Can you provide examples of UB that could be unspecified behaviour, where UB does anything other than cause harm?

Dereferencing a pointer to memory that doesn't point to a live valid object is undefined behaviour. You couldn't possibly specify what this does without instrumenting every single access to memory, like asan or valgrind does.

As I said, behaviour that could be unspecified rather than undefined.

This is an example of something that cannot be anything other than UB.

https://www.youtube.com/watch?v=yG1OZ69H_-o is a great talk on this

> C++ does not add runtime overhead to validate that programs are well-formed.

You have succinctly described the problem.

That's not a problem, that's a core policy of that language's design.

If you want a language that is slow, there are plenty of options.