Question: What platform was used to build both both the midsize Pontiac G6 and a compact executive car manufactured by a Swedish automaker?

Context paragraphs:
Saab 9-3: The Saab 9-3 is a compact executive car that was originally developed and manufactured by the Swedish automaker Saab.

Jaguar XE: The Jaguar XE (codename X760) is a compact executive car manufactured and marketed by Jaguar Cars and launched at the October 2014 Paris Auto Show, with production having begun in April 2015.

Mercedes-Benz C-Class (W202): Mercedes-Benz W202 is a compact executive car which was produced by the German automaker Mercedes-Benz in 1993–2000, under the C-Class model names.  In May 1993, the first generation Mercedes-Benz C-Class was introduced as a replacement for the 190.  The C-Class sedan was the company's entry-level model up until 1997, when Mercedes-Benz launched the smaller A-Class.  Styling themes were carried over from the previous W201 series, but the new series had a smoother and rounder design than the previous generation of the compact Mercedes-Benz.

Volvo S60: The Volvo S60 is a compact executive car manufactured and marketed by Volvo since 2000 and is now in its second generation.

Pontiac G6: The Pontiac G6 was a midsize car that was produced by General Motors.  It was introduced in 2004 for the 2005 model year to replace the Grand Am.  The car was built on the GM Epsilon platform which it shared with the Chevrolet Malibu and Saab 9-3 along with other General Motors vehicles.  Features included a remote starting system (standard on GT, optional on base model), traction control/ABS, electronic stability control, automatic headlights as well as a panoramic sunroof option.  Production ended in 2010.

Compact executive car: A compact executive car is a premium car smaller than an executive car.  In European classification, compact executive cars are part of the D-segment.  In North American terms, close equivalents are "compact premium car", "compact luxury car", "entry-level luxury car" and "near-luxury car".  Compact executive cars are usually available in saloon, estate, coupé, and cabriolet body styles.

Alfa Romeo Giulia: Alfa Romeo Giulia (] ) is the name of three not directly related models by the Italian car manufacturer Alfa Romeo.  The first is a line of sporty four-door compact executive cars (Type 105) produced from 1962 to 1978, the second is an updated, mainly up-engined Spider, Sprint and Sprint Speciale Giuliettas, and the third Giulia is a compact executive car (type 952) unveiled in 2015.

Pontiac Grand Am: The Pontiac Grand Am was a mid-size car and later a compact car produced by Pontiac.  The Grand Am had two separate 3-year runs in the 1970s: from 1973 to 1975, and again from 1978 to 1980.  It was based on the GM A platform.  Production of the Grand Am was canceled in 1980 when it was replaced by the Pontiac 6000.  The Grand Am was reintroduced in 1985 when it replaced the Pontiac Phoenix.  It was Pontiac's best selling car and later replaced by the Pontiac G6, so named as it was intended to be the 6th generation of the Grand Am.

Audi RS 4: The Audi RS4 quattro is the top tier and highest performing version of some specific generations of the Audi A4 range of automobiles.  It is a sports-focused compact executive car (often called sport compact in some countries ), produced by Audi's high-performance private subsidiary quattro GmbH, in limited numbers, for German car manufacturer AUDI AG, part of the larger Volkswagen Group.  It slots distinctly above the Audi S4, as the fastest, most sports-focused model based on the A4's "B" automobile platform.  The RS 4 made a comeback in 2012, in Avant form only based on the Audi A4 Avant.

Sports sedan: A sports sedan or a sports saloon is a sedan automobile (US) or saloon car (UK) that is designed to look and feel "sporty", offering the motorist more connection with the driving experience, while providing the comfort and amenities expected of a luxury sedan.  A wider definition that includes related coupé, convertibles, crossovers is known as sport luxury.  Most vehicles in this category overlap with the compact executive car and executive car classifications, while the sporty small family sedans are called sport compacts (mostly used in North America).

Supporting paragraphs: Pontiac G6 Saab 9-3
Answer: GM Epsilon platform