[GHC] #15517: haddock triggers panic in trimJoinCont

[GHC]

#15517: haddock triggers panic in trimJoinCont
-------------------------------------+-------------------------------------
           Reporter:  sjakobi        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.4.3
           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:
-------------------------------------+-------------------------------------
 Running haddock on the [https://github.com/VictorCMiraldo/generics-mrsop
 `generics-mrsop`] package results in

 {{{
 haddock: panic! (the 'impossible' happened)
   (GHC version 8.4.3 for x86_64-unknown-linux):
         completeCall

 fail_av9P
 Select nodup wild_00
 Stop[BoringCtxt] Rep Singl (El FamRoseInt) (Lkup ix CodesRoseInt)
 Call stack:
     CallStack (from HasCallStack):
       callStackDoc, called at compiler/utils/Outputable.hs:1150:37 in
 ghc:Outputable
       pprPanic, called at compiler/simplCore/Simplify.hs:1533:9 in
 ghc:Simplify
 }}}

 The same problem affects 8.6.1 and HEAD, but not 8.2.2.

 The panic seems to stem from
 [https://github.com/ghc/ghc/blob/64c71ce956af3af593a46ef0d615c7f6fe6ecece/compiler/simplCore/Simplify.hs#L1613
 this catch-all in `trimeJoinCont`].

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