Consider:

  struct S {
    enum B {
      A
    };
    using B::A;
    // or:
    // using enum B ;
  };

This is neither prohibited by 11.4.1 [class.mem.general] paragraph 6 (because using-declarators are not members per 11.4.1 [class.mem.general] paragraph 3) nor by 9.9 [namespace.udecl] paragraph 8.

Possible resolution:

1. Change in 9.9 [namespace.udecl] paragraph 8 as follows:

   [ Note: If a declaration is named by two using-declarators that inhabit the same class scope, the program is ill-formed (11.4.1 [class.mem.general]). [Example: ... -- end example ] -- end note ]

2. Change in 11.4.1 [class.mem.general] paragraph 6 as follows:

   A member An entity shall not be declared or named by a using-declarator (9.9 [namespace.udecl]) twice in the member-specification, except that

   • a nested class or member class template can be declared and then later defined, and
   • an enumeration can be introduced with an opaque-enum-declaration and later redeclared with an enum-specifier.

   [ Example:

     struct S {
       class C;
       class C {};      // OK
       enum E { e };
       using E::e;      // error: redeclaration of e
     };
   
   

   -- end example ]

   [Note 4: A single name can denote several member functions provided their types are sufficiently different (6.4.1 [basic.scope.scope]). -- end note]