[GHC] #15431: Coercible and Existential types don't play nicely

[GHC]

#15431: Coercible and Existential types don't play nicely
-------------------------------------+-------------------------------------
        Reporter:  NioBium           |                Owner:  (none)
            Type:  bug               |               Status:  merge
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.4.3
      Resolution:                    |             Keywords:  Roles
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #14333            |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by simonpj):

 * status:  new => merge


Comment:

 There was a palpable bug that meant the constraint was marked as
 insoluble, when it wasn't.  This fixes both the OP and coment:1.

 The fix in #14333 (comment:10) does the symmetry bit.  I agree that
 incompleteness is still lurking!

 The fix is a modest one; could be merged.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15431#comment:4>
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