Doesn't need to be UB, you can write expressions like: "some_s8_var < some_u32_var" and people will be had. Note that is not the same as "some_s8_var < some_u8_var".

-Wextra catches stuff like this, alas I know of a few people that think "-Wextra is evil" (even though annoying warnings can be selectively disabled)