DROP DOMAIN a_year CASCADE;

-- a simple domain
CREATE DOMAIN a_year AS INTEGER
CHECK (VALUE BETWEEN 1 AND 3000);

-- ok
SELECT 1::a_year;
SELECT CAST('2000' AS a_year);

-- fails as expected
SELECT 0::a_year; 

CREATE FUNCTION date2year(DATE)
RETURNS a_year IMMUTABLE STRICT AS $$
SELECT EXTRACT(YEAR FROM $1)::a_year;
$$ LANGUAGE sql;

-- ok
SELECT date2year(CURRENT_DATE);

-- fails as expected
SELECT date2year(DATE '3001-01-01');

CREATE CAST (DATE AS a_year) 
WITH FUNCTION date2year(DATE);

-- fails, I would expect 1970
SELECT (DATE '1970-03-20')::a_year;

-- fails, I would expect the current year
SELECT CURRENT_DATE::a_year;
SELECT CAST(CURRENT_DATE AS a_year);