[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:  patch
        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):  Phab:D4988
       Wiki Page:                    |
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Comment (by ppk):

 Some of the test cases in `backpack/should_fail/` are failing. Of which I
 think the bkpfail46.bkp is actually //not giving// a compilation failure
 (and hence the test is failing).
 Can you have a look at this test case ? Was this a previous limitation of
 backpack that was
 now fixed or is it the case that some thing that should not have been
 acceptable is now being
 accepted (thereby seriously questioning the type safety).

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15379#comment:14>
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