Question: Are there uses for Deep other than compatibility with non-GHC style generics? In other words, if I am writing a new library, is it an option I should look at?

@matthewleon That is the main use case, and I'm not sure if it's worth it to be honest. I don't think there is anything you can do with generics that you can't do with generics-rep (this PR is sort of a proof of that 😄)

I actually do have one use case which would work with old-style generics, but not with new-style generics, which is to reflect a generic signature as an actual type with a Generic instance.

So this PR is a proof that every type with a generics-rep instance also has an old Generic instance, but not the other way round, and when we're creating types at runtime, that's what we care about.

Should this PR be closed?

I think this hasn’t had enough interest to justify inclusion in core, yeah.