[GHC] #8984: Improve output of failed GeneralizedNewtypeDeriving coercion due to type roles

[GHC]

#8984: Improve output of failed GeneralizedNewtypeDeriving coercion due to type
roles
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              Reporter:  haasn       |            Owner:  goldfire
                  Type:  feature     |           Status:  new
  request                            |        Milestone:
              Priority:  normal      |          Version:  7.8.1
             Component:  Compiler    |         Keywords:
  (Type checker)                     |     Architecture:  Unknown/Multiple
            Resolution:              |       Difficulty:  Unknown
      Operating System:              |       Blocked By:
  Unknown/Multiple                   |  Related Tickets:
       Type of failure:              |
  None/Unknown                       |
             Test Case:              |
  deriving/should_fail/T8984         |
              Blocking:              |
Differential Revisions:              |
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Comment (by Richard Eisenberg <eir@…>):

 In [changeset:"02b4845e07ef7110b2f735f323eb8748903330ff/ghc"]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="02b4845e07ef7110b2f735f323eb8748903330ff"
 Consider equality contexts exotic, uninferrable by "deriving"

 See comments in #8984. This takes back the fix for #6088.
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8984#comment:11>
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