>
> In my opinion, 'DEPENDS ON' syntax is not actually appropriate here. It
> also looks like a not very good hack to me.
Hm...why do you think about it as a hack?
> Moreover user can implement subscripting to its own type without using
> 'DEPENDS ON' syntax. And he will face the bug mentioned above too.
Yes, but since it will require from a user to create few independent custom
functions for subscripting (as we discussed before, there were few reasons of
having them as a proper separate function), I don't see how to avoid this step
of explicitly marking all of them as related to a subscripting logic for
particular data type. And therefore it's possible to forget to do that step in
spite of what form this step will be. Maybe it's possible to make something
like `CREATE FUNCTION ... FOR SUBSCRIPTING`, then verify that assign/extract
functions are presented and notify user if he missed them (but I would rather
not do this unless it's really necessary, since it looks like an overkill).
But I'm open to any suggestions, do you have something in mind?