Test coverage for #13131 wasn't as good as I thought it was, because
although we infer a lot of types in stubs in typeshed, we don't check
typeshed, and therefore we don't do scope-level inference and pull all
types for a scope. So we didn't really have good test coverage for
scope-level inference in a stub. And because of this, I got the code for
supporting that wrong, meaning that if we did scope-level inference with
deferred types, we'd end up never populating the deferred types in the
scope's `TypeInference`, which causes panics like #13160.
Here I both add test coverage by running the corpus tests both as `.py`
and as `.pyi` (which reveals the panic), and I fix the code to support
deferred types in scope inference.
This also revealed a problem with deferred types in generic functions,
which effectively span two scopes. That problem will require a bit more
thought, and I don't want to block this PR on it, so for now I just
don't defer annotations on generic functions.
Fixes#13160.
29c36a56b2