[GHC] #8566: Panic with kindFunResult

[GHC]

#8566: Panic with kindFunResult
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        Reporter:  dreixel           |            Owner:
            Type:  bug               |           Status:  closed
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.7
      Resolution:  fixed             |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:  polykinds/T8566   |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by dreixel):

 I suspect this fix makes the following code fail to compile with HEAD:
 {{{
 {-# LANGUAGE DataKinds                  #-}
 {-# LANGUAGE PolyKinds                  #-}
 {-# LANGUAGE TypeFamilies               #-}
 {-# LANGUAGE GADTs                      #-}
 {-# LANGUAGE ScopedTypeVariables        #-}
 {-# LANGUAGE TypeOperators              #-}

 data Field = forall k. APP k [Field]

 data InField (u :: Field) :: * where
   A :: { unA :: AppVars t (ExpandField args) } -> InField (APP t args)
   -- Without the selector, the program compiles
   -- A ::          AppVars t (ExpandField args)   -> InField (APP t args)

 type family ExpandField (args :: [Field]) :: [*]
 type family AppVars (t :: k) (vs :: [*]) :: *
 }}}

 Note that simply removing the selector `unA` fixes the problem. The code
 compiles with 7.6. Is this a regression, or is it the expected
 behaviour?...

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8566#comment:5>
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