[GHC] #9788: `coerce` has an impossible type

[GHC]

#9788: `coerce` has an impossible type
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              Reporter:  goldfire    |            Owner:  goldfire
                  Type:  bug         |           Status:  patch
              Priority:  highest     |        Milestone:  7.10.1
             Component:  Compiler    |          Version:  7.8.3
            Resolution:              |         Keywords:
      Operating System:              |     Architecture:  Unknown/Multiple
  Unknown/Multiple                   |       Difficulty:  Easy (less than 1
       Type of failure:              |  hour)
  None/Unknown                       |       Blocked By:
             Test Case:              |  Related Tickets:
              Blocking:              |
Differential Revisions:  Phab:D468   |
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Changes (by simonpj):

 * priority:  normal => highest


Comment:

 I have no idea how this happens, but thank you for fixing it (for 7.10).
 Can you also add a regression test?

 Simon

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9788#comment:3>
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