If I look at the production for strict_mark in the parser

https://github.com/ghc/ghc/blob/master/compiler/parser/Parser.y#L1355

strict_mark :: { Located ([AddAnn],HsBang) }
        : '!'                        { sL1 $1 ([],HsUserBang Nothing      True) }
        | '{-# UNPACK' '#-}'         { sLL $1 $> ([mo $1,mc $2],HsUserBang (Just True)  False) }
        | '{-# NOUNPACK' '#-}'       { sLL $1 $> ([mo $1,mc $2],HsUserBang (Just False) True) } -- ***Correct?
        | '{-# UNPACK' '#-}' '!'     { sLL $1 $> ([mo $1,mc $2],HsUserBang (Just True)  True) }
        | '{-# NOUNPACK' '#-}' '!'   { sLL $1 $> ([mo $1,mc $2],HsUserBang (Just False) True) }