[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 simonpj):

 Maybe you want dis-aggregate `checkValidInstance`.

 * I think you need `checkInstTermination` on every iteration. Probably
 even if `UndecidableInstances` is on; I'm not sure that automatic deriving
 should ever generate a non-provably-terminating instance.

 * Then you can call `checkValidInstance` itself right at the end -- that
 will check the coverage conditions.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12860#comment:19>
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