I don't think emitting a warning is feasible in most cases. The compiler (generally) doesn't know at compile time that UB definitely occurs, only that in some UB exists on some path that may or may not be reachable in theory or practice.

Usually these are not reachable in practice so warnings that these exist would cause a tidal wave of pointless warnings. Instead, the compiler simply prunes those paths which can lead to better code generation for the paths that are taken.