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{-# LANGUAGE KindSignatures,TypeFamilies,FlexibleInstances #-}
{-# LANGUAGE CPP #-}
 
import Prelude
 
data Poo
data HooBar
data Bar
type family Hoo b :: *
type instance Hoo Bar = HooBar
 
class Pooable a where
  poo :: a -> Poo
 
f :: (Hoo b ~ HooBar) => b
f = undefined
 
instance {-# OVERLAPPABLE #-} (b ~ Bar) => Pooable b where
  poo = undefined
 
#if 1
  -- 0 にすると問題なく通る.
instance {-# OVERLAPPING #-} Pooable () where
  poo = undefined
#endif
 
x = poo f -- (f :: Bar) -- 型注釈入れると勿論通る.
main = print "ok"

[1 of 1] Compiling Main             ( prog.hs, prog.o )

prog.hs:27:9: error:
    • Couldn't match type ‘Hoo a0’ with ‘HooBar’
        arising from a use of ‘f’
      The type variable ‘a0’ is ambiguous
    • In the first argument of ‘poo’, namely ‘f’
      In the expression: poo f
      In an equation for ‘x’: x = poo f

Standard output is empty