names.hs

{-# LANGUAGE DuplicateRecordFields #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE StrictData #-}

{-

Something very similar to this works on 8.2.2, I have it in a
(non-free) codebase, but can’t quite minimize it… Hmm.

It stopped working when I updated that codebase’s compiler to 8.4.3.

-}

module Main where

data AB = A | B

class SomeClass (ab :: AB) where
  data SomeData ab

instance SomeClass 'A where
  data SomeData 'A = SomeData'A{someField :: Int} deriving Show

instance SomeClass 'B where
  data SomeData 'B = SomeData'B{someField :: Int}

main :: IO ()
main = print SomeData'A{someField = 5}