[GHC] #8773: Require -XIncoherentInstances to write role annotations on class definitions

[GHC]

#8773: Require -XIncoherentInstances to write role annotations on class
definitions
--------------------------------------------+------------------------------
        Reporter:  goldfire                 |            Owner:  goldfire
            Type:  bug                      |           Status:  merge
        Priority:  normal                   |        Milestone:  7.8.1
       Component:  Compiler                 |          Version:  7.8.1-rc1
      Resolution:                           |         Keywords:
Operating System:  Unknown/Multiple         |     Architecture:
 Type of failure:  None/Unknown             |  Unknown/Multiple
       Test Case:  roles/should_fail/T8773  |       Difficulty:  Unknown
        Blocking:  8745                     |       Blocked By:
                                            |  Related Tickets:  #8745
--------------------------------------------+------------------------------
Changes (by goldfire):

 * status:  new => merge
 * testcase:   => roles/should_fail/T8773


Comment:

 I think (once again) that GND and `coerce` are Safe.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8773#comment:5>
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