[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):

 Well I'm not sure what to do.

 1 You don't want 'deriving' to loop
 2 If the fundep coverage condition isn't satisfied, instance resolution
 can go into a loop,
 3 But comment:18 says that you want 'deriving' to work when fundep
 coverage doesn't hold.

 Perhaps (2) is over-conservative, somehow?  Presumably there is something
 special about (3) that makes it terminate, despite (2) not holding.  If
 you can identify what that is, you can make that into a new (more
 expressive) fundep coverage condition.

 Iavor is the fundep expert.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12860#comment:21>
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