[GHC] #12923: MultiParamTypeClasses + ExtendedDefaultRules

[GHC]

#12923: MultiParamTypeClasses + ExtendedDefaultRules
-------------------------------------+-------------------------------------
        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:                    |
-------------------------------------+-------------------------------------

Comment (by simonpj):

 I don't think that rule will work.  As the user manual page says, the list
 type `[] :: * -> *` is in the list of type defaults, so that higher-kinded
 variables can be defaulted too.   See #10971.

 So we can't restrict to *.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12923#comment:6>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler