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I don't see how this could work. Isn't loadTables() called before mGroups is filled?
loadTables() called after mGroups is filled. I have tested it on my own MediaWiki server (and passed) before I commit it.
Only because of luck, not good design. If any extension calls $wgParser->parse() before the primary wikitext is parsed, this will break. It's broken with memcached enabled, because the current set of group conversion tables is saved into memcached, merged with the default tables, and then used for every page regardless of the groups that that page defines. So it won't work on Wikimedia.