Incorrect handling of unions of generics

[hauntsaninja]

This was originally reported in #3644 (comment)

from typing import Callable, TypeVar, Generic, Union

_U = TypeVar('_U')
_T = TypeVar('_T')

class Future(Generic[_T]):
    
    def __init__(self, t: _T):
        self._t = t

    def then(self, fn: Callable[[_T], Future[_U]]) -> Future[_U]:
        return fn(self._t)
    
    def then2(self, fn: Callable[[_T], _U]) -> Future[_U]:
        return Future(fn(self._t))
        
    def then3(self, fn: Union[Callable[[_T], Future[_U]], Callable[[_T], _U]]) -> Future[_U]:
        ret = fn(self._t)
        if isinstance(ret, Future):
            return ret
        else:
            return Future(ret)

def foo(t: int) -> Future[str]:
    return Future("str")

fut = Future(5)

reveal_type(fut.then(foo)) # is a Future[str], good

def foo2(t: int) -> str:
    return "str"
    
reveal_type(fut.then2(foo2)) # also is a Future[str], good

reveal_type(fut.then3(foo)) # <<==== error, Future[<nothing>]
reveal_type(fut.then3(foo2)) 

[eltoder]

I have a simpler example of the same issue: https://mypy-play.net/?mypy=latest&python=3.11&flags=strict&gist=d1a9097549000c37b75cd1bbba495b8a

from typing import Generic, TypeVar

T = TypeVar("T")

class C(Generic[T]):
    value: T

def unwrap(x: T | C[T]) -> T:
    return x.value if isinstance(x, C) else x

def test(x: C[int]) -> int:
    return unwrap(x)

produces

main.py:12: error: Argument 1 to "unwrap" has incompatible type "C[int]"; expected "C[<nothing>]"  [arg-type]