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{-# LANGUAGE ViewPatterns #-}
{-# LANGUAGE PatternSynonyms #-}

pattern I :: Int -> Int
pattern I x <- (id -> x)
{-# COMPLETE I #-}

foo :: Int -> Int
foo (I x) = x + 3

bar :: Int -> Int
bar (id->I x) = x + 3

main :: IO ()
main = return ()

Raises a warning for bar:

Bug.hs:12:1: warning: [-Wincomplete-patterns]
    Pattern match(es) are non-exhaustive
    In an equation for 'bar': Patterns not matched: _
   |
12 | bar (id->I x) = x + 3
   | ^^^^^^^^^^^^^^^^^^^^^^^

but not for foo.