[GHC] #9046: Panic in GHCi when using :print

[GHC]

#9046: Panic in GHCi when using :print
-------------------------------------+------------------------------------
        Reporter:  quchen            |            Owner:
            Type:  bug               |           Status:  new
        Priority:  normal            |        Milestone:  7.8.3
       Component:  GHCi              |          Version:  7.8.2
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  GHCi crash        |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by chris.parks):

 I'm seeing the same thing on x86_64-apple-darwin in GHC HEAD:
 {{{
 > :t _t1
 ghc-stage2: panic! (the 'impossible' happened)
   (GHC version 7.9.20140520 for x86_64-apple-darwin):
         ASSERT failed! file compiler/typecheck/TcType.lhs line 640 t
 }}}

 That ASSERT is in `isMetaTyVar`:
 {{{
 isMetaTyVar tv
    = ASSERT2( isTcTyVar tv, ppr tv )
        case tcTyVarDetails tv of
          MetaTv {} -> True
          _         -> False
 }}}
 We're expecting that `tv` was constructed by the `TcTyVar` constructor of
 the `Var` datatype. However, what little printf-debugging I've been able
 to do indicates that we're getting a plain `TyVar` instead. Maybe someone
 with more debugging experience can shed some light on this?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9046#comment:1>
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