Rationale (November, 2014):

The current wording of the Standard makes this example ambiguous, and CWG did not find the similarities mentioned compelling enough to warrant a change.

Current implementations reject an example like:

  namespace X { int n; }
  namespace A = X;
  namespace { namespace A = X; }
  int k = A::n;

This seems curious, since a similar example with using-declarations or with alias-declarations is valid.