The text under "3.6 String Concatenation Expressions" states:

"String concatenation expressions allow the string representations of values to be concatenated. In XPath 3.1, $a || $b is equivalent to fn:concat($a, $b)."

In similar cases where we refer to a certain function in F&O from XPath we usually do two things:

* Link to that function, to ensure it is clear the F&O 3.1 version is meant
* Add a sentence something like "the same error conditions as for fn:concat" apply to this expression

Also, we don't explain in depth how a StringConcatExpr is translated to an fn:concat function, but I guess this leads little wiggle room in interpretation anyway.

I noticed that "||" is left-associative, and that "," (for function arguments) is marked as "neither associative". This may lead to evaluation-order differences in rare cases, if "||" gets directly translated to fn:concat (though I don't have an example of this, I just noticed the difference).

In the section "A.4 Precedence Order (Non-Normative)", it says that for "neither" the associativity is inconsequential, but for "||" is is.

Perhaps the proper way of interpreting this text is to see chained "||" as nested fn:concat#2 calls to ensure its left-associativity.