BUG #15960: ON CONFLICT Trying accessing to variables

The following bug has been logged on the website:

Bug reference: 15960
Logged by: Anton Dutov
Email address: anton(dot)dutov(at)gmail(dot)com
PostgreSQL version: 11.3
Operating system: Linux
Description:

Using upsert in stored procedure ON CONFLICT(id, ...) trows "ERROR: column
reference "id" is ambiguous" where id used as column name and as variable,
but ON CONFLICT can't use variables

Reproduce

CREATE TABLE IF NOT EXISTS test_table (id INT PRIMARY KEY, note TEXT);

CREATE OR REPLACE FUNCTION test_table_add (
in_id INT
,in_note TEXT
,OUT id INT
) RETURNS INT LANGUAGE plpgsql VOLATILE SECURITY DEFINER AS $$
BEGIN
INSERT INTO test_table (id, note) VALUES (in_id, in_note)
ON CONFLICT (id) DO UPDATE SET note = in_note;
END;
$$;

SELECT test_table_add (1, 'ONE');