[GHC] #11525: Using a dummy typechecker plugin causes an ambiguity check error

[GHC]

#11525: Using a dummy typechecker plugin causes an ambiguity check error
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        Reporter:  jme               |                Owner:  darchon
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler (Type    |              Version:  8.0.1-rc1
  checker)                           |             Keywords:
      Resolution:                    |  UndecidableSuperClasses, plugin
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:  T11525
      Blocked By:                    |             Blocking:
 Related Tickets:  #12780            |  Differential Rev(s):  Phab:D3105
       Wiki Page:                    |
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Comment (by Ben Gamari <ben@…>):

 In [changeset:"07292e958cb0c08705d9a694f09d9621058b16e6/ghc"
 07292e95/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="07292e958cb0c08705d9a694f09d9621058b16e6"
 zonkCt tries to maintain the canonical form of a Ct.

 For example,
  - a CDictCan should stay a CDictCan;
  - a CTyEqCan should stay a CTyEqCan (if the LHS stays as a variable.).
  - a CHoleCan should stay a CHoleCan

 Why?  For CDicteqCan see Trac #11525.

 Test Plan: Validate

 Reviewers: austin, adamgundry, simonpj, goldfire, bgamari

 Reviewed By: simonpj, bgamari

 Subscribers: thomie

 Differential Revision: https://phabricator.haskell.org/D3105
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11525#comment:12>
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