I was hacking GHC to see if it's able to derive Generic1 instance for datatypes with kind (Type -> Type) -> Type by omitting some checks. I noticed that the type parameter is substituted by GHC.Types.Any, preventing higher-kinded instances from typechecking. I found a comment implying that this is intentional; in GHC.Tc.Deriv.Generic.hs:977, Alternatively, we could have avoided this problem by expanding all type synonyms on the RHSes of Rep1 instances. But we might blow up the size of these types even further by doing this, so we choose not to do so. It wasn't obvious to me if binding a parameter in the type synonym instance declaration of Rep1 cause problems, because that's what I'd do if I were to define an instance by hand. Is there any offending example? I'm also trying to make such change myself (it doesn't add a type parameter and I'm not sure why) https://gitlab.haskell.org/fumieval/ghc/-/commit/bfe7766af80d90590b6b032d183...