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Description
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))