BUG #13587: Lag Default option does not work with floats and reals

The following bug has been logged on the website:

Bug reference: 13587
Logged by: Robert McGehee
Email address: rmcgehee@gmail.com
PostgreSQL version: 9.4.4
Operating system: Linux / Redhat 7
Description:

The lag() window function is documented to accept a default argument.
However, if lag is applied to either a float or a real (and possibly other
types as well), adding a default gives an error. As this behavior for
reals/floats does not match the documentation, I believe it to be a bug.
Below is an example showing that lag() works as document for integers, but
not on a real column.

CREATE TABLE _test1 (a INTEGER);
CREATE TABLE _test2 (a REAL);
INSERT INTO _test1 VALUES (1);
INSERT INTO _test1 VALUES (2);
INSERT INTO _test2 VALUES (1);
INSERT INTO _test2 VALUES (2);

SELECT a, lag(a, 1) OVER (ORDER BY a) FROM _test1;
SELECT a, lag(a, 1, 0) OVER (ORDER BY a) FROM _test1;

SELECT a, lag(a, 1) OVER (ORDER BY a) FROM _test2;
SELECT a, lag(a, 1, 0) OVER (ORDER BY a) FROM _test2;

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