[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:  8.0.2
       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 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?

 Currently GHC tries to do co/contra checking for subsumption checking. If
 we just used equality on the LHS of arrow, I think the negative/positive
 problem would disappear.  Which is an intesting strike in favour of that
 approach.  That would mean that this would fail:
 {{{
 f :: (Int -> Int) -> Int

 g :: (forall a. a-> a) -> Int
 g = f
 }}}
 On the other hand, with the same signatures
 {{{
 g x = f x
 }}}
 would succeed.  The current co/contra subsumption is clever, but tricky...
 eg every foray into impredicative polymorphism gives up on it.  Maybe we
 should give up on it now.

 Simon

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12466#comment:26>
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