[GHC] #13988: GADT constructor with kind equality constraint quantifies unused existential type variables

[GHC]

#13988: GADT constructor with kind equality constraint quantifies unused
existential type variables
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:                    |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect result  |  Unknown/Multiple
  at runtime                         |            Test Case:
      Blocked By:                    |             Blocking:  14131
 Related Tickets:                    |  Differential Rev(s):  Phab:D3902
       Wiki Page:                    |
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Comment (by Ben Gamari <ben@…>):

 In [changeset:"b9ac9e05b29c58efc93b78bf7fca43d61ead50c7/ghc" b9ac9e0/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="b9ac9e05b29c58efc93b78bf7fca43d61ead50c7"
 Fix egregious duplication of vars in RnTypes

 `RnTypes` contains a fairly intricate algorithm to extract
 the kind and type variables of an HsType. This algorithm carefully
 maintains the separation between type variables and kind variables
 so that the difference between `-XPolyKinds` and `-XTypeInType` can
 be respected.

 But after doing all this, `rmDupsInRdrTyVars` stupidly just
 concatenated the lists of type and kind variables at the end. If a
 variable were used as both a type and a kind, the algorithm would
 produce *both*! This led to all kinds of problems, including #13988.

 This is mostly Richard Eisenberg's patch. The only original
 contribution I made was adapting call sites of `rnImplicitBndrs` to
 work with the new definition of `rmDupsInRdrTyVars`. That is,
 `rnImplicitBndrs` checks for variables that are illegally used in
 both type and kind positions without using `-XTypeInType`, but in
 order to check this, one cannot have filtered duplicate variables out
 before passing them to `rnImplicitBndrs`. To accommodate for this, I
 needed to concoct variations on the existing `extract-` functions in
 `RnTypes` which do not remove duplicates, and use those near
 `rnImplicitBndrs` call sites.

 test case: ghci/scripts/T13988

 Test Plan: make test TEST=T13988

 Reviewers: goldfire, simonpj, austin, bgamari

 Reviewed By: goldfire, simonpj

 Subscribers: rwbarton, thomie

 GHC Trac Issues: #13988

 Differential Revision: https://phabricator.haskell.org/D3902
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13988#comment:5>
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