[GHC] #15379: Don't reject user-written instances of KnownNat and friends in hsig files

[GHC]

#15379: Don't reject user-written instances of KnownNat and friends in hsig files
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        Reporter:  ezyang            |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.4.3
      Resolution:                    |             Keywords:  backpack
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by ppk):

 I have started working on this bug at <https://github.com/piyush-
 kurur/ghc/tree/literal-instac>. Not yet on phab because I am waiting for
 the fix of #15138 to get merged. Will rebase and send the patches then.
 But here is a status report of what I have done

 1. Added a testcase

 2. Made the minimal modification that we discussed over IRC

 Unfortunately just 2 does not help as there is a overlapping instance
 error that is being thrown. I will investigate this and let you known.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15379#comment:2>
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