[GHC] #9792: map/coerce rule never seems to fire

[GHC]

#9792: map/coerce rule never seems to fire
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              Reporter:  dfeuer      |            Owner:  ekmett
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:  7.10.1
             Component:  Core        |          Version:  7.9
  Libraries                          |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:  Runtime     |  Related Tickets:
  performance bug                    |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by dfeuer):

 I think the `map` rewrite probably isn't a big problem, because if `map`
 fuses with anything, the mapped coercion will become free (if a mapped
 coercion fuses with another mapped coercion, those will end up a mapped
 composed coercion—that might need some special treatment, but I'm not
 sure). The expansion of `coerce`, however, which seems to relate to the
 fact that `Coercible` has only one member, looks problematic.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9792#comment:1>
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