[GHC] #12837: Disallow users to write instances for KnownNat and KnownSymbol

[GHC]

#12837: Disallow users to write instances for KnownNat and KnownSymbol
-------------------------------------+-------------------------------------
        Reporter:  adamgundry        |                Owner:  sean
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:  newcomer
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC accepts       |  Unknown/Multiple
  invalid program                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D2898
       Wiki Page:                    |
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Comment (by Matthew Pickering <matthewtpickering@…>):

 In [changeset:"c5609577fab8a214c50561bea861c70d4bfd47c7/ghc"
 c5609577/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="c5609577fab8a214c50561bea861c70d4bfd47c7"
 Disallow users to write instances of KnownNat and KnownSym

 As noted in #12837, these classes are special and the user should
 not be able to define their own instances.

 Test Plan: Validate

 Reviewers: adamgundry, goldfire, mpickering, austin, bgamari

 Reviewed By: goldfire, mpickering

 Subscribers: goldfire, mpickering, thomie

 Differential Revision: https://phabricator.haskell.org/D2898

 GHC Trac Issues: #12837
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12837#comment:6>
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