[GHC] #9222: Unexpected DataKind Panic

[GHC]

#9222: Unexpected DataKind Panic
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       Reporter:  ekmett                   |             Owner:
           Type:  bug                      |            Status:  new
       Priority:  normal                   |         Milestone:
      Component:  Compiler (Type checker)  |           Version:  7.8.2
       Keywords:                           |  Operating System:
   Architecture:  Unknown/Multiple         |  Unknown/Multiple
     Difficulty:  Unknown                  |   Type of failure:
     Blocked By:                           |  None/Unknown
Related Tickets:                           |         Test Case:
                                           |          Blocking:
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 Here's a simplified version of a panic I'm getting:

 {{{
 {-# LANGUAGE RankNTypes, GADTs, DataKinds, PolyKinds, TypeOperators,
 TypeFamilies #-}
 import Data.Proxy
 data Want :: (i,j) -> * where
   Want :: (a ~ '(b,c) => Proxy b) -> Want a
 }}}

 {{{
 GHCi, version 7.8.2: http://www.haskell.org/ghc/  :? for help
 Loading package ghc-prim ... linking ... done.
 Loading package integer-gmp ... linking ... done.
 Loading package base ... linking ... done.
 ghc: panic! (the 'impossible' happened)
   (GHC version 7.8.2 for x86_64-apple-darwin):
         tcTyVarDetails k{tv aAv} [tv]

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9222>
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