This is not too far to my proposal - and it is fully adequate alternative.
The ANSI form is related to SET or UPDATE commands - so in this case I see classic languages style https://en.wikipedia.org/wiki/Assignment_(computer_science) better. The assign statement in PLpgSQL is not related to embedded SQL. If we introduce SQL syntax and SET commands for schema variables then ( ) syntax is perfect, but for := PLpgSQL I am not sure
It is maybe strange, but
SET (a,b) = (SELECT a,b FROM foo)
a, b := fx()
are sentences from two independent worlds and different syntax can be correct (depends how much we would to integrate procedural and SQL worlds .. 100% T-SQL, 80% SQL/PSM, ..20% PLpgSQL or 5%PL/SQL)
Regards
Pavel
But what, if anything, does Ada do?
What I know, no, Ada has not this statement - but the design of OUT parameters in Ada absolutely different than PostgreSQL - so in this case we cannot to use Ada language as our base :(