[Resolved editorially by editorial issue 2353.]

According to 11.4 [class.mem] paragraph 1,

A member shall not be declared twice in the member-specification, except that a nested class or member class template can be declared and then later defined, and except that an enumeration can be introduced with an opaque-enum-declaration and later redeclared with an enum-specifier.

However, the grammar for member-declaration does not have a production that allows an opaque-enum-declaration.