It's more complicated than this. C++ compilers can't just "reject" UB code - they don't have enough information at all times to prove that code invokes UB, the languages is not powerful enough to represent this in all cases.
When people say "just fix this in compilers, don't compile when they run into UB" it's a fundamental misunderstanding of the problem. Yes, in some cases the compiler can prove UB, and uses it to write optimiations, but it doesn't prove that all code is not invoking UB.
I suppose that's true, yes. My point was more that your compiler is not hitting a line of code and saying "I know this is or is not UB" because it can not do that in the general case - it is definitely incorrect to say it is 'proving' this. Would you consider that an accurate representation?
> My point was more that your compiler is not hitting a line of code and saying "I know this is or is not UB" because it can not do that in the general case
I think it's still a (cause of) misunderstanding. For the compiler, UBs are situations which axiomatically can not occur. "This is UB" is not a concept, because the compiler assumes at all point that UBs can not occur.
The compiler doesn't go "oh you're dereferencing a pointer which may be null, fuck you", it goes "you're dereferencing a pointer so it can't be null, and thus I can remove anything assuming possible nullability".
> The compiler doesn't go "oh you're dereferencing a pointer which may be null, fuck you", it goes "you're dereferencing a pointer so it can't be null, and thus I can remove anything assuming possible nullability".
When people say "just fix this in compilers, don't compile when they run into UB" it's a fundamental misunderstanding of the problem. Yes, in some cases the compiler can prove UB, and uses it to write optimiations, but it doesn't prove that all code is not invoking UB.