[GHC] #9334: Implement "instance chains"

[GHC]

#9334: Implement "instance chains"
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        Reporter:  diatchki          |                Owner:  diatchki
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  7.9
  checker)                           |
      Resolution:                    |             Keywords:
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 Iceland_jack):

 Replying to [comment:6 diatchki]:
 >   2. "Fails instances", which are of the form `instance Num Char fails`;
 they enable programmers to state explicitly that an instance should never
 exits.  Interestingly, I just found a very related ticket asking for the
 same sort of thing (#7775).

 Using `Any` as a superclass we can effectively
 [https://gist.github.com/Icelandjack/5afdaa32f41adf3204ef9025d9da2a70
 #hackage-no-instances-allowed forbid instances], of course the compiler is
 not privy to this.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9334#comment:13>
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