class UStr(str):
    pass


class A:
    foo: str


class B(A):
    foo: UStr

This should and does not produce an error. Specifically, B is an invalid subclass of A. See: python/typing#1458 (comment) and python/mypy#3208 for context.

The unsafe nature shows up if B has a reliance on the subclass, but a function taking A modifies foo as only a string. This should be handled by users with a generic instead, and this should be invariant as lists are for the same reasons.