They might be able to get the compiler to accept their code, but that's not a good bar for competence in C. I personally wouldn't consider someone competent in

They might be able to get the compiler to accept their code, but that's not a good bar for competence in C. I personally wouldn't consider someone competent in it unless they're deeply familiar with UB and other language warts because it's too dangerous to do otherwise. UB can have nonlocal effects and "time travel" so any UB anywhere in the program legally invalidates the entire program, regardless of whether it would otherwise work. Most other languages have (wisely) decided not to carry that legacy forward. As we all know, the subject of UB and the practical ramifications of particular instances is a topic few people could reasonably learn in weeks or even months.