This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 112c. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.
According to 6.7.2 [intro.object] paragraph 7,
Unless it is a bit-field (11.4.10 [class.bit]), a most derived object shall have a nonzero size and shall occupy one or more bytes of storage. Base class subobjects may have zero size.
Base class objects of zero size is a misleading term, as sizeof such an object is non-zero. Size should not be a property of an object, rather of a type.