Instantiating concrete classes fails check in some cases #6586

#1843/#2853 added support for instantiating variables with an abstract type, under the assumption that it will always be applied to a concrete subclass. However, this doesn't seem to work in all cases:

Given

class Base(ABC):
    @abstractmethod
    def m(self) -> None:
        pass

class A(Base):
    def m(self) -> None:
        pass

class B(Base):
    def m(self) -> None:
        pass

Then

var: Type[Base]
if "foo" == "bar":
    var = A
elif "foo" == "baz":
    var = B
else:
    raise RuntimeError()

var()  # error: Cannot instantiate abstract class 'Base' with abstract attribute 'm'

As far as I understand it the last line shouldn't result in an error.

If I remove class B (and the corresponding elif) I don't get an error. Interestingly, if I remove the whole if block I also don't get an error.

This is with version 0.670.