Returning optional union from method complains about `got "object"`

[Garrett-R]

Bug Report

Mypy thinks my method returns "object" which is not right.

To Reproduce

Run Mypy on this file:

from typing import Optional


class Cat:
    pass


class Dog:
    pass


class MyClass:
    def __init__(self) -> None:
        self.pet: Optional[Dog | Cat] = None

    def get_pet(self) -> Optional[Dog | Cat]:
        if input() == 'yes':
            self.pet = Cat()
        else:
            self.pet = Dog()
        return self.pet  # (incorrect Mypy error on this line!)

Expected Behavior

This should have no errors.

Actual Behavior

It yields:

error: Incompatible return value type (got "object", expected "Union[Dog, Cat, None]")

Your Environment

• Mypy version used: v0.950
• Mypy command-line flags: none
• Mypy configuration options: none
• Python version used: 3.10.4
• Operating system and version: Ubuntu 20.04

[erictraut]

Looks like another case where a false positive results from using a join operation rather than a union.

[hauntsaninja]

Note the following works:

    def get_pet(self) -> Dog | Cat | None:
        pet: Dog | Cat | None = None
        if input() == 'yes':
            pet = Cat()
        else:
            pet = Dog()
        return pet

Looks like we're losing the type context of the instance variable.

[sterliakov]

Fixed in #18538 and passes on current master.