[GHC] #15264: Warn about implicit kind variables with -Wcompat

[GHC]

#15264: Warn about implicit kind variables with -Wcompat
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           Reporter:  int-index      |             Owner:  (none)
               Type:  task           |            Status:  new
           Priority:  high           |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.5
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Poor/confusing
  Unknown/Multiple                   |  error message
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 According to an accepted proposal https://github.com/ghc-proposals/ghc-
 proposals/blob/master/proposals/0024-no-kind-vars.rst:

   With `-Wcompat`, warn if a kind variable is brought into scope
 implicitly in a type with an explicit `forall`. This applies to type
 signatures and to other contexts that allow a `forall` with the forall-or-
 nothing rule in effect (for example, class instances).

 Creating ticket just for the record. Implementation coming soon. Setting
 priority as high because if the warning doesn't make it into the release,
 the actual change will have to be delayed by another half a year.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15264>
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