[GHC] #13391: PolyKinds is more permissive in GHC 8

[GHC]

#13391: PolyKinds is more permissive in GHC 8
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        Reporter:  crockeea          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.2
      Resolution:                    |             Keywords:  newcomer
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC accepts       |  Unknown/Multiple
  invalid program                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by RyanGlScott):

 * keywords:   => newcomer


Comment:

 Good catch. I suspect fixing this wouldn't be too challenging. If you look
 at the commitdiff for 6746549772c5cc0ac66c0fce562f297f4d4b80a2 (Add kind
 equalities to GHC), searching for `badGadtKindCon` will give you the check
 that used to be in place for catching datatypes that are GADT-like in
 their kind arguments. Fixing this issue should hopefully be a matter of
 reinstating that check, but guarded behind a check for the presence of
 `-XTypeInType`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13391#comment:1>
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