altering a column's collation leaves an invalid foreign key

Dear hackers,

I was looking at how foreign keys deal with collations, and I came across this comment about not 
re-checking a foreign key if the column type changes in a compatible way:

   * Since we require that all collations share the same notion of
   * equality (which they do, because texteq reduces to bitwise
   * equality), we don't compare collation here.

But now that we have nondeterministic collations, isn't that out of date?

For instance I could make this foreign key:

paul=# create collation itext (provider = 'icu', locale = 'und-u-ks-level1', deterministic = false);
CREATE COLLATION
paul=# create table t1 (id text collate itext primary key);
CREATE TABLE
paul=# create table t2 (id text, parent_id text references t1);
CREATE TABLE

And then:

paul=# insert into t1 values ('a');
INSERT 0 1
paul=# insert into t2 values ('.', 'A');
INSERT 0 1

So far that behavior seems correct, because the user told us 'a' and 'A' were equivalent,
but now I can change the collation on the referenced table and the FK doesn't complain:

paul=# alter table t1 alter column id type text collate "C";
ALTER TABLE

The constraint claims to be valid, but I can't drop & add it:

paul=# alter table t2 drop constraint t2_parent_id_fkey;
ALTER TABLE
paul=# alter table t2 add constraint t2_parent_id_fkey foreign key (parent_id) references t1;
ERROR:  insert or update on table "t2" violates foreign key constraint "t2_parent_id_fkey"
DETAIL:  Key (parent_id)=(A) is not present in table "t1".

Isn't that a problem?

Perhaps if the previous collation was nondeterministic we should force a re-check.

(Tested on 17devel 697f8d266c and also 16.)

Yours,

-- 
Paul              ~{:-)
pj@illuminatedcomputing.com