[GHC] #9569: Tuple constraints don't work right

[GHC]

#9569: Tuple constraints don't work right
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              Reporter:  simonpj     |            Owner:
                  Type:  bug         |           Status:  closed
              Priority:  normal      |        Milestone:
             Component:  Compiler    |          Version:  7.8.2
            Resolution:  fixed       |         Keywords:
      Operating System:              |     Architecture:  Unknown/Multiple
  Unknown/Multiple                   |       Difficulty:  Unknown
       Type of failure:              |       Blocked By:
  None/Unknown                       |  Related Tickets:
             Test Case:              |
  polykinds/T9569,                   |
  typecheck/should_compile/T9569a    |
              Blocking:              |
Differential Revisions:              |
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Comment (by basvandijk):

 Thanks for the advice. I'm now using:

 {{{
 mask :: MonadBaseControl IO m => ((forall a. m a -> m a) -> m b) -> m b
 mask f = control $ \runInBase ->
            E.mask $ \g -> runInBase $ f $ liftBaseOp_ g
 }}}

 without using `ImpredicativeTypes`.

 @luite I will make a point release of `lifted-base` with this fix.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9569#comment:15>
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