This is path-insensitive purely AST-based check.
At this point I'd like to get some feedback to decide whether I should polish this a bit more (split from the path-sensitive checker?, maybe a different warning?, ...) and land it as path-insensitive checker or if I should explore the path-sensitive option.
I've started working on this to gain more experience with checkers implementation - my familiarity with path-sensitive checkers is still pretty limited. My quick naive attempt of getting SVal of DeclRefExpr and checking if its MemRegion is in current stack frame didn't get very far because the returned SVal kind is undefined. My expectation (just a wild guess really) was that local variables in the current stack would be readily available as SVal-s. I'm going to explore a bit more (hoping I won't have to add every local variable defined to ProgramState for this to work).