[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
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        Reporter:  SimonHengel       |                Owner:  (none)
            Type:  bug               |               Status:  new
        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:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 Sigh. I don't see an easy way to fix this. The issue is that the entry-
 point into the pattern match coverage checker, `checkMatches`, requires a
 list of `LPat`s as its arguments. However, the AST for multi-way-`if`
 (`HsMultiIf`) has `GRHS`es, not `LPat`s. That's the same reason why this
 program doesn't emit any warnings under `-Wall`:

 {{{#!hs
 b :: Bool
 Just b | False = Nothing
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14773#comment:3>
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