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{-# LANGUAGE DataKinds #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE UndecidableInstances #-}
{-# OPTIONS_GHC -Wredundant-constraints -Wall -Werror #-}
import GHC.TypeLits (TypeError, ErrorMessage(..))
class C a where f :: a -> a
instance {-# OVERLAPPING #-} C Int where f _ = 42
instance {-# OVERLAPPABLE #-} TypeError ( 'Text "Only Int is supported" ) => C a where f = undefined
main :: IO ()
main = print $ f (42::Int)
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