[GHC] #15009: Float equalities past local equalities

[GHC]

#15009: Float equalities past local equalities
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        Reporter:  goldfire          |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.4.3
       Component:  Compiler          |              Version:  8.2.2
      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 simonpj):

 Actually GHC already does this, or something very like it.  See `Note
 [When does an implication have given equalities?]` and especially `Note
 [Let-bound skolems]` in `TcSMonad`.

 I don't say that it's perfectly implemented (eg I'm a bit worried about
 whether it's sensitive to whether the constraint looks like `a ~ b` or `b
 ~ a`) but it does more or less what you way.

 Your condition (%) doesn't look right.  After all, in your example the
 constraint `a ~ b` has both `a` and `b` free -- and the former is not
 locally bound.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15009#comment:1>
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