A QLabel can be set to label another widget using QLabel::setBuddy.
This not only allows moving keyboard focus to the buddy widget by pressing the label's accelerator key, but also sets the accessible labelled-by relation (e.g. ATSPI_RELATION_LABELLED_BY for Linux/AT-SPI: https://gnome.pages.gitlab.gnome.org/at-spi2-core/libatspi/enum.RelationType.html#labelled_by for the buddy widget's accessible object. This is e.g. needed for screen readers to announce the label text when the buddy widget receives focus.

However, the accessible relation is currently only set if the buddy is a sibling of the label, not otherwise.

It should also work when the label and its buddy are not siblings.

Sample steps to reproduce on Linux:

1. Compile and run the sample program below
2. start Accerciser and select the line edit's accessible object in Accerciser's treeview of the sample program's a11y tree
3) In Accerciser's Interface Viewer, check the relations for the selected object
-> None are shown
4) to double-check: query the relations using Accerciser's IPython console:

In [1]: acc.get_relation_set()
Out[1]: []

Actual result:

No relations are visible in steps 3 or 4.

Expected result:
A labelled-by relation should exist to the "My Label" label, as that one has the line edit set as its mnemonic widget.
As mentioned before, this works if both widgets are siblings.

Sample application:

#include <QApplication>
#include <QMainWindow>
#include <QVBoxLayout>
#include <QLabel>
#include <QLineEdit>

int main(int argc, char* argv[])
{
    QApplication a(argc, argv);
    QMainWindow w;
    QWidget* widget = new QWidget(&w);
    w.setCentralWidget(widget);

    QVBoxLayout* layout1 = new QVBoxLayout(widget);

    QWidget* widget2 = new QWidget(widget);
    layout1->addWidget(widget2);
    QVBoxLayout* layout2 = new QVBoxLayout(widget2);
    QLabel* label = new QLabel("My Label", widget);
    layout2->addWidget(label);

    QWidget* widget3 = new QWidget(widget);
    layout1->addWidget(widget3);
    QVBoxLayout* layout3 = new QVBoxLayout(widget3);
    QLineEdit* lineEdit = new QLineEdit(widget3);
    layout3->addWidget(lineEdit);

    label->setBuddy(lineEdit);

    w.show();
    return a.exec();
}