[GHC] #10971: ExtendedDefaultRules defaults to [] for Foldable/Traversable

[GHC]

#10971: ExtendedDefaultRules defaults to [] for Foldable/Traversable
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        Reporter:  kanetw            |                Owner:  kanetw
            Type:  feature request   |               Status:  patch
        Priority:  normal            |            Milestone:  8.0.1
       Component:  Compiler          |              Version:  7.10.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #10963            |  Differential Rev(s):  phab:D1329
       Wiki Page:                    |
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Comment (by simonpj):

 > Also, this implementation would seem to accept `default Either`, which
 is utterly meaningless, as `Either` has a non-defaultable kind. This is
 also utterly harmless, but it makes me suspicious of the design.

 It would not be utterly meaningless if we added some new type class of
 kind `(* -> * -> *) -> Constraint`, and wanted defaulting to work on it.

 Simon

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10971#comment:6>
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