[GHC] #14773: MultiWayIf makes it easy to write partial programs that are not catched by -Wall

[GHC]

#14773: MultiWayIf makes it easy to write partial programs that are not catched by
-Wall
-------------------------------------+-------------------------------------
        Reporter:  SimonHengel       |                Owner:  sighingnow
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.2
      Resolution:                    |             Keywords:
                                     |  PatternMatchWarnings
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
                                     |  T14773a,T14773b
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D4400
       Wiki Page:                    |
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Comment (by simonpj):

 Why do you need `vanillaLPat`?  Why not just have an empty list of
 patterns?

 Also
 {{{
 b :: Bool
 Just b | False = Nothing
 }}}
 No that is not ignored by design.  The thing that is ignored is this:
 {{{
   [ e | False <- blah, blah blah ]
 and
   do { False <- blah; ... }
 }}}
 I hope that `StmtCxt` is true of the latter two but not of the former.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14773#comment:11>
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