XPath 3 has introduced a syntactic sugar for a string concatenation, so following:

concat($a, $b)

can be now written as:

$a || $b

This is nice addition, except when you run into a trouble. Being rooted in C world we unintentionally have written a following xslt code:

<xsl:if test="$a || $b">
...
</xsl:if>

Clearly, we intended to write $a or $b. In contrast $a || $b is evaluated as concat($a, $b). If both variables are false() we get 'falsefalse' outcome, which has effective boolean value true(). This means that test condition of xsl:if is always true().

What can be done to avoid such unfortunate typo, which is manifested in no way neither during compilation nor during runtime?

The answer is to issue informational message during the compilation, e.g. if result of || operator is converted to a boolean, and if its arguments are booleans also then chances are high this is typo, and not intentional expression.

We adviced to implement such message in the saxon processor (see https://saxonica.plan.io/boards/3/topics/7305).