[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 nomeata):

 I was confused. I thought for a moment that the type of `$dCoercible_arz`,
 i.e. `Coercible` is the same as the `Coercion` thingy, and the latter can
 be undefined. But they are not, `Coercion` adds another box around
 `Coercible`, I guess we avoid `Coercion` to be undefined.

 But `Coercible` is a lifted type, so I’m not sure that we can guarantee
 that there is no way to produce a bottom here.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9792#comment:6>
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