According to 9.8.1 [dcl.enum] paragraph 5, the underlying type of an enum is an unspecified integral type, which could potentially be unsigned int. The promotion rules in 7.3.7 [conv.prom] paragraph 2 say that such an enumeration value used in an expression will be promoted to unsigned int. This means that a conforming implementation could give the value false for the following code:
enum { zero };
-1 < zero; // might be false
This is counterintuitive. Perhaps the description of the underlying
type of an enumeration should say that an unsigned underlying type
can be used only if the values of the enumerators cannot be
represented in the corresponding signed type. This approach would
be consistent with the treatment of integral promotion of
bitfields (7.3.7 [conv.prom] paragraph 3)
.
On a related note, 9.8.1 [dcl.enum] paragraph 5 says,
the underlying type shall not be larger than int unless the value of an enumerator cannot fit in an int or unsigned int.
This specification does not allow for an enumeration like
enum { a = -1, b = UINT_MAX };
Since each enumerator can fit in an int or unsigned int, the underlying type is required to be no larger than int, even though there is no such type that can represent all the enumerators.