[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:  closed
              Priority:  normal      |        Milestone:
             Component:  Compiler    |          Version:  7.8.3
            Resolution:  wontfix     |         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 goldfire):

 * status:  new => closed
 * resolution:   => wontfix


Comment:

 The way to handle the original case is to detect the redundant constraint
 and eliminate it, before doing any other processing. However, GHC does not
 do this currently, and the choice would have runtime consequences I think
 -- it would speed programs up, which now have fewer dictionaries to pass.
 I'm not inclined to make this change: perhaps the user has good reasons
 for the redundancy.

 Short of that, I don't see a way to fix this without contortions. Given
 the comments above, I'm closing as wontfix.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9443#comment:7>
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