Function sugnature with default parameter

Hello,

I find default values confusing when a function is overloaded, below is an example.

CREATE OR REPLACE FUNCTION default_test (a INT DEFAULT 1, b INT DEFAULT 1, C INT DEFAULT 1) RETURNS INT AS
$$
    BEGIN
        RETURN a+b+c;
    END;
$$
LANGUAGE 'plpgsql';

CREATE OR REPLACE FUNCTION default_test (a INT DEFAULT 1, b INT DEFAULT 1) RETURNS INT AS
$$
    BEGIN
        RETURN a+b;
    END;
$$
LANGUAGE 'plpgsql';

-- this will fail

--SELECT default_test(1,3);

--SELECT default_test(1);

test=# \df default_test
                                                 List of functions
 Schema |     Name     | Result data type |                      Argument data types                      |  Type 
--------+--------------+------------------+---------------------------------------------------------------+--------
 public | default_test | integer          | a integer DEFAULT 1, b integer DEFAULT 1                      | normal
 public | default_test | integer          | a integer DEFAULT 1, b integer DEFAULT 1, c integer DEFAULT 1 | normal
(2 rows)

SELECT default_test(1,3); -- match the function number 2

SELECT default_test(1); -- ERROR