The compiler doesn't do anything here that is hard to understand- its approximate solution works purely in terms of the control flow graph, so there is a very c

The compiler doesn't do anything here that is hard to understand- its approximate solution works purely in terms of the control flow graph, so there is a very clear and bright dividing line between "you have a use of this variable here" and not, which is never impacted by anything subtle about the values of conditions or whatever. The closest to head-scratching that I have ever seen here are the sorts of errors that include "note: value may be accessed in another iteration of this loop," or perhaps the corner cases that Polonius aims to solve if you count those.