ayg wrote:

{{PAGENAME}} means "return full page name unconditionally", {{SUBPAGENAME}} means "return subpage name if this is a subpage, otherwise return full page name". Some expected behavior is not necessarily going to work if the namespace doesn't support subpages. Not sure this is a good idea -- if you're moving pages to a namespace without subpages, you should either rename the (ex-)subpages to something that doesn't use subpage syntax, or enable subpages in the namespace.

It means that only because that's the way it has been coded. The name is {{SUBPAGENAME}} not {{SUBPAGENAMEIFNSALLOWSTHEM}} ;)
Bug 5167 just asks for a magic word to differentiate between page levels.

Either subpages exist in the namespace, or they don't. They need to *not* be broken so pages like [[OS/2]] don't think they're subpages of [[OS]].