BUG #8516: Calling VOLATILE from STABLE function

The following bug has been logged on the website:

Bug reference: 8516
Logged by: Dwayne Towell
Email address: dwayne(at)docketnavigator(dot)com
PostgreSQL version: 9.2.4
Operating system: CentOS
Description:

Why doesn't PostgreSQL give a warning when calling a volatile function from
a stable function?

For example:
CREATE TABLE x (val double);

CREATE FUNCTION g() RETURNS boolean AS $$
INSERT INTO x SELECT rand() RETURNING val>0.5;
$$ LANGUAGE SQL VOLATILE;

CREATE FUNCTION f() RETURNS boolean AS $$
SELECT g(); -- this is where the stability-violation happens
$$ LANGUAGE SQL STABLE; -- this is a lie

According to the documentation, f() should be marked VOLATILE also, since
calling f() produces side effects. PostgreSQL does not give a warning (or
better yet, an error); I think it should.