Saxon

I'll look into it. When xsl:message affects the outcome it's usually because the presence of the xsl:message disables an optimization, and the optimization is incorrect.

I haven't got to the bottom of this -- it's pretty complex -- but because base-uri() is effectively implemented as a memo function, the call on base-uri() within the "extra" xsl:message iinstruction in the "pass" stylesheet is causing data to be held on the containing document node, which somehow has the consequence that the (different) document node referenced in the "offending" call on base-uri() has a non-null base-uri property, whereas in the "fail" case that property is empty.

I've changed the "fail" case to output a message at the point where the "pass" case does: this message outputs "MBASE"+base-uri() in the pass case, and simply "MBASE" in the fail case.

Doing side-by-side debugging, at the point where we hit the xsl:message, the context node in both cases seems identical. In particular, the document node at the root of the tree has a correct non-null baseURI property.

We then step on to the first call on base-uri(), which is within the MBASE message for the pass case, but some time later in the fail case. We're calling base-uri() on the same node (the first element child of the document), but in the pass case the document node still has a correct non-null baseURI property, while in the fail case the baseURI property is now null.

If we look more carefully at the fail case, on entry to the base-uri() function, the element node has a parent node which is TinyDocumentImpl#4864, with a base URI of null, while the containing tree has a root node which is TinyDocumentImpl#4571, with a correct non-null baseURI property.

It's not intrinsically wrong that we have multiple Java objects representing the same XDM node, because these are "flyweight" objects created transiently on demand. But it's wrong that they should have different properties!

So the essence of the problem is that the two calls on base-uri() have used slightly different routes to get to the information and have found different properties. Since base-uri() is a memo function, the value found by the first call for any node is the one that matters.

Classic Java problem: TinyTree.java has a private property root, but it inherits from GenericTreeInfo which also has a private property root, and the two are out of sync.

Drop TinyTree.root and make GenericTreeInfo.root protected, and the problem is solved.

While we're about it, DominoTree also inherits from GenericTreeInfo and has the same problem.

Further fix committed (without testing).