RfD: more powerful "any" types

Hi,

I'm seeking more powerful "any" pseudotypes. In particular, consider a
function

foo(type1, type2) returns type3

where type1 and type2 can be both any element type, but not necessarily
both of the same type. Currently this cannot be made to work: you could
try to write the function this way:

foo(anyelement, anyelement) returns anyelement

but this will force them to be both of the same type, which is not what
we want. In my opinion this is a missing feature of our type system.

One seemingly trivial idea would be to have anyelement2, anyelement3,
and so on. This is not a very good solution, because we'd have to fill
the catalogs with a large bunch of new pseudotypes, and the code with a
bunch of hardcoded tests -- and there are already 27 cases of
ANYELEMENTOID in our code.

For a practical example, I am trying to write a function that returns
how many NULL arguments it has (this is useful in table CHECK
constraints). One simple idea is to use type "unknown":

CREATE FUNCTION a (unknown, unknown, unknown) RETURNS INT LANGUAGE plpgsql AS $$
DECLARE
count int = 0;
BEGIN
IF $1 IS NULL THEN count = count + 1; END IF;
IF $2 IS NULL THEN count = count + 1; END IF;
if $3 IS NULL THEN count = count + 1; END IF;
RETURN count;
END $$;

The problem is that it doesn't work. This trivial query does:

alvherre=# select a(null, '2', null);
a
---
2
(1 fila)

But this one does not:

alvherre=# select a(null, 2, null);
ERROR: function a(unknown, integer, unknown) does not exist

and you cannot cast the integer:
alvherre=# select a(null, 2::unknown, null);
ERROR: cannot cast type integer to unknown

Before I spend time trying to figure out how this works,
1. is there agreement that this is a problem and needs fixed, and
2. does anybody have an idea how to attack it?

--
Alvaro Herrera http://www.CommandPrompt.com/
The PostgreSQL Company - Command Prompt, Inc.