mypy fails to narrow a type when isinstance() is wrapped inside a function

[kprzybyla]

I stumbled upon this couple of times so I guess it's fairly reasonable to report this. Currently mypy understand isinstance() and will properly resolve types when isinstance() is used inside if statement:

from typing import Union

Complex = Union[int, str]

x: Complex

if isinstance(x, int):
    reveal_type(x)

example.py:8: note: Revealed type is 'builtins.int'

However, if you move isinstance() call to function that basically wraps it, mypy fails to narrow a type of variable.

from typing import Union

Complex = Union[int, str]

x: Complex

def is_integer(value: Complex) -> bool:
    return isinstance(value, int)

if is_integer(x):
    reveal_type(x)

example.py:11: note: Revealed type is 'Union[builtins.int, builtins.str]'

This is somewhat annoying if you have a complex isinstance() call that you don't want to copy in your code over and over again but simply hide it under a descriptive function name.

This might be somehow related to #4573.

Versions

mypy 0.761
Python 3.7.4

[ethanhs]

Duplicate of #5206