[GHC] #13125: GHC Panics when encoding Russel's paradox using GADTs

[GHC]

#13125: GHC Panics when encoding Russel's paradox using GADTs
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        Reporter:  tysonzero         |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:  panic, GADTs
Operating System:  MacOS X           |         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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Description changed by tysonzero:

@@ -37,1 +37,1 @@
- When I instead load it into GHCi and type "absurd `seq` ()" I get:
+ When I instead load it into GHCi and type "{{{absurd `seq` ()}}}" I get:

New description:

 The following encoding of Russel's paradox using GADTs causes GHC to
 panic, which is understandable as it involves creating a value of an
 uninhabited data type without using any recursion, which should not be
 possible as far as I can tell, so this should probably not type check.

 {{{#!hs
 {-# LANGUAGE GADTs #-}

 data False

 data R a where
     MkR :: (c (c ()) -> False) -> R (c ())

 condFalse :: R (R ()) -> False
 condFalse x@(MkR f) = f x

 absurd :: False
 absurd = condFalse (MkR condFalse)

 main :: IO ()
 main = absurd `seq` print ()
 }}}

 When compiled with optimization I get:

 {{{
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.0.1 for x86_64-apple-darwin):
         Simplifier ticks exhausted
   When trying UnfoldingDone condFalse
   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: 6441
 }}}

 When I instead load it into GHCi and type "{{{absurd `seq` ()}}}" I get:

 {{{
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.0.1 for x86_64-apple-darwin):
         atomPrimRep case absurd of _ [Occ=Dead] { }
 }}}

--

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13125#comment:1>
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