Overriding final method only raises error if overriding method is annotated

[antonagestam]

In the code below I expected both subclasses to raise errors due to overriding method marked with @final, but only the B class raises an error, it seems like adding the return type annotation triggers the check. The same behavior is not specific to __init__ but happens with any method name.

from typing_extensions import final


class Base:
    @final
    def __init__(self):
        pass


class A(Base):
    def __init__(self):  # no error raised
        pass


class B(Base):
    def __init__(self) -> None:  # raises an error
        pass

setup.cfg:

[mypy]
ignore_missing_imports = True
strict_optional = True
no_implicit_optional = True
check_untyped_defs = True
disallow_incomplete_defs = True
disallow_any_unimported = True
disallow_untyped_decorators = True

Same result with both master mypy 0.710+dev.44172caa122b01af5a63ec5787931b910f272eb6 and latest stable mypy 0.701 using Python 3.7.0.

[ilevkivskyi]

Yes, indeed, this happens with all methods, not just __init__(). I think mypy should give an error even for non-annotated methods.