[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:  closed
        Priority:  normal            |            Milestone:  8.4.3
       Component:  Compiler          |              Version:  8.2.2
      Resolution:  fixed             |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:  gadt/T15009
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 >  How is it that a_a2do occurs both as a_a2do[sk:2] and also as
 a_a2do[tau:2] in this implication tree?

 I strongly suspect that `a_a2do` started life as a unification variable
 `[tau:2]`, but we ended up generalising over it, so we skolemised it to
 `a_a2do`.   That is we unify `a_a2do := a_a2do[sk]`.   I believe that the
 implication tree is being displayed here without zonking; that is, without
 taking account of unifications.

 I'd be shocked if we had both, but ''without'' having `a_a2do[tau]` being
 unified with `a_a2do[sk]`.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15009#comment:17>
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