[GHC] #10318: Cycles in class declaration (via superclasses) sometimes make sense.

[GHC]

#10318: Cycles in class declaration (via superclasses) sometimes make sense.
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        Reporter:  ekmett            |                   Owner:
            Type:  feature request   |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler (Type    |                 Version:  7.10.1
  checker)                           |                Keywords:
      Resolution:                    |            Architecture:
Operating System:  Unknown/Multiple  |  Unknown/Multiple
 Type of failure:  GHC rejects       |               Test Case:
  valid program                      |                Blocking:
      Blocked By:                    |  Differential Revisions:
 Related Tickets:                    |
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Comment (by ekmett):

 Things like

 {{{
 instance Show (f (Fix f)) => Show (Fix f)
 }}}

 already work, we can ask for "undecidable instances", but not "undecidable
 superclasses".

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10318#comment:3>
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