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{-# OPTIONS_GHC -Wincomplete-patterns #-}
{-# LANGUAGE PatternSynonyms #-}
{-# LANGUAGE ViewPatterns #-}
{-# LANGUAGE MultiParamTypeClasses #-}
module Bug where
class Member a b where
prj :: b -> Maybe a
pattern P :: Member a b => a -> b
pattern P a <- (prj -> Just a)
{-# COMPLETE P :: Bool #-}
-- | Trying to instantiate P with 0 type arguments doesn't work, it takes 2.
-- This seemingly unrelated fact, only relevant through the COMPLETE set, may
-- not lead the compiler to crash or do shady stuff.
fun :: Bool -> ()
fun True = ()
fun _ = ()
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