[GHC] #12590: GHC panics, -XTypeInType, possibly "cobox" related

[GHC]

#12590: GHC panics, -XTypeInType, possibly "cobox" related
-------------------------------------+-------------------------------------
           Reporter:  heisenbug      |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 I get
 {{{
 GHCi, version 8.1.20160902: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Tree             ( src/Tree.hs, interpreted )
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.1.20160902 for x86_64-apple-darwin):
         tcTyVarDetails
   cobox_a15m :: (n_a159[ssk] :: Peano) ~# ('Z :: Peano)
 }}}
 on the attached file.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12590>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler