Proposed resolution (June, 2018):

Change 9.7.1 [dcl.enum] paragraph 8 as follows:

b_max is the smallest value greater than or equal to max(|e_min| - K, |e_max|) , e_max) and equal to 2^M - 1, where M is a non-negative integer.

[Accepted as a DR at the November, 2018 (San Diego) meeting.]

According to 9.7.1 [dcl.enum] paragraph 7,

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 e_min is the smallest enumerator and e_max is the largest, the values of the enumeration are the values in the range b_min to b_max, defined as follows: Let K be 1 for a two's complement representation and 0 for a one's complement or sign-magnitude representation. b_max is the smallest value greater than or equal to max(|e_min|-K,|e_max|) and equal to 2^M-1, where M is a non-negative integer. b_min is zero if e_min is non-negative and -(b_max+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 b_min is zero and M+1 otherwise.

The result of these calculations is that the number of bits required for

  enum { N = -1, Z = 0 }

is 1, but the number required for

  enum { N = -1 }

is 2. This is surprising. This could be fixed by changing |e_max| to e_max.