Prohibit use of names in annotations that are undefined in runtime

[pkch]

This type checks, but crashes in runtime with NameError:

class A:
    def f(x: A) -> None:
        pass

I think it's better if mypy reported an error Name "A" should be a forward reference.

[gvanrossum]

Oh, that's weird. I didn't even know mypy accepts that! Agreed it would be better to flag as error. (Outside stubs, and not in type comments of course.)

[emmatyping]

Closing as a duplicate in favor of #948, which has a bit more information.