CREATE COLLATION definitional questions for ICU

Reading over DefineCollation, I wondered:

* Should not the FROM code path copy the old collation's version?
It seems a little bit weird that "cloning" a collation takes the
liberty of installing a new version.

* Also (and this would be a pre-existing bug), why doesn't the FROM
path copy the old collation's encoding? For example, if you attempted
to clone the "C" encoding, you wouldn't get a true clone but something
that's specific to the current DB's encoding.

* For an ICU collation, should we not insist that collcollate and
collctype be equal? If not, what does it mean for them to be different?

* Now that it's possible for user-created collations to have encoding -1,
I do not think that the "shadowing" tests in CollationCreate and
IsThereCollationInNamespace are sufficient. They don't prevent a new
collation with encoding -1 from shadowing an existing encoding-specific
collation that doesn't happen to match the current DB's encoding.
Now, you'd have to work at it for that to cause problems --- say,
create such a situation in template0 and then copy template0 specifying
that other encoding. But none of that is forbidden.

regards, tom lane