[GHC] #10180: Lint check: Empty alternatives imply bottoming scrutinee

[GHC]

#10180: Lint check: Empty alternatives imply bottoming scrutinee
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        Reporter:  nomeata           |                   Owner:  nomeata
            Type:  task              |                  Status:  infoneeded
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.11
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:
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Comment (by simonpj):

 Well that's why I said (somewhere) that I wasn't certain this would work,
 but it would be interesting to know.

 The point is: to make the empty case, GHC '''knew''' at some point that
 the expression must diverge.  So how has it lost that knowledge?

 I think that in this case it's a use of `dataConCantMatch`.  So maybe we
 can beef up `isEmptyTy` to use that information?  (And ultimately perhaps
 beef up `exprIsBottom`.)

 My point is, this is a useful quest.

 Simon

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10180#comment:9>
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