[GHC] #7020: Kind variable generalization problem

[GHC]

#7020: Kind variable generalization problem
---------------------------------------+------------------------------------
 Reporter:  goldfire                   |          Owner:                  
     Type:  bug                        |         Status:  new             
 Priority:  normal                     |      Component:  Compiler        
  Version:  7.5                        |       Keywords:  PolyKinds       
       Os:  Unknown/Multiple           |   Architecture:  Unknown/Multiple
  Failure:  GHC rejects valid program  |       Testcase:                  
Blockedby:                             |       Blocking:                  
  Related:                             |  
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 Consider the following tangle of definitions:

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

 import GHC.Exts

 data family Sing (a :: k)
 class SingKind (Any :: k) => SingI (s :: k) where
   sing :: Sing s
 data SingInstance :: k -> * where
   SingInstance :: SingI a => SingInstance a
 class (b ~ Any) => SingKind (b :: k) where
   singInstance :: forall (a :: k). Sing a -> SingInstance a
 }}}

 When compiling (with 7.5.20120620), I get this error message:

 {{{
     Kind mis-match
     The first argument of `SingKind' should have kind `k1',
     but `Any' has kind `k'
     In the class declaration for `SingI'
 }}}

 I believe that the code should compile, though I'll admit I'm not 100%
 convinced.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7020>
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