[GHC] #14331: Overzealous free-floating kind check causes deriving clause to be rejected

[GHC]

#14331: Overzealous free-floating kind check causes deriving clause to be rejected
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.2.1
  checker)                           |
      Resolution:                    |             Keywords:  deriving
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by goldfire):

 I assume you really meant

 {{{#!hs
 data D = D deriving (forall k a. C (a :: k))
 }}}

 but that GHC doesn't quantify `k` the right way. So it's a bug in kind
 quantification, not the free-floating kind variable check.

 (Sidenote: that `forall` isn't allowed there. But perhaps it should be
 incorporated into [https://github.com/ghc-proposals/ghc-
 proposals/blob/master/proposals/0007-instance-foralls.rst the recent
 proposal expanding where `forall` can be used.)

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14331#comment:2>
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