Optional generic not accepted with --strict-optional

[FuegoFro]

When running with --strict-optional, mypy complains about the last statement here. However, this seems like it should work, both because it does work when --strict-optional is left out, and because the revealed type looks like it should match.

from typing import Optional, Type, TypeVar


class BaseThing: pass
class ChildThing(BaseThing): pass


ThingClass = TypeVar("ThingClass", bound=BaseThing)


def get_thing(thing_class: Type[ThingClass]) -> Optional[ThingClass]:
    """ Get the thing with the given class. Stub implementation. """
    return None

thing = None  # type: Optional[ChildThing]

# Fails, complaining about `Type argument 1 of "get_thing" has incompatible value "Optional[ChildThing]"`
thing = get_thing(ChildThing)

# Revealed type is `Union[scratch_1.ChildThing*, builtins.None]`, which seems like it should 
# match Optional[ChildThing].
reveal_type(get_thing(ChildThing))

[gvanrossum]

Hm, I wonder if the problem is due to a bad interaction between the ThingClass TypeVar and the Optional from the context? I think it ought to infer that if the context is Optional[ChildThing] and the return type (to be matched to that context) is Optional[ThingClass], then it must solve this to ThingClass==ChildThing, but it is then not correctly concluding that the argument type must be Type[ChildThing].

[ddfisher]

This problem isn't Optional-specific -- it'll happen with any Union. For example:

from typing import *

class A: pass
class B(A): pass


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

def f(x: Type[T]) -> Union[T, str]:
    return "foo"

x = None  # type: Union[B, str]

reveal_type(x)
reveal_type(f(B))
x = f(B)

test.py:14: error: Revealed type is 'Union[test.B, builtins.str]'
test.py:15: error: Revealed type is 'Union[test.B*, builtins.str]'
test.py:16: error: Type argument 1 of "f" has incompatible value "Union[B, str]"

We've known for a while that TypeVars and Unions don't play well together. There's been a bit of spot-patching so far, but I think we're at the point where a larger careful rethinking makes sense.