A list or union simple type is derived only from xsd:anySimpleType (XSD Part 2),
so an xsi:type naming a list or union can validly stand in (cvc-elt.4.3.2.1) only
for an element whose declared type is a ur-type (anySimpleType/anyType). Naming
one on an element of a more specific atomic or complex type, such as
xsi:type="myList" on an xs:int element, is not a valid derivation and is now
rejected.
listUnionSubstitutionError flags the case; declaredAdmitsAnySimpleType suppresses
it when the declared type is a ur-type, is itself a list or union (whose inline
identity cannot be matched), or is an unresolvable reference, so the check biases
to under-reject and never over-rejects. A genuinely derived restriction
substitute, a list/union under an anySimpleType declaration, and a restriction of
a list (which keeps list variety) are all left accepted.
This widening became safe once the derivation backbone was keyed by namespaced
identity (#186); the conservative atomic and built-in not-derived cases remain
under-rejected pending the full type-identity relation.
invalid-instances-accepted 81 to 78, no false positive (valid-instances-rejected
and valid-schemas-rejected held at 0), no other bucket moved. An adversarial
review exercised a list-restriction substitute, a union declared type, ur-type
and typeReference-chain declarations, and cross-namespace list types, and found
no over-rejection.
