It is not possible to fix this at the moment, because Parser::addTrackingCategory is "protected" and cannot called from the ParserFunctions extension to add the tracking category.

It is possible to copy all the checks and adding it manuell, but that sounds not so good.

Someone did it this way: http://it.wikipedia.org/w/index.php?title=Speciale%3APrefissi&prefix=Pfunc+expr&namespace=8

(In reply to comment #1)

It is not possible to fix this at the moment, because
Parser::addTrackingCategory is "protected" and cannot called from the
ParserFunctions extension to add the tracking category.

public since r108628

Only for documentation: When using #iferror around a #expr a tracking category should not be added, because that is a known error case, which also has a way to handle with ('then' of iferror)

But when having a #iferror and at another place a #expr with error on one page, the tracking category should be there.

You can already do this by adding the category to [[MediaWiki:Pfunc expr unexpected number]] (and the other expression error messages).