It's not really surprising that invalid code doesn't go along with compiler optimizations.

The correct way to deal with this here is probably either _NOT_ using nullptr, and instead using a special null object (similar to the end iterator); or using pointers instead of references (because you really want pointer behaviour).

Their fix is insane, and not something I'd want in production code. Unless you carefully vet your code and compiler version, you should never rely on undefined behaviour.

(There are some cases in the c standard that technically undefined but all compiler vendors have essentially agreed to handle in the same way; so those are fine. IIRC unions are a common example.)