isinstance(instance, SubClass) loses type instead of narrowing

[jeremyBanks]

Given an instance of a SuperClass, I would expect isinstance(instance, SubClass) to narrow the type of instance to SubClass in the following branch. Instead, the type appears to have been lost and replaced with NoneType.

class SuperClass:
    def __init__(self) -> None:
        self.property: bool = True

class SubClass(SuperClass):
    pass

def function(instance: SuperClass) -> None:
    if isinstance(instance, SubClass):
        print(instance.property)

$ mypy --strict example.py && echo "no errors"
no errors

$ pyright example.py
Finding source files
Found 1 source files
Analyzed 1 file in 1.229sec
/Users/jbanks/Desktop/example.py
  'property' is not a known member of 'None' (10, 24)
1 error, 0 warnings

For a slightly smaller example, we get the same error narrowing from the base object type:

class MyClass:
    def __init__(self) -> None:
        self.property: bool = True


def function(instance: object) -> None:
    if isinstance(instance, MyClass):
        print(instance.property)

[erictraut]

This is fixed in 1.0.4, which I just published. Thanks for the bug report.