The type of items in a list is not inferred when using an isinstance check in an all call.

from typing import List

class A:
    pass

class B(A):
    pass

def test(arg: List[A]) -> None:
    if all([isinstance(a, B) for a in arg]):
        test2(arg)

def test2(arg: List[B]) -> None:
    pass

• What is the actual behavior/output?

error: Argument 1 to "test_2" has incompatible type "List[A]"; expected "List[B]"
Found 1 error in 1 file (checked 1 source file)

• What is the behavior/output you expect?

I expect mypy to infer the type of the list items based on the if check.

• What are the versions of mypy and Python you are using?

mypy: v0.740
python: 3.6.7