[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:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.5
      Resolution:                    |             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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Changes (by int-index):

 * priority:  high => normal


Old description:

> 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.

New description:

 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.

--

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15264#comment:2>
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