Can't find the right generated column syntax

Bzzzz <[email protected]> Tue, 26 Nov 2019 03:30:41 +0100

Newsgroups	gmane.comp.db.postgresql.novice
Organization	Anyone, anywhere BUT in banana demokratik republik of france and UERSS.
Message-ID	<[email protected]>

Hi list,

I've got this table:
	date_start	timestamptz,
	date_end	temstamptz

and I'd like to auto-generate a 3rd column with a tstzrange:
	date_start	timestamptz,
	date_end	temstamptz,
	duration	tstzrange GENERATED ALWAYS AS ( ? ) STORED

but I can't find the right syntax to do so :/ (is it even possible ?)

Jean-Yves