Proposed resolution (February, 2016):

Add the following as a new paragraph at the end of 9.7.1 [dcl.enum]:

If an enum-head contains a nested-name-specifier, the enum-specifier shall refer to an enumeration that was previously declared directly in the class or namespace to which the nested-name-specifier refers, or in an element of the inline namespace set (9.8.2 [namespace.def]) of that namespace (i.e., not merely inherited or introduced by a using-declaration), and the enum-specifier shall appear in a namespace enclosing the previous declaration. In such cases, the nested-name-specifier of the enum-head of the definition shall not begin with a decltype-specifier.

[Adopted at the February, 2016 meeting.]

The description of enumeration declarations in 9.7.1 [dcl.enum] does not, but should, contain similar wording to that preventing a class definition from defining a class type named by a using-declaration:

If a class-head-name contains a nested-name-specifier, the class-specifier shall refer to a class that was previously declared directly in the class or namespace to which the nested-name-specifier refers, or in an element of the inline namespace set (9.8.2 [namespace.def]) of that namespace (i.e., not merely inherited or introduced by a using-declaration), and the class-specifier shall appear in a namespace enclosing the previous declaration. In such cases, the nested-name-specifier of the class-head-name of the definition shall not begin with a decltype-specifier.