BUG #16547: ECPG can't CALL the procedure which has INOUT parameter

The following bug has been logged on the website:

Bug reference: 16547
Logged by: yusuke egashira
Email address: egashira(dot)yusuke(at)jp(dot)fujitsu(dot)com
PostgreSQL version: 11.8
Operating system: Red Hat Enterprise Linux Server release 6.10
Description:

I created the procedure with INOUT parameter as :
CREATE OR REPLACE PROCEDURE inout_proc(a in int, b inout int)
LANGUAGE plpgsql AS $$
BEGIN
b = a + b;
RETURN;
END;
$$;

If I called it in ECPG by "EXEC SQL CALL inout_proc(1,2);", "SQL error: too
few arguments on line xx" is caused.

On the other hand, if the procedure don't have INOUT parameter (IN only),
"EXEC SQL CALL inout_proc(1,2);" statement succeeded.

This error seems to occur because ECPG doesn't expect the value to be
returned by CALL statement.
Is this a bug?
Or, is the current version limited to calling procedures with INOUT
parameters from ECPG?

I found this behavior in PostgreSQL 11.8 and 12.3.

Also, I found the patch[1] introducing "INTO" clause in CALL of ECPG.
However, the patch changes the EXC SQL CALL statement to always require INTO
clause.

[1]
https://www.postgresql.org/message-id/CAE9k0P%3DGQFjfy9jHCPDw4Eh_xE%3DNw%3D9y4f-CuTZp1L%2B6vEHwFg%40mail.gmail.com