cvc-elt.4.3 defers to cos-st-derived-OK, whose clause 2.2.4 makes a type
validly derived from a union when it is validly derived from one of the
union's member types. The block check followed only the named base chain,
so when the declared type was a union and the substitute reached it through
a member by a blocked derivation method, the check found no path and stayed
silent, wrongly accepting the substitution (W3C XSTS elemT074: block on the
element is restriction and the xsi:type restricts a union member).
Record a union's direct member types during schema compilation and follow a
derivation through a member when the base chain does not reach the union.
The member clause is existential: bar the substitution only when every
confirmed derivation path to a member is blocked, so a substitute that is
itself a member (its self-membership path uses no method) or reaches one
member by an unblocked method is accepted whatever other members it reaches
by a blocked method. The check adds a rejection only on a positively
confirmed path, so an unresolved, inline-anonymous, or nested-union member
leaves it silent: a bounded, disclosed under-rejection, never a false
positive.
Drops XSTS invalid-instances-accepted 15 to 14 with valid-schemas-rejected
and valid-instances-rejected held at 0 over the full archive. An adversarial
over-rejection review caught a first cut that order-shadowed the
self-membership path; the existential form rules it out and the case is a
regression test.
Mihaela Mihaljevic
94e16b1d7d