The definition logically implies that compilers are allowed to assume UB doesn't happen. The definition is:

> undefined behavior: behavior, upon use of a nonportable or erroneous program construct or of erroneous data, for which this International Standard imposes no requirements.

With these optimisations, either:

1. The program contains no UB, the transformed code acts as expected, and the compilation is valid.

2. There's UB; the standard imposes no requirements on what behaviour that translates to; and so the transformed code is valid, regardless of what it does.