BUG #19034: Recursive function with sql_body can replace an existing function but can not be created on it's own

The following bug has been logged on the website:

Bug reference: 19034
Logged by: Katja Henke
Email address: katja(dot)henke(at)foo(dot)ag
PostgreSQL version: 15.3
Operating system: Linux
Description:

An existing function can be converted into a recursive function that uses
sql_body. But it is not possible to create this same recursive function
using CREATE FUNCTION. In other words: if you managed to have a recursive
function with sql_body in your database, you can't simply restore a dump of
this database.

The following statement results in
ERROR: 42883: function pg_temp.do_something(integer) does not exist

CREATE FUNCTION pg_temp.do_something(int) RETURNS int
LANGUAGE sql
RETURN CASE WHEN $1 % 2 = 1 THEN pg_temp.do_something($1 + 1) ELSE $1 END;

But the combination of the following statements works. It does not matter if
the first function uses sql_body or a string constant.

CREATE FUNCTION pg_temp.do_something(int) RETURNS int
LANGUAGE sql
RETURN 42;

CREATE OR REPLACE FUNCTION pg_temp.do_something(int) RETURNS int
LANGUAGE sql
RETURN CASE WHEN $1 % 2 = 1 THEN pg_temp.do_something($1 + 1) ELSE $1 END;