[GHC] #7347: Existential data constructors should not be promoted

[GHC]

#7347: Existential data constructors should not be promoted
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    Reporter:  simonpj           |       Owner:                  
        Type:  bug               |      Status:  new             
    Priority:  normal            |   Milestone:                  
   Component:  Compiler          |     Version:  7.6.1           
    Keywords:                    |          Os:  Unknown/Multiple
Architecture:  Unknown/Multiple  |     Failure:  None/Unknown    
  Difficulty:  Unknown           |    Testcase:                  
   Blockedby:                    |    Blocking:                  
     Related:                    |  
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Comment(by kosmikus):

 It seems that all that GHC 7.6.1 currently allows is to promote
 existentially quantified variables of kind *. Such constructors, if
 promoted, lead to simply kinded datatypes. No polymorphism, no
 existentials on that level. So why not keep allowing them? There's no
 danger of escaping variables even for type families, afaics.

 The bug Stefan reports seems to be a missing check whether the type is
 used according to its inferred kind.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7347#comment:3>
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