Saxon

I've added a unit test which demonstrates the bug: src/test/java/bugs/bug6117

Around June 2022, we made changes to introduce the "durability" property of node trees, classifying them as durable or transient. The idea is that in a memo function cache, we avoid having the cache hold strong links to transient nodes; we do this in effect by holding the generateId of the node as the cache key rather than the node itself (that's a simplification, there are some optimisation tweaks).

The crash occurs because some tree implementations - in this case SpaceStrippedNode - are returning null for the getDurability() property.

To fix this I suggest (a) reviewing implementations of TreeInfo to check that they return a non-null value for the property, and (b) making the call in MemoFunction.substitute() resilient to getting a null value back,

That's working fine on SaxonJ, but C# is fussy about not allowing an enumeration value to be null, so the transpilation is failing. I think I've solved this in the past by adding an extra value such as UNDEFINED to the enumeration.