Violating ODR doesn't introduce UB it's IFNDR, Ill-formed No Diagnostic Required which is much worse in principle and in such cases probably also in practice.

Violating ODR doesn't introduce UB it's IFNDR, Ill-formed No Diagnostic Required which is much worse in principle and in such cases probably also in practice. UB is a runtime phenemenon, it happens, or it doesn't, and we may be able to ensure the case where it happens doesn't occur with ordinary human controls. But IFNDR is a property of the compiled program, if you have IFNDR (by some estimates that's most C++ programs) your program has no defined behaviour and never did, so there is no possible countermeasure, too bad game over.