It turns out that aliases are defined as class-independent.
Oh, it's worse than that.
$ fgrep -i "class independent" rfc????.txt
rfc3845.txt: The NSEC RR RDATA format is class independent and defined for all
rfc4025.txt: This resource record [IPSECKEY] is class independent.
rfc4034.txt: The DNSKEY RR is class independent.
rfc4034.txt: The RRSIG RR is class independent.
rfc4034.txt: The NSEC RR is class independent.
rfc4034.txt: The DS resource record is class independent.
rfc5155.txt: The NSEC3 RR RDATA format is class independent and is described
rfc5155.txt: The NSEC3PARAM RR RDATA format is class independent and is
described
rfc6698.txt: The TLSA RR is class independent.
rfc6742.txt: The NID RR is class independent.
rfc6742.txt: The L32 RR is class independent.
rfc6742.txt: The L64 RR is class independent.
rfc6742.txt: The LP RR is class independent.
rfc7043.txt: The EUI48 RR is class independent.
rfc7043.txt: The EUI64 RR is class independent.
rfc7553.txt: The URI resource record is class independent.
I think this means that all of the other RR's are only valid in class
IN, but I don't really know.
R's,
John