TypeVar not narrowed when using isinstance or issubclass on final classes

[DetachHead]

from typing import TypeVar, final

@final
class A:
    def __init__(self,value: str) -> None: ...

@final
class B: 
    def __init__(self,value: int) -> None: ...

T = TypeVar("T", A, B)

def foo(cls: type[T]) -> T:
    if issubclass(cls, A):
        return cls("") # error: Argument 1 to <subclass of "B" and "A"> has incompatible type "str"; expected "int
        # return cls(1) # error: Argument 1 to "A" has incompatible type "int"; expected "str
        # return A("") # error: Incompatible return value type (got "A", expected "B")
        # return B("") # error: Incompatible return value type (got "B", expected "A")
    else:
        return cls(1)

https://mypy-play.net/?mypy=master&python=3.10&flags=show-error-codes%2Callow-redefinition%2Cstrict%2Ccheck-untyped-defs%2Cdisallow-any-decorated%2Cdisallow-any-expr%2Cdisallow-any-explicit%2Cdisallow-any-generics%2Cdisallow-any-unimported%2Cdisallow-incomplete-defs%2Cdisallow-subclassing-any%2Cdisallow-untyped-calls%2Cdisallow-untyped-decorators%2Cdisallow-untyped-defs%2Cno-implicit-optional%2Cno-implicit-reexport%2Cstrict-equality%2Cwarn-incomplete-stub%2Cwarn-redundant-casts%2Cwarn-return-any%2Cwarn-unreachable%2Cwarn-unused-configs%2Cwarn-unused-ignores&gist=43b6f21a9c890c7da5e41e2591b321b0

(the error messages are wrong due to #11536)

[KotlinIsland]

Here is the minified version:

from typing import final

@final
class A: ...

class B: ...

a: A
assert isinstance(a, B)
reveal_type(a)  # note: Revealed type is "__main__.<subclass of "A" and "B">"
print("hi") # no unreachable error

[pwuertz]

This issue is probably the same reason for this error here?

import typing

class A: ...

@typing.final
class AA(A): ...

T = typing.TypeVar("T", bound=A)

def test(x: T) -> T:
    if isinstance(x, AA):
        return AA()  # error: Incompatible return value type (got "AA", expected "T")
    raise NotImplementedError()

The expected return type within the isinstance(x, AA) branch is guaranteed to be AA and the typing should be valid, right?

Using T = typing.TypeVar("T", AA, AB, ...) doesn't help either, although T should be constrained to exactly those types even without interpreting the final decorator.

[brianschubert]

Fixed in #16330, released with mypy v1.8.0