If doesn't matter what 'p' is in their example. The point is: if 'f' is undefined behavior (rather than just impl-defined), then the optimizer concludes that the "if p { f() }" can never happen... which means that we're allowed to assume that 'if p { ... } else { ... }' (in the first part of the example) will always take the else branch. The compiler will optimize accordingly and just always call g() unconditionally.