[GHC] #11427: superclasses aren't considered because context is no smaller than the instance head

[GHC]

#11427: superclasses aren't considered because context is no smaller than the
instance head
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        Reporter:  phadej            |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1-rc1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 I agree that the error message could be better, but I don't see an easy
 way to ''make'' it better.  All the error-message generator sees is that
 `(SListI xss)` is unsolved.  It hard for it to figure out that it might
 have been solved from superclasses but in fact wasn't because of size.

 The "arising from" is easier to improve perhaps.  What should it say?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11427#comment:8>
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