This is a necessary consequence (in the formal logic sense) of the "principle of explosion"[1] for optimizing compilers. If the compiler assumes some undefined

This is a necessary consequence (in the formal logic sense) of the "principle of explosion"[1] for optimizing compilers. If the compiler assumes some undefined behavior does not occur, but in fact that undefined behavior does occur, all of conclusions it makes which depend, directly or indirectly, on the contradicted assumption are invalid. Since, in the process of optimization, optimizing compilers make conclusions about parts of a program based on other parts of the program which may be executed later, undefined behavior in one part of a program causes invalid conclusions about other parts which may be executed earlier. Therefore undefined behavior cannot be contained to just parts of a program executed after a particular part, if the compiler tries to optimize parts of the program based on parts that may be executed later. [1]: https://en.wikipedia.org/wiki/Principle_of_explosion https://en.wikipedia.org/wiki/Principle_of_explosion