[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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Comment (by dfeuer):

 Ryan mentions

 {{{#!hs
 newtype XMLGenT m a = XMLGenT (m a) deriving (MonadRWS r w s)`
 }}}

 I suspect what we want to do is collect the fundeps from the ''superclass
 constraints'' of `MonadRWS` before doing anything much else. I have the
 feeling (maybe wrong?) that we can get these more safely than we can
 fundeps of instance constraints. These should (at least in this case) be
 sufficient to satisfy the coverage condition.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12860#comment:23>
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