[GHC] #9357: Kind-polymorphic type family accepts unlifted type arguments

[GHC]

#9357: Kind-polymorphic type family accepts unlifted type arguments
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       Reporter:  kosmikus           |                   Owner:
           Type:  bug                |                  Status:  new
       Priority:  normal             |               Milestone:
      Component:  Compiler (Type     |                 Version:  7.8.2
  checker)                           |        Operating System:
       Keywords:                     |  Unknown/Multiple
   Architecture:  Unknown/Multiple   |         Type of failure:  GHC
     Difficulty:  Unknown            |  accepts invalid program
     Blocked By:                     |               Test Case:
Related Tickets:                     |                Blocking:
                                     |  Differential Revisions:
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 The following code is accepted by ghc-7.8.2 and ghc-7.6.3:
 {{{#!hs
 {-# LANGUAGE TypeFamilies, MagicHash, PolyKinds #-}

 import GHC.Exts

 type family F (a :: k) :: *
 type instance F Int# = ()
 }}}

 In nearly all other places, it seems that unlifted types are explicitly
 forbidden. I'm also not allowed to actually apply `F` to `Int#` after
 defining the type family like this. So I think the compiler should
 probably reject the `type instance`.

 (BTW, is it actually clear that using `#` as an argument kind to a type
 family or as an index kind for a GADT is in any way harmful?)

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9357>
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