Upstreaming: Use greatestLowerBound in CFAbstractValue about checker-framework HOT 3 CLOSED

(But presumably that has some performance cost. So maybe upstreaming even a modified patch is not a slam dunk.)

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(I may have overstated that: The usual implementation of greatestLowerBound is primarily just some Map lookups -- not free, but probably not the most expensive part of dataflow.)

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I experimented with a possible alternative to this: I could ensure that our CFAbstractValue instances always use the CFAnalysis (and therefore the NullSpecAnnotatedTypeFactory, etc.) from the least convenient world.

(I had some problems with getting everything initialized in the right order. The approach I settled on was to override createFlowAnalysis to return a CFAnalysis subclass that overrides createAbstractValue to call defaultCreateAbstractValue but to pass withLeastConvenientWorld().analysis instead of this. That seemed like it was at least avoiding throwing exceptions, but I didn't test without my CF change to see if it rendered the CF change unnecessary. (I had been trying to fix a different problem, and my createFlowAnalysis hack didn't help with that.))

I don't know if anything could go wrong as a result of such a change, but I wouldn't be shocked if it violated some assumption somewhere. Then again, maybe even my change to CF itself can violate some assumption somewhere :)

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