Saxon

I meant "$a || $b"

Are $a and $b declared as xs:boolean? I guess if we know that then an advisory warning is possible - as you say, no-one is likely to concatenate two booleans. It's not possible with other expressions like @a || @b -- that's a perfectly sensible concatenation operation, and if you intended a boolean "or", there's not much we can do about it.

I don't recall why string concatenation does strong casting of its operands to strings, unlike other functions and operators. Presumably it's because of expectations set by other languages.

In my case both variables were declared as xs:boolean, so expression turned to 'falsefalse', with effective boolean value true().

I've added the warning for the next release - if all arguments of a call to concat() are statically known to be singleton booleans. (a || b gets turned into a call on concat() long before type-checking can happen, so we can't distinguish then between a||b and concat(a, b)).