#7527: Couldn't match kind `*' with `*' with PolyKinds & GADTs.
    Reporter:  Ashley Yakeley    |        Owner:                           
        Type:  bug               |       Status:  closed                   
    Priority:  normal            |    Component:  Compiler (Type checker)  
     Version:  7.6.1             |   Resolution:  fixed                    
    Keywords:                    |           Os:  Unknown/Multiple         
Architecture:  Unknown/Multiple  |      Failure:  GHC rejects valid program
   Blockedby:                    |     Blocking:                           
     Related:                    |  
Changes (by goldfire):

  * status:  new => closed
  * resolution:  => fixed


 Fixed in HEAD:

     Couldn't match type `t0' with `f'
       `t0' is untouchable
         inside the constraints ('WrapType * a ~ 'WrapType * a1)
         bound by a pattern with constructor
                    MkW :: forall (k :: BOX) (f :: k -> *) (a :: k).
                           f a -> W k f ('WrapType k a),
                  in a case alternative
         at /Users/rae/temp/Scratch.hs:21:9-14
       `f' is a rigid type variable bound by
           the type signature for thing :: T ('WrapType * a) -> f a
           at /Users/rae/temp/Scratch.hs:19:30
     Expected type: f a
       Actual type: t0 a1
     Relevant bindings include
       thing :: T ('WrapType * a) -> f a
         (bound at /Users/rae/temp/Scratch.hs:20:5)
       cw :: t0 a1 (bound at /Users/rae/temp/Scratch.hs:21:13)
     In the expression: cw
     In a case alternative: MkW cw -> cw
     In the expression: case (get t) of { MkW cw -> cw }

 Are you claiming that this should type check? Or, just stating that the
 error message is unhelpful? I agree with GHC that it shouldn't, and I can
 explain if that would be helpful...

Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7527#comment:1>
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