Extra parens in error message's displayed code

I'm forwarding the bug #499137 from Debian's BTS.

With this example:

import System.Environment

main = do args <- filter (/= "--nofork") getArgs
          return ()

I get this error message:

    Couldn't match expected type `[[Char]]'
           against inferred type `IO [String]'
    In the second argument of `filter', namely `getArgs'
    In a 'do' expression: args <- filter ((/= "--nofork")) getArgs
    In the expression:
        do args <- filter ((/= "--nofork")) getArgs
           return ()

At some turn, the expression gained an extraneous pair of parenthesis.

Trac metadata

Trac field	Value
Version	6.8.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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