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

[GHC]

#15431: Coercible and Existential types don't play nicely
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        Reporter:  NioBium           |                Owner:  (none)
            Type:  bug               |               Status:  new
        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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Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"f0d27f515ffbc476144d1d1dd1a71bf9fa93c94b/ghc" f0d27f5/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="f0d27f515ffbc476144d1d1dd1a71bf9fa93c94b"
 Stop marking soluble ~R# constraints as insoluble

 We had a constraint (a b ~R# Int), and were marking it as 'insoluble'.
 That's bad; it isn't.  And it caused Trac #15431. Soultion is simple.

 I did a tiny refactor on can_eq_app, so that it is used only for
 nominal equalities.
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15431#comment:3>
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