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{-# LANGUAGE Haskell2010 #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE QuantifiedConstraints #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE InstanceSigs #-}
module T16502 where
import Data.Kind
class (forall c. c ~ T a => Show (c b)) => ShowT a b
instance Show (T a b) => ShowT a b
class (forall b. Show b => ShowT a b) => C a where
type family T a :: Type -> Type
data D a = MkD (T a Int)
instance C a => Show (D a) where
showsPrec p (MkD x)
= (showParen (p > 10) $ showString "MkD " . showsPrec 11 x)
-- This is rejected because we cannot derive (Show (D a)) from (C a)
-- due to troublesome quantified constraints. And then the error
-- message mentions the internal name of the default method for show.
-- Simon thinks that we should accept the fact that this doesn't get
-- accepted, given that a quantified constraint for (Show (c b)) is far
-- too applicable to be useful. So we're left with the suboptimal
-- error message.
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