Proposed resolution (March, 2008): Add the indicated sentence to the end of 9.8.1 [dcl.enum] paragraph 5:
...It is possible to define an enumeration that has values not defined
by any of its enumerators. If the enumerator-list is empty,
the values of the enumeration are as if the enumeration had a single
enumerator with value 0.
N2800 comment
UK 96 According to 9.8.1 [dcl.enum] paragraph 6, the underlying
type of an enumeration with an empty enumeration-list is
determined as if the enumeration-list contained a single
enumerator with value 0. Paragraph 7, which specifies the values of
an enumeration and the minimum size of bit-field needed represent
those values needs a similar provision for
empty enumeration-lists.