Check constraints on non-immutable keys

We currently allow this:

postgres=# create table t(a timestamptz not null primary key, check(a > now()));
NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index
"t_pkey" for table "t"
CREATE TABLE


Which seems very wrong. For one thing, a dump of this database can not
be restored if now() has advanced enough into the future (which it
will eventually). It also makes impossible to do things like SET a=a
on the table.

Yes, this is clearly a stupidly defined constraint, but why do we allow it?

Shouldn't we disallow anything that's not IMMUTABLE in a check constraint?

-- Magnus HaganderMe: http://www.hagander.net/Work: http://www.redpill-linpro.com/