[GHC] #5400: GHC loops on compiling with optimizations

[GHC]

#5400: GHC loops on compiling with optimizations
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        Reporter:  noschinl          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  ⊥
       Component:  Compiler          |              Version:  7.0.4
      Resolution:                    |             Keywords:
Operating System:  Linux             |         Architecture:  x86_64
 Type of failure:  Compile-time      |  (amd64)
  crash                              |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by sgraf):

 Note that the reproduction lacks a `{-# LANGUAGE ConstrainedClassMethods
 #-}` to compile with GHC 8.2.1 and then compiles flawlessly with -O2.

 The typical Russel's paradox example from the user's guide just crashes
 with a panic (similar to #5448):

 {{{
 ghc.exe: panic! (the 'impossible' happened)
   (GHC version 8.2.1 for x86_64-unknown-mingw32):
         Simplifier ticks exhausted
   When trying UnfoldingDone russel
   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: 6361
   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'd advocate this to be a warning rather than a panic, but otherwise
 consider the whole issue fixed.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/5400#comment:3>
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