[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:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:                    |             Keywords:
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 Simon Peyton Jones <simonpj@…>):

 In [changeset:"eb55ec2941239dee05afc6be818b129efe51660e/ghc" eb55ec2/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="eb55ec2941239dee05afc6be818b129efe51660e"
 Refactor functional dependencies a bit

 * Rename CoAxiom.Eqn = Pair Type to TypeEqn,
   and use it for fundeps

 * Use the FunDepEqn for injectivity, which lets us share a bit
   more code, and (more important) brain cells

 * When generating fundeps, take the max depth of the two
   constraints.  This aimed at tackling the strange loop in
   Trac #12860, but there is more to come for that.

 * Improve pretty-printing with -ddump-tc-trace
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12860#comment:4>
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