[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 request          |           Status:  new
        Priority:  normal                   |        Milestone:
       Component:  Compiler (Type checker)  |          Version:  7.8.1
      Resolution:                           |         Keywords:
Operating System:  Unknown/Multiple         |     Architecture:
 Type of failure:  None/Unknown             |  Unknown/Multiple
       Test Case:                           |       Difficulty:  Unknown
        Blocking:                           |       Blocked By:
                                            |  Related Tickets:
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Comment (by haasn):

 More specifically, the exact type that causes a problem is
 ‘WrappedSchoenfinkel cat’ and ‘cat’ not being nominally equal.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8984#comment:1>
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