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Check class type parameter variance violations in inheritance #736
Description
In generic inheritance, we can't change the variance of a type from invariant to covariant/contravariant. This should probably be rejected:
IV = TypeVar('IV')
CV = TypeVar('CV', covariant=True)
class A(Generic[IV]): ... # Bunch of methods
class B(A[CV], Generic[CV]): pass # Error, type variable should be invariant
Going from covariant to invariant is fine (for example, List has to do this since it subclasses Sequence, which is covariant).
Activity
jhance commentedon Oct 10, 2015
Seems like we break this in our own tests but fixing the
lib-stubto makeSequencecovariant onTshould fix that.jhance commentedon Oct 10, 2015
I have this done on a branch, will submit PR later when my other stuff related to variance is in (since fixing this depends on fixing variance in general)
jhance commentedon Oct 11, 2015
It just struck me that we should reject things like
List[Sequence[T]]whereTis covariant. This is going to be fun...JukkaL commentedon Oct 12, 2015
Yeah,
List[Sequence[T]]would also be invalid.I'm sure there's a research paper that goes through all the special cases. Some of these might be relevant, in case you want to dig deeper (warning greek alert):
http://research.microsoft.com/en-us/um/people/akenn/generics/
Fix unsound variance (#13714)