When the release group primary type is single, the secondary type compilation is ignored/not displayed/etc. This isn't a common situation, but the single (it says so on the cover) (link below) released as a direct to the consumer promotion contains 2 tracks 1 from The Police and 1 from Bryan Adams. This promotional single has apparently become a $16 collectible. Given the expectations for the track count of single vs EP vs album and the cover artwork, a work-around policy to classify this type of release as an album would be awkward at best.

http://musicbrainz.org/release/9b129a10-f7d9-42ba-b05f-40e1b009cf5b