[GHC] #3872: New way to make the simplifier diverge

[GHC]

#3872: New way to make the simplifier diverge
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        Reporter:  simonpj           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  ⊥
       Component:  Compiler          |              Version:  6.12.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #5400 #5448       |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by sgraf):

 * related:   => #5400 #5448


Comment:

 This doesn't loop infinitely anymore with GHC 8.2.1 and instead panics:

 {{{
 ghc.exe: panic! (the 'impossible' happened)
   (GHC version 8.2.1 for x86_64-unknown-mingw32):
         Simplifier ticks exhausted
   When trying UnfoldingDone rir_s1bm
   To increase the limit, use -fsimpl-tick-factor=N (default 100)
   If you need to do this, let GHC HQ know, and what factor you needed
   To see detailed counts use -ddump-simpl-stats
   Total ticks: 11721
   Call stack:
       CallStack (from HasCallStack):
         prettyCurrentCallStack, called at
 compiler\utils\Outputable.hs:1133:58 in ghc:Outputable
         callStackDoc, called at compiler\utils\Outputable.hs:1137:37 in
 ghc:Outputable
         pprPanic, called at compiler\simplCore\SimplMonad.hs:199:31 in
 ghc:SimplMonad
 }}}

 I think this should result in a warning rather than in a crash.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/3872#comment:3>
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