The parser in GHC 6.11 currently accepts this:

thing :: (?dude :: Int) -> Int
thing = undefined

Where the type should really be written with => like

thing :: (?dude :: Int) => Int
thing = undefined

Running hlint on the malformed-but-accepted-by-GHC version crashes haskell-src-exts:

benl@humboldt:~$ cat tmp/Main.hs 
main = undefined

thing :: (?dude :: Int) -> Int
thing = undefined

benl@humboldt:~$ hlint tmp/Main.hs 
hlint: src/Language/Haskell/Exts/ParseUtils.hs:(841,18)-(863,53): Non-exhaustive patterns in case