Template:If/errmsg/doc

The Template:If/errmsg is used to display advisory messages for either the Template:If or also for Template:Ifexpr which can process if-expressions without removing the leading or trailing spaces within then/else clauses of the logic.

Usage:  {{If/errmsg |{{{2|}}}|{{{3|}}}|{{{1|}}}|t=templatename}}

The template passes the values of parameters {1}, {2}, {3} and {t} (the template name, such as t=Ifexpr) so that they can be displayed to clue the reader into spotting which instance (of using Template:If or Template:Ifexpr) had triggered the message.