[GHC] #15517: -O0 and pattern synonyms triggers panic in trimJoinCont

[GHC]

#15517: -O0 and pattern synonyms triggers panic in trimJoinCont
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        Reporter:  sjakobi           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.4.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"ce6ce788251b6102f5c1b878ffec53ba7ad678b5/ghc" ce6ce78/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="ce6ce788251b6102f5c1b878ffec53ba7ad678b5"
 Set strictness correctly for JoinIds

 We were failing to keep correct strictness info when eta-expanding
 join points; Trac #15517.   The situation was something like

   \q v eta ->
      let j x = error "blah
          -- STR Lx   bottoming!
      in case y of
            A -> j x eta
            B -> blah
            C -> j x eta

 So we spot j as a join point and eta-expand it.  But we must
 also adjust the stricness info, else it vlaimes to bottom after
 one arg is applied but now it has become two.

 I fixed this in two places:

  - In CoreOpt.joinPointBinding_maybe, adjust strictness info

  - In SimplUtils.tryEtaExpandRhs, return consistent values
    for arity and bottom-ness
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15517#comment:6>
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