C++ standard library UB "that works in practice" in Qt

I've found a couple of places where we have "library UB" (precondition violations).

I don't think there's anything that we need to do, strictly speaking, just something worth documenting.

1. In qjsonobject.cpp and qjsonparsers.cpp we use algorithms (std::lower_bound and std::stable_sort) on iterators that are not forward iterators. For instance

template<typename String>
static qsizetype indexOf(const QExplicitlySharedDataPointer<QCborContainerPrivate> &o,
                         String key, bool *keyExists)
{
    const auto begin = QJsonPrivate::ConstKeyIterator(o->elements.constBegin());
    const auto end = QJsonPrivate::ConstKeyIterator(o->elements.constEnd());

    const auto it = std::lower_bound(
                begin, end, key,
                [&](const QJsonPrivate::ConstKeyIterator::value_type &e, const String &key) {
        return o->stringCompareElement(e.key(), key) < 0;
    });

    *keyExists = (it != end) && o->stringEqualsElement((*it).key(), key);
    return it.it - begin.it;
}

QJson iterators are only input, not forward, because dereferencing them gives back proxy types and not actual references, violating https://eel.is/c++draft/forward.iterators#1.3

if X is a mutable iterator, reference is a reference to T; if X is a constant iterator, reference is a reference to const T

2. In QLalr there's a Node type which has a std::set<Node> as a member. That's illegal because std::set requires its value type to be a complete type (only vector, list, forward_list have support for incomplete types).