[GHC] #12907: ExtendedDefaultRules-related regression in GHC 8.0.2

[GHC]

#12907: ExtendedDefaultRules-related regression in GHC 8.0.2
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        Reporter:  RyanGlScott       |                Owner:
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.0.2
       Component:  Compiler (Type    |              Version:  8.0.2-rc1
  checker)                           |
      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 goldfire):

 I agree with comment:2. The change to reject the program reported was
 intentional, following the Report. When `-XExtendedDefaultRules` is in
 effect, the program given is accepted, as `Bool` is a member of several of
 the classes defaultable under the extended rules.

 I'd be willing to consider other designs here, but I believe what's
 implemented is the most faithful to the Report.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12907#comment:3>
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