Decorated method type-incompatibility with superclass

[joelberkeley]

I'm raising a bug.

It's also posted on SO at https://stackoverflow.com/q/52744713/5986907

I get

$ mypy file.py
python.py:34: error: Signature of "fn" incompatible with supertype "B"

for file.py

from typing import Callable, TypeVar, cast

T = TypeVar('T')


def dec(f: Callable[..., T]) -> T:
    def p(self):
        return getattr(self, '_' + f.__name__)

    return cast(T, property(p))


class X:
    pass


class Y(X):
    pass


class A:
    _fn = Y()


class B(A):
    @dec
    def fn(self) -> X:
        return X()


class C(B):
    @dec
    def fn(self) -> Y:
        return Y()

I'd expect there to be no error as Y is a subclass of X.

I've tried this with mypy 0.630 and Python 3.5.2, 3.6.6 and 3.7.0. Also with mypy git master for python 3.5.2 and 3.7.0

It's possible these are related but I couldn't quite connect them
#220
#4430

[ilevkivskyi]

Hm, it still fails on master, this is because of this code:

            elif is_equivalent(original_type, typ):
                # Assume invariance for a non-callable attribute here. Note
                # that this doesn't affect read-only properties which can have
                # covariant overrides.
                #
                # TODO: Allow covariance for read-only attributes?
                pass
            else:
                # ...give error

I think this is easy to fix by replacing is_equivalent with is_subtype at least for effectively read-only attributes. Anyway mypy currently allows covariant overriding for mutable attributes although this is technically unsafe, see #3208