[GHC] #12991: panic when using IrredPreds in a type checker plugin.

[GHC]

#12991: panic when using IrredPreds in a type checker plugin.
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        Reporter:  owen              |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      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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Comment (by simonpj):

 Works ok in HEAD and the 8.0 branch, so maybe it's fixed.
 {{{
 ~/5builds/ghc-8.0-branch/inplace/bin/ghc-stage2 -dynamic --make -dcore-
 lint -package ghc *hs
 [1 of 3] Compiling Test.Plugin      ( Plugin.hs, Plugin.o )
 [2 of 3] Compiling Test.Compare     ( Compare.hs, Compare.o )
 [3 of 3] Compiling Test.Fails       ( Fails.hs, Fails.o )
 irred compare
   [W] irred_aaYl :: l_aaVc[tau:3] ≽ l'_aaVd[tau:3] (CNonCanonical)
 irred compare
   [W] irred_aaYm :: l'_aaVd[tau:3] ≽ l_aaVc[tau:3] (CNonCanonical)
 simonpj at cam-05-unx:~/tmp/ghc-plugin-test/Test$
 }}}
 I had to change `import Data.Constraint` to `import GHC.Exts`.  Where is
 the "right" place to import `Constraint` from?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12991#comment:1>
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