Am I correct in thinking that {{subst:CURRENTMONTH}} and {{subst:CURRENTYEAR}} not being substituded in this edit is due to this bug?

Yes, substituting within refs won't work if this bug is not resolved!

@Thryduulf use {{#tag:ref|}} or {{subst:#tag:ref|}} and it will work

and I would love to see this added to the projects for fixing that are coming up. It is truly time that this got resolved for all wikis.

@Billinghurst add an entry to https://meta.wikimedia.org/wiki/2017_Community_Wishlist_Survey (I guess it would fit in the editing section?).

(Notice: Proposed to Community-Wishlist-Survey-2017 by @Bdijkstra)

Not sure why this needs triage now. It's a long standing broken part of the system, which should have been fixed a long time ago. If the foundation can't fix it in 14 years, and with, I suppose, about $1bn to spent, I despair of having anythung useful done. Certainly lowering the priority won't help, and raising it seems unlikely to either.

TOKEN FOR RICH!

@Rich_Farmbrough: Please bring up meta-level discussions on priorities or who should spend money on what in a better suited place. Also see https://www.mediawiki.org/wiki/Bug_management/Development_prioritization about prioritization and wrong expectations. Thanks for your understanding.

@Aklapper You refer @Rich_Farmbrough to https://www.mediawiki.org/wiki/Bug_management/Development_prioritization - but that's a very long essay with multiple claims. Which of them do you think applies to this bug?

"Hence there are many tasks which will stay unresolved forever. It is unrealistic to expect all tasks to ever get fixed.", perhaps?

Or perhaps Rich has seen it, and has noted that it suggests "sometimes the best bet is ... to convince someone else (maintainers, developers, external developers) to write that software patch."

More generally, please be specific, not vague, just as you enjoin others to be

@Pigsonthewing: I'm not sure if I get the question correctly, sorry... I replied to a comment mentioning "should have been a fixed long time ago" and "Priority".
Hence all of that page applies to this ticket, and for example also the two sentences that you quoted.

The whole page cannot apply, since the two quotes I gave involve mutually exclusive outcomes.

Here's another possible use case that is blocked by this bug.

This problem has just been raised again in a discussion at https://en.wikipedia.org/wiki/Wikipedia:Administrators%27_noticeboard/Incidents#Srich32977_and_FAITACCOMPLI where the subst'ed template "Format ISBN" is seen as a solution to a problem ... but does not, currently, work within <ref> tags. So I add my voice here to urge that this problem should be fixed ASAP for the benefit of readers and editors.

Change 981193 had a related patch set uploaded (by Thomas-topway-it; author: Thomas-topway-it):

[mediawiki/core@REL1_39] parsing of nested tabs with 'parse-wikitext' attribute, see phabricator T4700 and T3310

https://gerrit.wikimedia.org/r/981193

hello, I have created the following patch https://gerrit.wikimedia.org/r/c/mediawiki/core/+/981193 that may solve the issue without modifying the parser

see also https://phabricator.wikimedia.org/T3310
and https://wikisphere.org/wiki/Template:Page_upload
for more information (I will post a pure-mediawiki example soon)

Once resolved, please correct/remove the mention of the bug in the lede of Wikipedia:Substitution. Thank you!