Is it undefined behaviour to change sort order while sorting?

Question

Imagine the following scenario:

std::atomic<int>  values[10];
std::size_t      indices[10];

void sort() {
    std::iota(indices, indices+10, 0);

    std::sort(indices, indices+10,
        [&](size_t lhs, size_t rhs) { return values[lhs] < values[rhs]; });
}

While running the sort(), another thread is changing values. Will this merely result in indices not being correctly ordered afterwards, or is it actually undefined behaviour?

Answer 1

Probably (see below) it's undefined behavior; in practice, I've seen plain crashes (for out of container boundary access) even just for incorrect (=not inducing a total ordering) comparators, and < with indexes changed while sorting surely fails to induce a total ordering

Interestingly, the standard doesn't explicitly mention undefined behavior regarding this issue; C++11 §5.4 ¶3 just states that

For algorithms other than those described in 25.4.3 to work correctly, comp has to induce a strict weak ordering on the values.

and I fail to see a formal definition of "work correctly" looking around there; even just the word "undefined" isn't ever uttered in the whole chapter 25 (which describes <algorithm>).