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__eq__ methods disable narrowing in the else block #17229
Description
from typing import Literal, override
class Foo:
@override
def __eq__(self, value: object, /) -> bool:
return True
def fun(a: Foo | Literal["a"]) -> None:
if a == "a":
reveal_type(a) # Foo | Literal['a']
else:
reveal_type(a) # Foo | Literal['a']in this case, it's safe to narrow to Foo in the else block, because the Literal["a"] cannot have a custom __eq__. this means if a == "a" returns False, there's no chance that a is a Literal["a"], but if it returns True then it's still possible for it to be a Foo because of the Foo.__eq__.
therefore, the narrowing should work like this:
def fun(a: Foo | Literal["a"]) -> None:
if a == "a":
reveal_type(a) # Foo | Literal['a']
else:
reveal_type(a) # Foothis would also be consistent with pyright's behavior: playground
Activity
Better narrowing with custom equality (python#20643)