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{-# LANGUAGE TypeApplications, PatternSynonyms, GADTs, ViewPatterns #-}
module T22328 where
import Data.Typeable
data Gadt x y where
ExistentialInGadt :: Typeable a => a -> Gadt x x
pattern CastGadt :: Typeable a => x ~ y => a -> Gadt x y
pattern CastGadt a <- ExistentialInGadt (cast -> Just a)
test :: Gadt i o -> Bool
test gadt = case gadt of
CastGadt @Bool a -> a
_ -> False
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