[GHC] #7386: Panic "tcTyVarDetails" in GHCi when :forcing a data family instance

[GHC]

#7386: Panic "tcTyVarDetails" in GHCi when :forcing a data family instance
------------------------------+---------------------------------------------
 Reporter:  goldfire          |          Owner:                  
     Type:  bug               |         Status:  new             
 Priority:  normal            |      Component:  GHCi            
  Version:  7.6.1             |       Keywords:  PolyKinds       
       Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
  Failure:  GHCi crash        |       Testcase:                  
Blockedby:                    |       Blocking:                  
  Related:                    |  
------------------------------+---------------------------------------------
 I loaded the following code into GHCi:

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

 data Nat = Zero | Succ Nat
 data family Sing (a :: k)
 data instance Sing (a :: Nat) where
   SZero :: Sing Zero
   SSucc :: Sing n -> Sing (Succ n)
 }}}

 Then. I run these commands at the prompt:

 {{{
 *Main> let x = SSucc SZero
 *Main> :f x
 }}}

 The response is

 {{{
 ghc: panic! (the 'impossible' happened)
   (GHC version 7.7.20121031 for x86_64-apple-darwin):
         tcTyVarDetails
 <<details unavailable>>
 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7386>
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