Saxon

During type-checking, Atomizer.computeSingleValued() has wrongly concluded that the result of data(DIMENSIONS) will be a singleton, and is therefore passing a single number to the XPath JAXP extension function, which crashes because it expects a list.

Saxon 11 is making the same incorrect inference (the code is unchanged), but isn't making use of the information at run-time. The new expression elaboration code does make use of the information. Specifically, in 11.x, Atomizer.oneToOne is computed as false, but Atomizer.getAtomizingIterator() ignores this and processes the value as a sequence regardless. In 12.x, AtomizerElaborator.lazily() constructs return new LearningEvaluator(expr, new MemoClosureEvaluator(expr, elaborateForPull()));, and the MemoClosureEvaluator calls Cardinality.allowsMany() on the Atomizer, which thinks it will return a singleton.

The incorrect logic is in Atomizer.computeSingleValued() where it does:

if ( .... || st.isComplexType() && st != AnyType.getInstance())) {
   singleValued = true;
}

In this case st is a complex type with simple content, and the simple content type is a list type, so the atomised result is sequence-valued.