plpgsql - variable's names conflict with table field names

Hello

When I declare variable with same name as field of table, then I
have a problem with insert cmd in plpgsql procedure. I can't use this name
of columns list in insert cmd; I get syntax error.

When I use equal names in SELECT cmd, I didn't get error msg, but stored
prodedure don't work.

CREATE TABLE fog2(
idx SERIAL PRIMARY KEY,
cas TIMESTAMP
);

-- work fine
CREATE OR REPLACE FUNCTION errdemo() RETURNS TIMESTAMP AS '
DECLARE _cas TIMESTAMP;
BEGIN SELECT INTO _cas cas FROM fog2 LIMIT 1;
RETURN _cas;
END; ' LANGUAGE plpgsql;

-- don't work
CREATE OR REPLACE FUNCTION errdemo() RETURNS TIMESTAMP AS '
DECLARE cas TIMESTAMP;
BEGIN SELECT INTO cas cas FROM fog2 LIMIT 1;
RETURN cas;
END; ' LANGUAGE plpgsql;

-- works fine
CREATE OR REPLACE FUNCTION errdemo() RETURNS TIMESTAMP AS '
DECLARE cas TIMESTAMP;
BEGIN cas := CURRENT_TIMESTAMP;
INSERT INTO fog2 VALUES(DEFAULT, cas);
RETURN cas;
END; ' LANGUAGE plpgsql;

-- don't work - syntax error

CREATE OR REPLACE FUNCTION errdemo() RETURNS TIMESTAMP AS '
DECLARE cas TIMESTAMP;
BEGIN cas := CURRENT_TIMESTAMP;
INSERT INTO fog2 (cas) VALUES(cas);
RETURN cas;
END; ' LANGUAGE plpgsql;

intra=# select errdemo();
ERROR: syntax error at or near "$1" at character 20
CONTEXT: PL/pgSQL function "errdemo" line 3 at SQL statement
intra=#

Is it plpgsql error or my bug?

Regards
Pavel Stehule