Deriving Generic1 for a type containing Either

With the following code:

{-# LANGUAGE DeriveGeneric #-}
import GHC.Generics
data T a = C (Either Int a) deriving Generic1

I get the error message:

Main.hs:3:38:
    Can't make a derived instance of `Generic1 T':
      must not apply type constructors that cannot be represented with `Rep1'
      (such as `Either') to arguments that involve the last type parameter
    In the data declaration for `T'

But when I look at the instances of Generic1, I see the following (summarized):

*Main> :i Generic1
...
instance Generic1 (Either a) -- Defined in `GHC.Generics'
...

So, it seems that Either a can be represented with Rep1. Consequently, the error message is a bit confusing.

Similarly, I can define my own version of Either -- e.g. data U a b = A a | B b deriving Generic1 -- replace Either with U and still get the message. I also get this problem using U as data U a b = U a b deriving Generic1 but not with T as data T a = C (Int, a) deriving Generic1.

Lastly, I try this:

data T a = C (Either Int (T a)) deriving Generic1

but the error is even stranger here:

Main.hs:3:38:
    Can't make a derived instance of `Generic1 T':
      must not apply type constructors that cannot be represented with `Rep1'
      (such as `T') to arguments that involve the last type parameter
    In the data declaration for `T'

What's the general problem with these examples?

• Can the error message be improved?
• Can the deriving mechanism be extended for these types?

I'm assuming I can write my own Generic1 instance, though I haven't actually tried.

Trac metadata

Trac field	Value
Version	7.6.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture