[GHC] #8557: Panic "tcTyVarDetails" when :forcing a kind-polymorphic data family instance

[GHC]

#8557: Panic "tcTyVarDetails" when :forcing a kind-polymorphic data family
instance
------------------------------------+-------------------------------------
       Reporter:  goldfire          |             Owner:
           Type:  bug               |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  Compiler          |           Version:  7.7
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
------------------------------------+-------------------------------------
 I have

 {{{
 {-# LANGUAGE TypeFamilies, DataKinds, PolyKinds #-}

 data family Sing (a :: k)
 data instance Sing (a :: [k]) = SNil
 x :: Sing '[]
 x = SNil
 }}}

 When I load into GHCi and tried to force `x`, this happened:

 {{{
 *Main> :force x
 ghc: panic! (the 'impossible' happened)
   (GHC version 7.7.20131031 for x86_64-apple-darwin):
         tcTyVarDetails k{tv a3Gv} [tv]
 }}}

 This was tested on the build from Oct 31 -- apologies if it has since been
 fixed. If I make the `[k]` in the instance declaration `k`, the file
 compiles fine.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8557>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler