TYPO3 Core

After checking the code it is clear to me why that happens:
typo3/sysext/frontend/Classes/ContentObject/Menu/MenuContentObjectFactory.php::getMenuObjectByType($type) expects $type to be a menu type name like TMENU, but in case of a reference this will be '< lib.....'.

Reference support has to be implemented into
typo3/sysext/frontend/Classes/ContentObject/HierarchicalMenuContentObject.php
before calling the MenuContentObjectFactory.

Unfortunately I don't know the TS API to handle such references, so somebody else needs to take a look.

References only work on content object level, nowhere else.

So this works:

lib.foo = TEXT
lib.foo.value = Bar

page.10 =< lib.foo

This does not work:

lib.foo = TEXT
lib.foo.value = Bar

page.10 = TEXT
page.10.value =< lib.foo.value

The reason for that is, that the implementation of references is only done in ContentObjectRenderer::cObjGetSingle() and the only case that is checked there is if the first level is a reference.

A TMENU is not a content object, but a menu object, thus it does not work there, same as it does not work in any other place in TS (except top level content objects).

Implementing this now only for menu objects might be possible at cost of performance (TS-Parser needs to be triggered) and at cost of consistency.
Implementing this on a parser level would be consistent, but I'm not sure if this is really doable with the current parser code.

So what should we do with this one?

Thanks Helmut for clarifying this.
I agree that we should strive for consistency first place!

Moreover the menu generation is already really slow, so don't slow it down even further.

General feature request/evaluation in #63894.