[GHC] #12837: Disallow users to write instances for KnownNat and KnownSymbol (was: KnownNat and KnownSymbol should be abstract)

[GHC]

#12837: Disallow users to write instances for KnownNat and KnownSymbol
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        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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Description changed by sean:

@@ -9,3 +9,3 @@
- This is accepted (albeit with a warning about the missing `natSing`
- method). I think it should be rejected, as should an instance of
- `KnownSymbol`. Compare doing the same thing with `Typeable`, which says:
+ Originally this was accepted (albeit with a warning about the missing
+ `natSing` method). It is now rejected, as with instances of `KnownSymbol`,
+ in the same way as instances of `Typeable`, which says:

New description:

 Consider the following patent nonsense:

 {{{#!hs
 {-# LANGUAGE FlexibleInstances #-}
 import GHC.TypeLits
 instance KnownNat n
 }}}

 Originally this was accepted (albeit with a warning about the missing
 `natSing` method). It is now rejected, as with instances of `KnownSymbol`,
 in the same way as instances of `Typeable`, which says:

 {{{
     Class ‘Typeable’ does not support user-specified instances
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12837#comment:5>
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