https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62096
--- Comment #5 from Jason Merrill <jason at gcc dot gnu.org> --- 7.2/8: For an enumeration whose underlying type is fixed, the values of the enumeration are the values of the underlying type. Otherwise, for an enumeration where emin is the smallest enumerator and emax is the largest, the values of the enumeration are the values in the range bmin to bmax, defined as follows: Let K be 1 for a two’s complement representation and 0 for a ones’ complement or sign-magnitude representation. bmax is the smallest value greater than or equal to max(|emin| − K, |emax|) and equal to 2M − 1, where M is a non-negative integer. bmin is zero if emin is non-negative and −(bmax + K) otherwise. The size of the smallest bit-field large enough to hold all the values of the enumeration type is max(M, 1) if bmin is zero and M + 1 otherwise. 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. 5.2.9/10: A value of integral or enumeration type can be explicitly converted to a complete enumeration type. The value is unchanged if the original value is within the range of the enumeration values (7.2). Otherwise, the behavior is undefined. Since the underlying type is not fixed, the values of the enumeration are 0 and 1 regardless of the underlying type. Since ~1 is outside that range, the behavior is undefined under DR 1766 (http://wg21.link/cwg1766).
