Thanks for posting an example—that's very helpful to figure out what is going on.

> Currently, GHC rejects the following code:
>
> {-# LANGUAGE DeriveGeneric #-}
> {-# LANGUAGE PolyKinds #-}
> import GHC.Generics
>
> data HKD f = Foo (f Int) (f Double)
>     | Bar (f Bool)
>     deriving Generic1
> The compilation error is
>
> • Can't make a derived instance of ‘Generic1 HKD’:
>     Constructor ‘Foo’ applies a type to an argument involving the last parameter
>                       but the applied type is not of kind * -> *, and
>     Constructor ‘Foo’ applies a type to an argument involving the last parameter
>                       but the applied type is not of kind * -> *, and
>     Constructor ‘Bar’ applies a type to an argument involving the last parameter
>                       but the applied type is not of kind * -> *
> • In the data declaration for ‘HKD’

I see. That error message could be worded better, in my opinion. The issue really isn't so much about the kind of `f`. If you had written `data HKD (f :: Type -> Type) = Baz (Proxy f)`, for instance, I would expect it to work. The real issue is _where_ `f` appears in the data constructors. In `Bar`, for instance, you have:

> Bar (f Bool)

`Generic1` is limited to data types where the last type parameter only appears as the last type argument in any field. This means that a field like `Proxy f` would be fine, as that would be represented as `Rec1 Proxy` in a `Rep1` instance. `f Bool`, on the other hand, is problematic. `GHC.Generics` doesn't have a representation type that simultaneously allows representing this field while also "focusing" on the last type parameter like `Rec1`, `Par1`, etc. would allow.

The `Foo` constructor has similar issues. The error-reporting machinery for `DeriveGeneric` essentially just accumulates every issue it encounters and reports everything at once, which is why there is a duplicate error message involving `Foo`. Needless to say, this kind of error message could be improved.

> Although it is possible to define a hand-rolled instance of Generic1

Really? I'm not sure how you would define a correct `Generic1` instance for `HKD` at all. What did you have in mind?

Best,

Ryan