I don't think that's an extension, it falls out directly from how Haskell builds records on top of ADTs and is specified in the standard as such.  See http://www.haskell.org/onlinereport/decls.html at the end of the "Labelled Fields" section for Haskell98, or the middle of section 3.15.2 in http://www.haskell.org/onlinereport/haskell2010/haskellch3.html#x8-490003.15 for the H'2010 version.