[GHC] #15248: Coercions from plugins cannot be stopped from floating out

[GHC]

#15248: Coercions from plugins cannot be stopped from floating out
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        Reporter:  goldfire          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.4.3
      Resolution:                    |             Keywords:
                                     |  TypeCheckerPlugins
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 Yes: see comment:15 on #8095, and the discussion on Phab:D4766.
 `PlugInProv` should have a `TyCoVarSet` just as the (upcoming)
 `ZappedProv` does, and for the same reasons.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15248#comment:4>
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