[GHC] #15757: Coercion Variable Almost Devoid Checking in ForAllCo

[GHC]

#15757: Coercion Variable Almost Devoid Checking in ForAllCo
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        Reporter:  ningning          |                Owner:  ningning
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #15497            |  Differential Rev(s):  Phab:D5231
       Wiki Page:                    |
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Comment (by Ningning Xie <xnningxie@…>):

 In [changeset:"879db5595208fb665ff1a0a2b12b9921d3efae0e/ghc"
 879db559/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="879db5595208fb665ff1a0a2b12b9921d3efae0e"
 Adding almost devoid check for covar in ForAllCo

 Summary:
 For the sake of consistency of the dependent core, there is a restriction
 on
 where a coercion variable can appear in ForAllCo: the coercion variable
 can
 appear nowhere except in coherence coercions.

 Currently this restriction is missing in Core. The goal of this patch is
 to add
 the missing restriction.

 After discussion, we decide: coercion variables can appear nowhere except
 in
 `GRefl` and `Refl`. Relaxing the restriction to include `Refl` should not
 break
 consistency, we premuse.

 Test Plan: ./validate

 Reviewers: goldfire, simonpj, bgamari

 Reviewed By: goldfire

 Subscribers: rwbarton, carter

 GHC Trac Issues: #15757

 Differential Revision: https://phabricator.haskell.org/D5231
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15757#comment:4>
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