[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:
                  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:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by simonpj):

 Yes, I agree with your proposal.  We should not blithely accept random
 (and perhaps unsatisfiable) equality constraints in `deriving` contexts.
 You can always specify the context with standalone-deriving if you want.

 Weill you execute on this?

 Simon

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8984#comment:7>
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