Proposed resolution (April, 2015):

Change 9.8.1 [dcl.enum] paragraph 1 as follows:

A : following “enum identifier” within the decl-specifier-seq of a member-declaration is parsed as part of an enum-base. [Note: This resolves a potential ambiguity between the declaration of an enumeration with an enum-base and the declaration of an unnamed bit-field of enumeration type...