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

[GHC]

#12860: GeneralizedNewtypeDeriving + MultiParamTypeClasses sends typechecker into
an infinite loop
-------------------------------------+-------------------------------------
        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:"f8c966c70bf4e6ca7482658d4eaca2dae367213f/ghc" f8c966c/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="f8c966c70bf4e6ca7482658d4eaca2dae367213f"
 Be a bit more selective about improvement

 This patch makes [W] constraints not participate in
 improvement.   See Note [Do not do improvement for WOnly]
 in TcSMonad.

 Removes some senseless work duplication in some cases (notably
 Trac #12860); should not change behaviour.
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12860#comment:5>
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