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{-# Language DataKinds, TypeInType, TypeFamilies, UndecidableInstances #-}
module T15412 where
import Data.Kind
newtype I a = I a
type C = Constraint
type family
UnitC :: C where
UnitC = ()
instance UnitC => Functor I where
-- The UnitC type family in the context needs UndecidableIntances
fmap = undefined
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