[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 think I have fixed it. It turned out that the patch is simpler than I
 though. Essentially I just made the matchGlobalInstance function to first
 actually search for an instance and failing which try out the KnownNat and
 KnownSymbol cases.

 The commit is https://github.com/piyush-
 kurur/ghc/commit/364de810f315faba50a4fbe4a68fba3876eb0555
 I have also written up an explanation. See if it makes sense.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15379#comment:6>
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