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{-# LANGUAGE PatternSynonyms #-}
{-# LANGUAGE ViewPatterns #-}
-- | Derived from T11336. Note that the pattern synonym makes it so that the
-- pattern match checker can't see any complete sets: It only sees @s a@
-- instead of @Proxy a@ for the match in 'PProxy'.
module Bug where
import Data.Proxy
class Prj s where
prj :: Proxy a -> s a
instance Prj Proxy where
prj = id
pattern PProxy :: Prj s => s a -> Proxy a
pattern PProxy s <- (prj -> s)
-- | Although this is technically a complete match, the pattern match checker
-- can't in general look through the pattern synonym. So, it should warn that
-- some pattern wasn't matched. It should still flag the redundant second
-- clause, though!
fun :: Proxy a -> ()
fun (PProxy Proxy) = ()
fun (PProxy Proxy) = ()
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