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{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE EmptyDataDecls #-}
module T6088 where
class C a
newtype A n = A Int
type family Pos n
data True
instance (Pos n ~ True) => C (A n)
newtype B n = B (A n) deriving (C)
-- This should work, giving
-- instance C (B n)
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