[GHC] #7974: Panic on a context with additional variable

[GHC]

#7974: Panic on a context with additional variable
-------------------------------+--------------------------------------------
Reporter:  monoidal            |          Owner:                  
    Type:  bug                 |         Status:  new             
Priority:  normal              |      Component:  Compiler        
 Version:  7.7                 |       Keywords:                  
      Os:  Unknown/Multiple    |   Architecture:  Unknown/Multiple
 Failure:  Compile-time crash  |      Blockedby:                  
Blocking:                      |        Related:                  
-------------------------------+--------------------------------------------
 File:

 {{{
 {-# LANGUAGE GADTs #-}
 data X a where
   N :: (a ~ b) => X a

 k :: X a -> X b
 k N = N
 }}}

 Result with HEAD:

 {{{
 X.hs:6:7:
     Couldn't match type ‛b’ with ‛b1’
       ‛b’ is untouchable
         inside the constraints (a ~ b)
         bound by a pattern with constructor
                    N :: forall a. a ~ b => X a,
                  in an equation for ‛k’
         at X.hs:6:3ghc: panic! (the 'impossible' happened)
   (GHC version 7.7.20130608 for x86_64-unknown-linux):
         No skolem info: b{tv ae1} [sk]

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

 I got it by attempting to compile [http://hackage.haskell.org/package
 /data-category data-category] with HEAD.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7974>
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