Mutable foreign key constraints

I happened to notice that Postgres will let you do

regression=# create table foo (id timestamp primary key);
CREATE TABLE
regression=# create table bar (ts timestamptz references foo);
CREATE TABLE

This strikes me as a pretty bad idea, because whether a particular
timestamp is equal to a particular timestamptz depends on your
timezone setting. Thus the constraint could appear to be violated
after a timezone change.

I'm inclined to propose rejecting FK constraints if the comparison
operator is not immutable. Among the built-in opclasses, the only
instances of non-immutable btree equality operators are

regression=# select amopopr::regoperator from pg_amop join pg_operator o on o.oid = amopopr join pg_proc p on p.oid = oprcode where amopmethod=403 and amopstrategy=3 and provolatile != 'i';
amopopr
---------------------------------------------------------
=(date,timestamp with time zone)
=(timestamp without time zone,timestamp with time zone)
=(timestamp with time zone,date)
=(timestamp with time zone,timestamp without time zone)
(4 rows)

A possible objection is that if anybody has such a setup and
hasn't noticed a problem because they never change their
timezone setting, they might not appreciate us breaking it.
So I certainly wouldn't propose back-patching this. But
maybe we should add it as a foot-gun defense going forward.

Thoughts?

regards, tom lane