Comment by Quekid5

It's definitely less likely with that technique (in practice!), but still very possible, via e.g. ODR violation or even the classic SIOF.

So, ideally you'd want each separate test case to be compiled separately, but even then you wouldn't be safe! ... because any UB in that test (or the code it's testing!) could lead to a random pass.

UB is good in some ways, but other ways it's really really bad.

EDIT: I will say: If you have a UBSAN turned on for testing, etc. you're reasonably safe... but not fully. There's a lot of stuff they don't catch because it's essentially impossible.