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Make sure that adding a path condition clears negative caches. #2549
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Release note: none
When a new path condition is added to an existing path conditions object, the negative caches must be cleared since the new condition might allow previously failed implications to now succeed. This was already done in one case, but a few others were missed. The clearing code is now centralized and all cases go through it.
While debugging the internal test case that failed because of the caching mistake, I also realized that PathConditions.implies and PathConditions.impliesNot can profitably deconstruct expressions of the form !x before doing the implication check. This also has the advantage that it eliminates a subtle endless recursion situation in a nicer way than the current code.
I've also added some comments in places that caught my eye during debugging.