[GHC] #16391: "Quantified type's kind mentions quantified type variable" error with fancy-kinded GADT

[GHC]

#16391: "Quantified type's kind mentions quantified type variable" error with
fancy-kinded GADT
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:  8.10.1
       Component:  Compiler (Type    |              Version:  8.6.3
  checker)                           |
      Resolution:  fixed             |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  GHC rejects       |            Test Case:
  valid program                      |  dependent/should_compile/T16391a,
                                     |  dependent/should_fail/T16391b
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |  https://gitlab.haskell.org/ghc/ghc/merge_requests/503
-------------------------------------+-------------------------------------

Comment (by Marge Bot <ben+marge-bot@…>):

 In [changeset:"068b7e983f4a0b35f453aa5e609998efd0c3f334/ghc"
 068b7e98/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="068b7e983f4a0b35f453aa5e609998efd0c3f334"
 Fix #16391 by using occCheckExpand in TcValidity

 The type-variables-escaping-their-scope-via-kinds check in
 `TcValidity` was failing to properly expand type synonyms, which led
 to #16391. This is easily fixed by using `occCheckExpand` before
 performing the validity check.

 Along the way, I refactored this check out into its own function,
 and sprinkled references to Notes to better explain all of the moving
 parts. Many thanks to @simonpj for the suggestions.

 Bumps the haddock submodule.
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16391#comment:6>
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