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{-# LANGUAGE TypeFamilies #-}
module T22331 where
import Data.Coerce
data family Fool a
-- This works
joe :: Coercible (Fool a) (Fool b) => Fool a -> Fool b
joe = coerce
-- This does not
bob :: Coercible (Fool a) (Fool b) => Fool b -> Fool a
bob = coerce
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