Saxon

The constructor for SequenceType has this logic explicitly:

public SequenceType(ItemType primaryType, int cardinality) {
        this.primaryType = primaryType;
        if (primaryType instanceof ErrorType) {
            this.cardinality = StaticProperty.EMPTY;
        } else {
            this.cardinality = cardinality;
        }
    }

It feels wrong, but we need to see if anything breaks when we change it.

It also feels wrong that ErrorType extends NodeTest. Perhaps this is because at one time we implemented empty-sequence() as having an item type of ErrorType? We need to think carefully about the consequences of making changes here.

Maybe also worth adding that:

You can also modify the query so the parameter has one-or-mode cardinality e.g. "$x as xs:error+", and it will still evaluate with an empty sequence as the argument.

e.g. the following still evaluates (note the +):

declare function local:test1($x as xs:error+) { }; local:test1(())

QT3 actually contains quite a number of similar tests cases, but it doesn't include the case of supplying an empty sequence to a function that expects xs:error. I've added this as test xs-error-054.

Changing the constructor described in comment #1 leads to a test failure in xs-error-007. This test does

xs:error#1 instance of function(xs:anyAtomicType?) as empty-sequence()

and expects "true".

The F+O spec says the signature of xs:error#1 is xs:error($arg as xs:anyAtomicType?) as xs:error?

Now, §2.5.6.1 in the XPath book says that xs:error? is "treated in the same way" as empty-sequence() as regards the subtype relationship, and I guess it's the subtype relationship that matters here.

I get the correct results on all the xs:error tests (including the new one) if I change the SequenceType constructor to

public SequenceType(ItemType primaryType, int cardinality) {
        this.primaryType = primaryType;
        if (primaryType instanceof ErrorType && Cardinality.allowsZero(cardinality)) {
            this.cardinality = StaticProperty.EMPTY;
        } else {
            this.cardinality = cardinality;
        }
    }

which is basically as it was before except that it doesn't permit an empty sequence as an instance of xs:error or xs:error+.

This is passing all tests so I'll run with it.