In the second example, the constant is propagated across expression/statement boundaries. It is likely, that this happened on IR level, rather than on the AST level.

I'd imagine the generic case becomes a non-trivial problem if you don't want to produce fluke/useless diagnostic messages.

The compiler might already be several optimization passes in at this point, variables long since replaced by chained SSA registers, when it suddenly discovers that an IR instructions produces UD. This itself might end up being eliminated in a subsequent pass or entirely depend on a condition you can't statically determine. In the general case, by the point you definitely know, there might not be enough information left to reasonably map this back to a specific point in the input code, or produce useful output why the problem happens here.