[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:
 Type of failure:  Incorrect         |  Unknown/Multiple
  warning at compile-time            |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Blaisorblade):

 Possibly silly question: the extra expressivity here is behind a language
 flag, so why does its support reduce the normal language? Is the extra
 check (for non-bottomness) needed without recursive superclasses?
 - Is it hard to reproduce the old behavior with the new algorithm?
 - Or is it a bad idea, because then the examples in this bug would work
 normally and break with UndecidableSuperClasses?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11427#comment:13>
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