Chicken/egg problem with range types

I'm trying to create a discrete range type and I'm having trouble with
the canonical function.

--Create shell type
CREATE TYPE dt_range;

--Create subtype diff
CREATE OR REPLACE FUNCTION dt_subtype_diff(timestamptz, timestamptz)
RETURNS float8 AS
$$
SELECT EXTRACT(EPOCH FROM $1 - $2);
$$ LANGUAGE 'sql' IMMUTABLE STRICT;

-- Create the canonical function
CREATE OR REPLACE FUNCTION dt_range_canonical(dt_range)
RETURNS dt_range AS
$$
SELECT dt_range(
CASE WHEN lower_inc($1)
THEN lower($1)::timestampTz(0)
ELSE lower($1)::timestampTz(0) - INTERVAL '1s' END,
CASE WHEN NOT upper_inc($1)
THEN upper($1)::timestampTz(0)
ELSE upper($1)::timestampTz(0) + INTERVAL '1s' END
);
$$ LANGUAGE 'sql' IMMUTABLE STRICT;

Fails with ERROR: SQL function cannot accept shell type dt_range. So I
add the type and try to alter it later.

-- Create the type any way
CREATE TYPE dt_range AS RANGE (
SUBTYPE = timestamptz,
SUBTYPE_DIFF = dt_subtype_diff
-- CANONICAL = dt_range_canonical -- can't use, fn doesn't exist
);

ALTER TYPE dt_range SET CANONICAL = dt_range_canonical;

This doesn't work either. I'm stuck.

Scott Bailey