[GHC] #12593: ghci spins forever

[GHC]

#12593: ghci spins forever
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        Reporter:  Iceland_jack      |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by simonpj):

 * cc: goldfire (added)


Comment:

 Yes, I can reproduce this with HEAD. It seems that we are updating a
 unification variable so that it points to itself.  I see
 {{{
 Following filled tyvar
   k_arL[tau:1] = (k_apN[sk] |> Sym
                                  U(hole:{arZ}, *, (((k_arR[tau:1]
                                                      -> k_arU[tau:1] ->
 TYPE t_arG[tau:1])
                                                     -> Constraint)
                                                    -> (k_arL[tau:1]
                                                        -> k_arO[tau:1] ->
 TYPE t_arJ[tau:1])
                                                    -> *)
                                                   -> Constraint)_N)
 }}}
 And that is disaster.  Remarkable example!

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12593#comment:2>
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