagOverride(FlagBase):
+    __dunder__ = 2
+    __labels = {1: "1"}
+
+FlagOverride.__dunder__ = 3
+FlagBase.__dunder__ = 3
+FlagOverride.__labels__ = {2: "2"}
 [builtins fixtures/dict.pyi]
 
 [case testCanNotInitialize__members__]