[GHC] #15436: Compile-time panic, Prelude.!!: negative index

[GHC]

#15436: Compile-time panic, Prelude.!!: negative index
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        Reporter:  pbrisbin          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.5
      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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Changes (by RyanGlScott):

 * cc: simonpj (added)


Comment:

 This was probably caused by commit
 193664d42dbceadaa1e4689dfa17ff1cf5a405a0 (`Re-engineer caseRules to add
 tagToEnum/dataToTag`), considering that this panic does not occur on GHC
 8.2 or earlier (see also #14680).

 simonpj, do you know what is happening here?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15436#comment:4>
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