Is something undefined behavior by omission?

Question

The standard says:

1.3.24                                                          [defns.undefined]
undefined behavior         


        

Answer 1

The last sentence of the note you quoted explains the may in its first sentence:

Many erroneous program constructs do not engender undefined behavior; they are required to be diagnosed.

Thus, may is quite correct.

Anything not defined is undefined, though there are many ways it can be defined (even if it is only slightly constrained).

Also, as notes are not normative, they cannot define behavior.