Incorrect handling of unions of generics

This was originally reported in https://github.com/python/mypy/issues/3644#issuecomment-825494986

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)) 

Start by running the supplied Python reproducer through mypy and checking the reveal_type results for then3. Trace inference for the Union of callable types, and consider the issue complete when both foo and foo2 are accepted and infer Future[str] rather than producing Future[nothing].