Unreachable White Region Bug

[charlestian23]

In the second step, use of the Unreachable White Region justification is incorrectly marked as valid. It seems to be that the unreachable square rule is sensitive to existing whites in a bad way, as the second step shows that changing any square to black will still make the rule say everything is valid.

nurikabe2 - Unreachable (checks out when it should not).zip

[Bram28]

I think the bug is this: The Unreachable White Region rule is a basic rule and is probably implemented on the basis of the Can't Reach White Cell contradiction rule (wow! That is already confusing ... maybe we should call the basic rule: Unreachables Are Black). Specifically, the Unreachable White Region rule is probably coded to work as follows: assume that all the squares that the user just made black are white instead, and see if there is now a Unreachable White Region contradiction. And in this case: yes, there is! Because that existing white square that was placed in the top left in the first step is indeed Unreachable. And so therefore the Unreachable White Region basic rule checks out. .... OK, so that should be changed to: see if any of the squares that the user just made black would be unreachable if we assume they are white (i.e. it should not see if any existing white squares are unreachable).