[GHC] #8767: Add rules involving `coerce` to the libraries

[GHC]

#8767: Add rules involving `coerce` to the libraries
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              Reporter:  nomeata     |            Owner:
                  Type:  task        |           Status:  new
              Priority:  normal      |        Milestone:  7.10.1
             Component:              |          Version:  7.9
  libraries/base                     |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:  8718
       Type of failure:              |  Related Tickets:  #2110
  None/Unknown                       |
             Test Case:              |
  tests/simplCore/should_run/T2110.hs|
              Blocking:              |
Differential Revisions:              |
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Comment (by nomeata):

 >  Is this thread about making a workaround for that?

 No. Or, not mainly.

 > Is the reason that it's complicated because some functors have nominal
 roles? (Maybe a SortedList would break if you changed out Ord instances
 under it?) Or is there more to it than that?

 Precisely: We don’t know about the roles of type variables, so we cannot
 lift coercions through them.

 I believe that a subsequent design of the system that will be able to cope
 with Monad’s `join` might be able to handle that, but I’m not up on that.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8767#comment:18>
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