[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 nfrisby):

 If I correctly understand what the "inert substitution" is, then I think I
 can summarize the intent of my draft rule as follows.

 > Let dis[i] be the domain of the inert substitution active in the wanteds
 of TcLevel i. We can safely float a wanted CTyEqCan {{{w : alpha[L] ~
 <t>}}} from level K to level L if dis[K] can never change (which implies
 dis[L] can never change -- ie no CTyEqCan might flip to reveal a new LHS)
 and if dis[K] - dis[L] does not include any metavars, any flattening
 skolems, or any skolems of level <= L.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15009#comment:21>
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