[GHC] #12081: TypeInType Compile-time Panic

[GHC]

#12081: TypeInType Compile-time Panic
-------------------------------------+-------------------------------------
           Reporter:  MichaelK       |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1-rc4
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Compile-time
  Unknown/Multiple                   |  crash
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 I've been playing around with GHC 8.0.1-rc4 release and got a panic from
 the following (stripped down) code:

 {{{#!hs
 {-# LANGUAGE TypeInType #-}

 data Nat = Z | S Nat

 class C (n :: Nat) where
   type T n :: Nat
   f :: (a :: T n)
 }}}

 {{{
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.0.0.20160421 for x86_64-apple-darwin):
         isInjectiveTyCon sees a TcTyCon T
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12081>
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