Spliced together from reflector messages c++std-lib-8294 and -8295:

18.1, paragraph 5, reads: "The macro offsetof accepts a restricted set of type arguments in this International Standard. type shall be a POD structure or a POD union (clause 9). The result of applying the offsetof macro to a field that is a static data member or a function member is undefined."

For the POD requirement, it doesn't say "no diagnostic required" or "undefined behavior". I read [intro.compliance], paragraph 1, to mean that a diagnostic is required. It's not clear whether this requirement was intended. While it's possible to provide such a diagnostic, the extra complication doesn't seem to add any value.