Interestingly PostgreSQL's pl/pgsql does that.

There are two operators, equality = and assignment :=

But where it is clear that assignment was meant, = is treated as assignment. this means you can:

    a = 5; -- exactly equivalent to a := 5 here
    if a = 6 then
        raise exception 'this should never happen';
    end if;

Interestingly this is entirely a Postgres-ism. Oracle's PL/SQL offers no such "feature."