A :ref: to a label outside a section renders an empty link

AnchorReferenceResolver fills a reference that has no children with $target->getTitle() ?? ''. A target only carries a title when it is a section, or an anchor whose parent in the shadow tree is a SectionNode — CollectLinkTargetsTransformer sets it from $parentSection->getTitle() and leaves it null otherwise. A label declared anywhere else therefore resolves to a link with no text at all.

Minimal reproduction:

..  note::

    .. _nested-label:

    A label declared inside a directive body.

A plain reference to it: :ref:`nested-label`

Rendered with this repository's own default templates at c9868443:

<p>A plain reference to it: <a href="/index.html#nested-label"></a></p>

A resolved link, a correct href, and nothing between the tags. Nothing is logged, because from the resolver's point of view the reference resolved fine. The same page with the label on a section renders <a href="/index.html#section-label">A section with a label</a>, so it is specifically the titleless target that produces the empty link.

Two consequences. For a reader the link is invisible; for a screen reader it has no accessible name at all, which fails WCAG 2.4.4 Link Purpose (In Context) and 4.1.2 Name, Role, Value, both Level A. And since it happens only for labels outside a section, it tends to appear in exactly the places an author reached for a label deliberately.

InterlinkReferenceResolver has the same shape but cannot hit it: InventoryLink::$title is a non-nullable string.

The fix I would suggest is to fall back to the reference the author wrote rather than to the empty string. PR: #1395.

Start with AnchorReferenceResolver and CollectLinkTargetsTransformer, then reproduce the nested-label example using the repository's default templates. Verify that a titleless target still gets the correct href but uses the written reference as visible and accessible link text, and add or run a regression test for the rendered link.