[GHC] #12466: Typechecker regression: Inaccessible code in a type expected by the context

[GHC]

#12466: Typechecker regression: Inaccessible code in a type expected by the context
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        Reporter:  RyanGlScott       |                Owner:
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.2.1
       Component:  Compiler (Type    |              Version:  8.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by dfeuer):

 Replying to [comment:26 simonpj]:
 > I agree with your reasoning, but it's quite painful to implement the
 positive/negative distinction (given the way the constraint solver works).
 Anything is possible, but this all seems a bit exotic, so I'm inclined to
 look for a simple solution.
 >
 > I think it would be simple to:
 >
 > * Not complain about inaccessible code in instance methods
 >
 > Would that do?

 That doesn't seem likely to solve the problem in general, and leads to an
 odd inconsistency. Unfortunately, I don't know enough about type checking
 to understand the problem with the positive/negative approach. If you
 can't make that work, I would personally prefer dialing back the
 inaccessible code error to a warning and being stingier about emitting it.
 Having to manually defer GHC's checking using `Dict` and such always
 struck me as unnatural and potentially fragile. If I state ex falso, I'm
 usually doing it on purpose.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12466#comment:30>
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