Type of list items not inferred when using `isinstance` inside `all`

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

No source file or test is named. First reproduce the example with mypy v0.740, then trace how the isinstance check inside all and the list comprehension are analyzed. Done means the example narrows List[A] to List[B] without the reported error and has regression coverage.