The Comparator That Crashes

The C++ standard requires comp to induce a strict weak ordering, where strict means the relation is irreflexive: !comp(x, x) must hold for every x. A developer writes:

std::sort(tasks.begin(), tasks.end(),
          [](const Task &a, const Task &b) { return a.priority <= b.priority; });

reasoning that <= is "the same order, just including ties". The code passes on small test data and later crashes in production. What is the correct account?

It fails to compile, since std::sort requires a predicate that models a strict ordering

The comparator violates a precondition, so the behavior is undefined; the partition scan loses its sentinel and walks off the end of the range

The sort is still correct, it just stops being stable, so ties come out in an arbitrary order

It sorts in descending order, because <= inverts the sense of the predicate