[sv-ec] Associative array with enum index

If the index type of an associative array is an enumeration type, and an
expression not equivalent to the enum type is used instead, such as
integer, is that an error?  I hope so and that the expression would need
to be cast.

If an expression is cast to the enumeration type, but does not have a
valid value for that enumeration type, is this treated as in 5.9.6?
That is, return default initial value on read and do nothing on write?

-- Brad