[GHC] #14710: GHC 8.4.1-alpha allows the use of kind polymorphism without PolyKinds

[GHC]

#14710: GHC 8.4.1-alpha allows the use of kind polymorphism without PolyKinds
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.4.1
       Component:  Compiler (Type    |              Version:  8.4.1-alpha1
  checker)                           |
      Resolution:                    |             Keywords:  PolyKinds
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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Comment (by RyanGlScott):

 One way to summarize this issue in that at the moment, GHC believes it can
 spot any code that requires `PolyKinds` at the granularity of individual
 type variable telescopes (for instance, `forall (a :: k). <...>` would
 require `PolyKinds` since the telescope `forall (a :: k)` implicitly
 quantifies `k`). But really this isn't true—a type variable might seem
 perfectly normal //within// a telescope, but elsewhere in its body it
 might be used as a kind variable (as in the examples above)! So it feels
 like we should be validity checking //uses// of type variables as well,
 not just their binding sites.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14710#comment:3>
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