[GHC] #12785: GHC panic, `tcTyVarDetails` is missing a case

[GHC]

#12785: GHC panic, `tcTyVarDetails` is missing a case
-------------------------------------+-------------------------------------
        Reporter:  heisenbug         |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:  yes
      Blocked By:                    |             Blocking:
 Related Tickets:  #12590            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 Here's a much simpler program that also trips up the same error:

 {{{#!hs
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE TypeInType #-}
 {-# LANGUAGE TypeFamilies #-}
 module Bug (foo) where

 import Data.Kind (Type)

 foo :: forall (dk :: Type) (c :: Type -> Type) (t :: dk -> Type) (a ::
 Type).
        (dk ~ Type)
     => (forall (d :: dk). c (t d)) -> Maybe (c a)
 foo _ = Nothing
 }}}

 To make things more interesting, on GHC HEAD this errors with:

 {{{
 $ /opt/ghc/head/bin/ghc Bug.hs
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.1.20161218 for x86_64-unknown-linux):
         tcTyVarDetails
   cobox_aCE :: (dk_aCl[sk:2] :: *) ~# (* :: *)
   Call stack:
       CallStack (from HasCallStack):
         prettyCurrentCallStack, called at
 compiler/utils/Outputable.hs:1133:58 in ghc:Outputable
         callStackDoc, called at compiler/utils/Outputable.hs:1137:37 in
 ghc:Outputable
         pprPanic, called at compiler/basicTypes/Var.hs:461:22 in ghc:Var
 }}}

 But on GHC 8.0.1 and 8.0.2, it compiles fine! So this is actually a
 regression.

 I don't know if heisenbug's patch fixes it, nor what commit caused this
 regression. I'll look later today.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12785#comment:7>
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