This seems wrong, both before and after these changes. If the destructor is virtual and not pure virtual, it must be defined regardless of whether any objects exist. I think the intent is to talk about pure virtual destructors here, in which case "any objects of that class or any derived class are created" is not the right constraint either! Merely deriving from the class with a class that has a non-pure virtual destructor is enough to (indirectly) require the pure virtual base class destructor to exist, because the derived class destructor is virtual and therefore odr-used, and it in turn odr-uses the base class destructor.

So I think instead we should say something like:

[Note: A pure virtual destructor must still be defined in most cases, because it is invoked by the destructor of any derived class.]

We shouldn't use "must" in notes, and certainly not in this way: we're not talking about an existing, external requirement here, but we're explaining the reason for the rule in [basic.def.odr].

@jensmaurer, @opensdh, any ideas for how to phrase an "explanation of a rule"? How about:

"Since a pure virtual destructor will be invoked by the destructor of a derived class, it has a definition in most cases; see [basic.def.odr]."

"Since a pure virtual destructor will be invoked by the destructor of a derived class, a definition is generally required; see [basic.def.odr]."

Another alternative would be "usually required".

@zygoloid suggested:

"Because a pure virtual destructor will be invoked by the destructor of a derived class, [basic.def.odr] requires a definition in most cases."

I'm a bit hesitant about having "requirements in notes", which we aren't allowed to, and maybe this latter form is a bit further from the forms ISO considers "requirements" than "is required" (https://www.iso.org/sites/directives/current/part2/index.xhtml#_idTextAnchor082).

How about

[Note: Even a pure virtual destructor is odr-used by any derived concrete class. — end note]

to avoid talking about any requirement at all? Otherwise, you can use statements like "… is necessary" that more clearly appear to be (mere) statements of fact.

I don't think that's quite correct, since an abstract derived class will call the destructor of a direct non-virtual abstract base class. I'm also not a fan of starting the sentence with "Even", as it implies elaboration on a previous broader statement. How about:

[Note: The destructor of a derived class will odr-use ([basic.def.odr]) the selected destructor of a direct base class -- even if that destructor is pure virtual. -- end note]

Or

[Note: A pure virtual selected destructor will be odr-used ([basic.def.odr]) by any derived classes. -- end note]

The note doesn't have to be exhaustive; having a concrete class is a sufficient but not necessary condition for the odr-use. If you like it better, though, the first version you gave seems correct with minor edits:

[Note: A definition of a destructor of a derived class odr-uses ([basic.def.odr]) the selected destructor of a direct base class, even if that destructor is pure virtual. — end note]

I don't think the second is right, since those derived classes could all have pure virtual destructors with no definition.

The second version is intended to be more concise but less precise... I'll apply your suggested changes to the first version next week :)