[GHC] #9880: ghc crashes on th splices in instance context

[GHC]

#9880: ghc crashes on th splices in instance context
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              Reporter:  Philonous   |            Owner:
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:
             Component:  Compiler    |          Version:  7.8.3
            Resolution:              |         Keywords:
      Operating System:              |     Architecture:  Unknown/Multiple
  Unknown/Multiple                   |       Difficulty:  Unknown
       Type of failure:              |       Blocked By:
  None/Unknown                       |  Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by goldfire):

 The behavior you see -- that error -- is what I'd expect.

 I had a brief go at implementing local declaration splices but soon
 realized that implementing this involves quite a bit of replumbing. The
 splices would have to break up mutually-recursive groups within class,
 instance, and `let` declarations, and that would require changes to
 several bits of HsSyn. So, I settled for just fixing the panic and waiting
 for someone to scream loudly that they need this feature.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9880#comment:2>
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