Assignment to an instance backed by a descriptor doesn't discard optional #5851
Description
Universal example:
from typing import Type, TypeVar, Optional
T = TypeVar('T')
class IntDescr:
def __get__(self, obj: T, typ: Type[T]) -> Optional[int]: ...
def __set__(self, obj: T, value: Optional[int]) -> None: ...
class C:
reg = 0
spec = IntDescr()
def meth_reg(self) -> int:
if self.reg is None:
self.reg = 0
return self.reg # OK
def meth_spec(self) -> int:
if self.spec is None:
self.spec = 0
# assert self.spec is not None
return self.spec # Error: Incompatible return value type (got "Optional[int]", expected "int")If I uncomment the assert on the line before the error, the error goes away. I suspect that something is missing for assignment to descriptors. Or perhaps this is intentional, because we can't be sure that __set__ doesn't just throw the value away? Then the assert would be an unfortunate workaround needed 99% of the time.