document-node(a|b) is the same type as document-node(a)|document-node(b)

[michaelhkay]

document-node(a|b) is the same type as document-node(a) | document-node(b) but the current subtyping rules don't say this.

Revealed by test case misc/subtyping-076

Note, this problem existed before we introduced document-node(X), the same is true of the expansion:

document-node(element(a|b)) is the same type as document-node(element(a)) | document-node(element(b))

[michaelhkay]

In fact there are other cases where A | B ⊆ C is false despite the value space of A | B being contained within the value space of C, and we identify some of these cases in §3.3.2.4. This is just another example, which we could add to the notes in §3.3.2.4.

Under the current rules document-node(a) | document-node(b) ⊆ document-node(a|b) is true, but the converse document-node(a|b) ⊆ document-node(a) | document-node(b) is not. The same applies to element(a|b) ⊆ element(a) | element(b).