Inconsistent narrowing of Union and TypeVar with constraints and neglect of @final

I had a short discussion about this with @JelleZijlstra on Gitter.

When narrowing the constrained type variable, Mypy seems to ignore the final decorator. When removing the final decorator, Mypy's output changes neither for the non-generic function f nor the generic function g. Shouldn't Mypy consistently report an error or not in both cases (regardless of marking the classes as final or not)? If I understand correctly, Mypy allows changing the signature of __init__ when subclassing. So the reports are correct for f when using final and correct for g when not using final, aren't they?

from typing import final, Type, TypeVar, Union

@final
class A:
    def __init__(self, a: int) -> None:
        ...

@final
class B:
    def __init__(self, b: str) -> None:
        ...

U = Union[A, B]
T = TypeVar("T", A, B)

def f(x: Type[U]) -> U:
    if issubclass(x, A):
        reveal_type(x)
        # Revealed type is "A"
        return x(a=1)
    return x(b="a")

def g(x: Type[T]) -> T:
    if issubclass(x, A):
        reveal_type(x)
        # Revealed type is "A*"
        # Revealed type is "<subclass of "B" and "A">"
        return x(a=1)
        # Unexpected keyword argument "a" for <subclass of "B" and "A">
    return x(b="b")

Start by running the issue's Python snippet through mypy and compare the diagnostics and revealed types with and without @final. Trace the handling of constrained TypeVars, Union narrowing, subclass checks, and constructor signatures; done means the behavior is consistent for both f and g when the classes are final or non-final.