Manu Sridharan

This is outside of JSpecify, but looks like Checker Framework supports this with an (unchecked) [`Covariant` annotation](https://checkerframework.org/api/org/checkerframework/framework/qual/Covariant.html) ([use in Supplier](https://github.com/typetools/jdk/blob/ecc60338caa93c8713d4ae35ae8c858b23ce4f1f/src/java.base/share/classes/java/util/function/Supplier.java#L45)). We could consider supporting something similar in the future.

Thanks so much @jeffrey-easyesi for looking into these and for the report! We'll try to dig deeper as soon as we can.

Also, if you happen to have cycles to open a PR on any of these that would be welcome. But understood if you can't, we still appreciate the report.

@jeffrey-easyesi did you make both `Iterable` and `Iterator` null-marked via a library model to reproduce those cases?

I'm just trying to figure out the best way, for the moment, to write a regression test for some of these cases involving `Iterable`s. At some point (hopefully soon-ish) we...

@chemicL how often does this one come up in your codebase? It's a valid issue, but an annoying one to fix. Under the hood, `javac` generates a synthetic subclass of...

Thanks, @jeffrey-easyesi, super useful!

@chemicL my guess is the JLS design decision is deliberate and that they are unlikely to revise it. If someone has time maybe they can search around for the discussion...

Hi @Marinell0 thanks for the report. At least some of these issues are due to missing JDK models (#950), which we are actively working on and hope to have some...

This PR (which needs some work still) will address a good deal of #950. But, it's also going to potentially cause _a lot_ more errors to be reported in existing...