[GHC] #12923: MultiParamTypeClasses + ExtendedDefaultRules

[GHC]

#12923: MultiParamTypeClasses + ExtendedDefaultRules
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        Reporter:  amindfv           |                Owner:
            Type:  feature request   |               Status:  patch
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D2816
       Wiki Page:                    |
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Comment (by amindfv):

 By the way, to get the current code to work we need to add:

 ` {-# LANGUAGE KindSignatures #-} `

 and

 ` data A (b :: Bool) = A deriving Show `

 to testsuite/tests/typecheck/should_compile/T12923.hs

 and

 ` import Kind `

 to compiler/typecheck/TcSimplify.hs

 It does regress for Foldable at the moment, as Simon predicted, but in the
 test case I provided it does in fact work, beautifully!

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12923#comment:8>
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