Issue 1411 added :: as a production for nested-name-specifier. However, the grammar for using-declarations should have been updated but was overlooked:

using-declaration:
using typename_opt nested-name-specifier unqualified-id ;
using :: unqualified-id ;

In addition, there is some verbiage in 6.5.5.3 [namespace.qual] paragraph 1 and 9.10 [namespace.udecl] paragraph 9 that should probably be revised.