:t zがダメだけど、D (X "123") 123 (1::Y)やD (X "123") 123 (X "456")ではデータ構築可能

Test2.hs

{-# LANGUAGE TypeSynonymInstances #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE ExistentialQuantification #-}
{-# LANGUAGE GADTs #-}
module Test where

data X = X String 
       deriving Show
                
type Y = Int

class Z a where
instance Z X
instance Z Y
  
data D = forall a. (Show a, Z a) => D { x :: X
                                      , y :: Y
                                      , z :: a
                                      }
--       deriving Show