[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 hvr):

 Just for the record, I consider this GHC being faithful to H2010 when it
 rejects the code

 {{{#!hs
 module Bug where

 default (Bool)
 }}}

 since
 [https://www.haskell.org/onlinereport/haskell2010/haskellch4.html#x10-790004.3.4
 4.3.4] says:

 > Ambiguities in the class Num are most common, so Haskell provides
 another way to resolve them—with a default declaration: default (t1 , … ,
 tn) where n ≥ 0, and each ti must be a type for which Num ti holds.

 And `Bool` is not an instance of `Num`, hence the code above is not legal
 plain Haskell2010 code. Personally, I consider this a bugfix rather than a
 regression :-)

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12907#comment:2>
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