[GHC] #7805: Panic: promoteType with higher-rank datatype

[GHC]

#7805: Panic: promoteType with higher-rank datatype
-------------------------------+--------------------------------------------
Reporter:  goldfire            |          Owner:                  
    Type:  bug                 |         Status:  new             
Priority:  normal              |      Component:  Compiler        
 Version:  7.7                 |       Keywords:  DataKinds       
      Os:  Unknown/Multiple    |   Architecture:  Unknown/Multiple
 Failure:  Compile-time crash  |      Blockedby:                  
Blocking:                      |        Related:                  
-------------------------------+--------------------------------------------
 The following code causes the panic in HEAD:

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

 data HigherRank = HR (forall a. a -> a)

 type family F (x :: HigherRank)
 type instance F (HR x) = Bool
 }}}

 Here is the panic text:

 {{{
 ghc-stage2: panic! (the 'impossible' happened)
   (GHC version 7.7.20130329 for x86_64-apple-darwin):
         promoteType
 }}}

 This panic does not happen with GHC 7.6.1.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7805>
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