[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 goldfire):

 But comment:1 seems to require a constrained type in the `~` relation,
 which is generally not allowed. I suppose nothing stops a plugin from
 producing such things (and GHC might even consume them correctly), but I
 don't think that's explicitly a supported feature. (You can't say that in
 source Haskell, for example. Even with quantified constraints.)

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15248#comment:3>
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