BUG #7685: last_value() not consistent throughout window partition

The following bug has been logged on the website:

Bug reference: 7685
Logged by: Wes Devauld
Email address: wes(at)devauld(dot)ca
PostgreSQL version: 9.2.1
Operating system: Windows 7 Enterprise
Description:

-- The last_value(0 window function appears to be changing values
-- mid window. An example is detailed below.

----SELECT version();
-- version
-------------------------------------------------------------
-- PostgreSQL 9.1.6, compiled by Visual C++ build 1500, 32-bit

-------------------------

-- **** DB DUMP ****
--
-- PostgreSQL database dump
--

-- Dumped from database version 9.1.4
-- Dumped by pg_dump version 9.1.4
-- Started on 2012-11-19 15:01:41

SET statement_timeout = 0;
SET client_encoding = 'UTF8';
SET standard_conforming_strings = on;
SET check_function_bodies = false;
SET client_min_messages = warning;

SET search_path = public, pg_catalog;

SET default_tablespace = '';

SET default_with_oids = false;

--
-- TOC entry 170 (class 1259 OID 25186)
-- Dependencies: 5
-- Name: bug_report; Type: TABLE; Schema: public; Owner: postgres;
Tablespace:
--

CREATE TABLE bug_report (
d1 date NOT NULL,
d2 date NOT NULL,
v real NOT NULL
);

ALTER TABLE public.bug_report OWNER TO postgres;

--
-- TOC entry 1880 (class 0 OID 25186)
-- Dependencies: 170
-- Data for Name: bug_report; Type: TABLE DATA; Schema: public; Owner:
postgres
--

COPY bug_report (d1, d2, v) FROM stdin;
2012-11-19 2012-11-07 358363
2012-11-18 2012-11-07 358363
2012-11-17 2012-11-07 358363
2012-11-16 2012-11-07 257572
2012-11-15 2012-11-07 257572
2012-11-14 2012-11-07 257572
2012-11-13 2012-11-07 257572
2012-11-12 2012-11-07 257572
2012-11-11 2012-11-07 257572
2012-11-10 2012-11-07 257572
\.

-- Completed on 2012-11-19 15:01:42

--**** Description ****

select d2, last_value(v) over (partition by d2 order by d1),
first_value(v) over (partition by d2 order by d1 desc)
from bug_report

--Returns:
--"2012-11-07";358363;358363
--"2012-11-07";358363;358363
--"2012-11-07";358363;358363
--"2012-11-07";257572;358363
--"2012-11-07";257572;358363
--"2012-11-07";257572;358363
--"2012-11-07";257572;358363
--"2012-11-07";257572;358363
--"2012-11-07";257572;358363
--"2012-11-07";257572;358363

--You can see that the last_value()
--switches in the middle of the
--partition for the window function
--when both columns should be
--identical since the first/last
--and asc/desc changes should
--cancel each other out.

--Thanks, and good work with the product.