This code reproduces the bug: https://mypy-play.net/?gist=7c59359096250bb558136c4595fa1a47

I expect that I won't be able to create classes B and C because of incompatible __init_subclass__() arguments, however mypy doesn't find any errors in this. Second case (class C) will raise an exception at import time, but the first (class B) will pass.