[GHC] #9518: Improve error message for unacceptable role annotations

[GHC]

#9518: Improve error message for unacceptable role annotations
-------------------------------------+-------------------------------------
       Reporter:  dmcclean           |                   Owner:
           Type:  feature request    |                  Status:  new
       Priority:  low                |               Milestone:
      Component:  Compiler           |                 Version:  7.8.3
       Keywords:                     |        Operating System:
   Architecture:  Unknown/Multiple   |  Unknown/Multiple
     Difficulty:  Unknown            |         Type of failure:
     Blocked By:                     |  None/Unknown
Related Tickets:                     |               Test Case:
                                     |                Blocking:
                                     |  Differential Revisions:
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 It would be nice if this message:

 {{{
 Role mismatch on variable a:
   Annotation says representational but role nominal is required
 while checking a role annotation for `Point'
 }}}

 Could be extended to say where the requirement arises.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9518>
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