Tell mypy that all elements of list are of same type

Say we have

from typing import List

a: List[int] | List[str]

if all(isinstance(i, int) for i in a):
    reveal_type(a)

This gives us

main.py:6: note: Revealed type is "Union[builtins.list[builtins.int], builtins.list[builtins.str]]"

Is there a way to get mypy to recognise that a is a list of integers, or is the only solution here to use cast?