So what about future cases of preprocessor breakage? When users type <code>-{</code> in template arguments or in an URL without the intention to start a "language converter" construct, will they get an error message? If not, and if the result is not what the user expects, then how are they supposed to "debug" this? Bdijkstra (talk) 11:49, 9 May 2017 (UTC)
Topic on Talk:Parsoid/Language conversion/Preprocessor fixups
My 2 cents: Code gets deprecated and breaks up all the time. I expect that any documentation where that syntax is mentioned labels it clearly as invalid, suggests workarounds etc. Tech-savvy people like template editors should also be aware, so they can tell others and fix future problems.
If they use VE, it will get auto-wiki'ed automatically. For wikitext editors, the intent is to add a parser warning (php side) or automatic linter rule (parsoid side) to prevent regressions in the future. Unfortunately, there's a chicken-and-egg issue, in that I can't add the warning until the PHP/Parsoid code is merged, and we're currently trying to clean up as many cases as possible *before merge* so that things don't break too much.
Parsoid patch isn't quite merged yet ( https://gerrit.wikimedia.org/r/140235 ), but when it is it will protect language converter markup when used inadvertently in VE. I'm going to look at a linter warning and/or a Parser warning to flag these for source editors.