swift

4c7e13d3 - [region-isolation] Fix join to be a true union operation including the symmetric difference.

[region-isolation] Fix join to be a true union operation including the symmetric difference. We were performing a union on the intersection of the lhs/rhs but were dropping the parts of lhs/rhs that were in the symmetric difference of the two sets. Without this, we would not diagnose cases like this where we had elements on the lhs/rhs that were not in the intersection. ``` var closure: () -> () = {} await transferToMain(closure) if await booleanFlag { closure = { print(self.klass) } } else { closure = {} } // At this point we would lose closure since they were different elements await transferToMain(closure) // We wouldn't error on this! ``` rdar://117437059