Old question - failed to find conversion function from "unknown"

Hi!

I've already asked this question a month ago -
it remains unanswered.
Dare to repeat since this issue is rather important for me.

So.

When I invoke

select 1 where 5 in (null)

everything is Ok,
though when I try

select 1 where 5 in (select null)

I get

ERROR: failed to find conversion function from "unknown" to integer.

Is it desired behavior or subject to change
in future versions of PG?

I realize my simplified example looks radiculous while it came from reality.
Sometimes my engine generates SQL queries does not know correct type
and unable to cast, while PostgreSQL should have all information required.

Thanks.

--
Best regards
Ilja Golshtein