[GHC] #11091: Fix MonadFail warnings

[GHC]

#11091: Fix MonadFail warnings
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           Reporter:  quchen         |             Owner:
               Type:  task           |            Status:  new
           Priority:  low            |         Milestone:  8.2.1
          Component:  Compiler       |           Version:  7.11
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 #10751 implements the MonadFail warning mechanism.

 Using `-fwarn-missing-monadfail-instance`, GHC's internal warnings created
 by the patch can be shown and fixed.

 A second pass with `-XMonadFailDesugaring` then provides a hard check on
 whether any issues remain, by switching on the new desugaring mechanism
 properly.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11091>
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