foreign keys with constant part

Hello,

I have to define a foreign relation to something like a unique partial
index.
I could do achieve this with triggers, but I already have too much of
them, which make the model hard to understand.
for now, I will just add a constant column which allows to define a
standard foreign key.

Would an extension of the foreign keys declaration like following make
sense, or does it too much break SQL standards ?

best regards,

Marc Mamin

create temp table t (
a int,
b int,
constraint t_pk primary key (a, b)
);
create unique index t_partial on t (a) where b=5;

create temp table f (
a int
)

alter table f add constraint f_fk FOREIGN KEY ( a, 5 ) REFERENCES t
(a,b); --> t_pk
-- or
alter table f add constraint f_fk FOREIGN KEY ( a ) REFERENCES t (a)
WHERE t.b=5; --> t_partial

for now:

create temp table f (
a int,
five int default 5 -- :-(
)
alter table f add constraint f_fk FOREIGN KEY ( a, five ) REFERENCES t
(a,b);