[GHC] #11811: GHC sometimes misses a CUSK

[GHC]

#11811: GHC sometimes misses a CUSK
-------------------------------------+-------------------------------------
           Reporter:  goldfire       |             Owner:  goldfire
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 {{{
 {-# LANGUAGE TypeInType #-}

 data Test (a :: x) (b :: x) :: x -> *
   where K :: Test Int Bool Double
 }}}

 fails, because GHC thinks that `Test` does not have a CUSK.

 It should have a CUSK, because while there is no `forall x` in the result
 kind, the `x` is in scope from previous use in kinds.

 Fix en route.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11811>
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