[sv-bc] Casting to enum type with 4-state base type (4.10.4)

In 4.10.4, the SV LRM says

    "Casting to an enum type shall cause a conversion of the expression
to its base type without checking the validity of the value"

That's great if it's a 2-state base type, but if it's a 4-state
base-type wouldn't we want the result of the cast to be x's, so that x's
could propagate into any consumers of the bogus enum value?  Why else
would the user have declared the base type as 4-state?

-- Brad