[GHC] #9443: Regression from 7.6.3 to 7.8.3: could not coerce because argument "has role Nominal"

[GHC]

#9443: Regression from 7.6.3 to 7.8.3: could not coerce because argument "has role
Nominal"
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              Reporter:  bernalex    |            Owner:
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:
             Component:  Compiler    |          Version:  7.8.3
            Resolution:              |         Keywords:
      Operating System:              |     Architecture:  x86_64 (amd64)
  Unknown/Multiple                   |       Difficulty:  Unknown
       Type of failure:  GHC         |       Blocked By:
  rejects valid program              |  Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Changes (by bernalex):

 * cc: slyfox (removed)
 * cc: eir@…, slyich@…, haskell@… (added)


Comment:

 Nilkas and Trofi helped me out a bit. It seems this is due to the new
 role-inference mechanism. As such, it's perhaps not a regression per se. I
 do think GHC should accept this as a valid program though.

 CC'ing Richard, as Trofi mentioned that he was the GND/roles guy.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9443#comment:2>
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