A lot of UB is runtime behaviour that would be difficult to statically analyse. UBSan (-fsanitize=undefined -fno-sanitize-recover) does a good job of detecting

A lot of UB is runtime behaviour that would be difficult to statically analyse. UBSan (-fsanitize=undefined -fno-sanitize-recover) does a good job of detecting it at runtime. A bunch of UB is of course static, and often comes with compiler warnings. Some UB doesn't have compiler warnings, but is found by linters and analysers (such as clang static analyzer).