[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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Comment (by nomeata):

 It was commit 2f3ea95285d0cccc2a999e7572d8fb78dc2ea441 that made this type
 signature issue visible. Note that you even updated the users guide with
 this particular example, so I guess you’ll need to find one that is not
 horribly wrong :-).

 The original cause was 976a1087ae75accb4ad9d869d14641b2581c1606
 implementing #8541. I back then just assumed that if `Coercible` has to be
 higher-kinded, then `coerce` has to be too. I guess I was wrong...

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9788#comment:4>
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