More than month has passed since we have reported a problem to the saxon forum (see Saxon optimizer bug and Saxon 9.2 generate-id() bug).

The essence of the problem is that we have constructed argumentless function to return a unique identifiers each time function is called. To achieve the effect we have created a temporary node and returned its generate-id() value.

Such a function is nondetermenistic, as we cannot state that its result depends on arguments only. This means that engine's optimizer is not free to reorder calls to such a function. That's what happens in Saxon 9.2, and Saxon 9.3 where engine elevates function call out of cycle thus producing invalid results.

Michael Kay, the author of the Saxon engine, argued that this is "a gray area of the xslt spec":

If the spec were stricter about defining exactly when you can rely on identity-dependent operations then I would be obliged to follow it, but I think it's probably deliberate that it currently allows implementations some latitude, effectively signalling to users that they should avoid depending on this aspect of the behaviour.

He adviced to raise a bug in the w3c bugzilla to resolve the issue. In the end two related bugs have been raised:

• Bug 13494 - Node uniqueness returned from XSLT function;
• Bug 13747 - [XPath 3.0] Determinism of expressions returning constructed nodes.

Yesterday, the WG has resolved the issue:

The Working Group agreed that default behavior should continue to require these nodes to be constructed with unique IDs. We believe that this is the kind of thing implementations can do with annotations or declaration options, and it would be best to get implementation experience with this before standardizing.

This means that the technique we used to generate unique identifiers is correct and the behaviour is well defined.

The only problem is to wait when Saxon will fix its behaviour accordingly.