I’m assuming you are referring to this part:

> If the underlying type is not fixed and the source value is out of range, the behavior is undefined.

Note the fine print about the meaning of ”out of range”:

> (The source value, as converted to the enumeration's underlying type if floating-point, is in range if it would fit in the smallest bit field large enough to hold all enumerators of the target enumeration.)

So this is not undefined:

  enum E { A = 0, B = 1, C = 2 };
  E valid = static_cast<E>(3);