An attribute or element whose type is or derives from xs:ID may not carry a
default or fixed value (a-props-correct.3 / e-props-correct.5). Such schemas
were accepted.
The check runs after named simple types are resolved, so a named user type
derived from xs:ID is recognized (not just the built-in and inline forms). The
type reference's namespace is resolved from its prefix: only an ID in the XSD
namespace is the built-in, and a user type is ID only when it resolves to one
whose base is xs:ID. An adversarial critic caught a namespace-blind over-
rejection (a user type named ID, or an imported foo:ID, mistaken for the
built-in); the namespace resolution fixes it, with a regression test.
XSTS: invalid schemas accepted 489 to 481 (attZ013a/b, elemZ032a/b,
valueconstraint01001m2/m3/m5/m6 now rejected), no other bucket raised.