<i>they do get a benefit from the performance gained by assuming away undefined behavior as dead cases</i><p>I think this is a highly suspicious assumption. Programmers don&#x27;t write code for no reason; they have a mental model of the current state of parameters and variables, and establish invariants as they write loops and conditionals. If you&#x27;re going to start throwing out chunks of code the programmer wrote, you need to be <i>certain</i> - not merely unsure, but <i>certain</i> - that the code is dead. Infectious undefined values that propagate through data flow to break code flow isn&#x27;t a sane programming environment.