[GHC] #10288: -flate-dmd-anal triggers "Entered absent arg"

[GHC]

#10288: -flate-dmd-anal triggers "Entered absent arg"
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        Reporter:  yongqli           |                   Owner:
            Type:  bug               |                  Status:  infoneeded
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.10.1
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  Runtime crash     |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:
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Comment (by simonpj):

 Oh VERY interesting!  I have reproduced the problem with 7.10, and it is
 YET ANOTHER bug in the demand analyser.  Here is the offending code
 {{{
 dmdAnal' sigs dmd (App fun (Coercion co))
   = (fun_ty, App fun' (Coercion co))
   where
     (fun_ty, fun') = dmdAnal sigs dmd fun
 }}}
 The incoming demand `dmd` is not modified before passing it on to `fun`.

 But that is wrong wrong wrong.  That gets the demands from the function
 out of step, so the wrong demand goes to the wrong argument. It's a
 miracle that this has not been causing chaos for ages.  Horror.  Coercions
 are ''value'' arguments and so they must be treated as such.

 I think the solution is simply to delete these lines so that coercion
 arguments are handled uniformly; but I want to tread carefully.

 Thank you for extracting this example.  More anon.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10288#comment:5>
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