Better error message for assignments incompatible due to invariance

[silviogutierrez]

There seems to be a problem with enums when inferred and used inside lists. This happens with both mypy 0.540 and mypy 0.520.

Code:

class Magic(Enum):
    ISSUER = 1
    ISSUER_EXCHANGE = 2
    ASSIGNEE = 3
    ALL = 3

boo = [c for c in Magic if c not in [Magic.ISSUER]]
boo_explicit: List[Enum] = [c for c in Magic if c not in [Magic.ISSUER]]
spam1: Dict[str, List[Enum]] = {'abc': boo} # Fails with: Dict entry 0 has incompatible type "str": "List[Magic]"; expected "str": "List[Enum]"
spam2: Dict[str, List[Enum]] = {'abc': boo_explicit} # works when typed explicitly
spam3: Dict[str, List[Enum]] = {'abc': [Magic.ISSUER]} # works when done inline
spam4: Dict[str, List[Enum]] = {'abc': [c for c in Magic if c not in [Magic.ISSUER]]} # works when done inline, even with a list comp.
spam5: Dict[str, Enum] = {'abc': Magic.ISSUER} # works Works for single enums.

Let me know what other info you need, or any other steps I should take.

Thanks for all the work on such an awesome project.

[JelleZijlstra]

This is not related to enums and I don't think it's a bug; it's because of invariance. Because the List type is invariant, List[B] is not allowed for List[A] when B is a subclass of A. As your examples, show mypy uses special logic to infer List[A] instead of List[B] in some cases.

[ilevkivskyi]

This is not a bug, List is invariant. It is however strange that you didn't get a note explaining this. Also the bug is that you don't get an error on the comprehension line, see #3210

I am keeping this open so that we can give a better error message, for some reasons it does not work with enums.

[ilevkivskyi]

OK, the problem is that we give the better error message only for incompatible arguments, not in assignments. This is therefore easy to fix.

[lipemorais]

I taking a look at this.