[GHC] #12860: GeneralizedNewtypeDeriving + MultiParamTypeClasses sends typechecker into an infinite loop

[GHC]

#12860: GeneralizedNewtypeDeriving + MultiParamTypeClasses sends typechecker into
an infinite loop
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |             Keywords:  FunDeps,
      Resolution:                    |  deriving
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by RyanGlScott):

 * cc: diatchki (added)


Comment:

 Indeed, I suspect we need a more permissive check than `checkInstCoverage`
 which would reject `C b a` but not `Monoid w` in the `XMLGenT` example.
 Alas, I have no idea what that check would be.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12860#comment:22>
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