Annex G (informative) Ill-formed, no diagnostic required [ifndr]

G.8 [class]: Classes [ifndr.class]

G.8.2[ifndr:class.virtual.pure.or.defined]
Specified in: [class.virtual]

If a virtual function that is not pure has no definition, the program is ill-formed, no diagnostic required.
[Example 1: class A { virtual void f(); }; int main() { A a; // IFNDR, virtual function that is not pure but has no definition } — end example]