... BTW, a dependency loop is also possible without using this feature,
by abusing default-value expressions:
create function f1(x int, y int) returns int language sql
as 'select $1 + $2';
create function f2(x int, y int default f1(1,2)) returns int language sql
as 'select $1 + $2';
create or replace function f1(x int, y int default f2(11,12)) returns int language sql
as 'select $1 + $2';
The actual use-case for that seems pretty thin, so we never bothered
to worry about it before. But if we're going to build loop-breaking
logic to handle function body dependencies, it should deal with this
too. I think that all that's required is for the initial dummy
function declaration to omit defaults as well as providing a dummy
body.
regards, tom lane