xdg-shell: clarify whether an xdg_popup's parent must have a role

Assuming the case when a non-null xdg_surface is passed to xdg_surface.get_popup as parent, is it mandatory for it to have a role? If not, it's possible to create an xdg_popup loop, e.g. https://gitlab.freedesktop.org/vyivel/randfall/-/blob/main/cases/xdg_popup_parent_cycle.c

In other words, is the following sequence legal:

<...>.get_popup(<...>, parent, <...>)
parent.get_toplevel(<...>)