[Voted into the WP at the March, 2011 meeting as paper N3259.]

Proposed resolution (February, 2011):

The proposed resolution will be submitted as a separate document.

Suggested resolution:

It would simplify the grammar, and apparently better reflect existing practice, to factor the global-scope operator into the rule for nested-name-specifier.

In looking at a large handful of core issues related to elaborated-type-specifiers and the naming of classes in general, I discovered an odd fact. It turns out that there is exactly one place in the grammar where nested-name-specifier is not immediately preceded by "::_opt": in class-head, which is used only for class definitions. So technically, this example is ill-formed, and should evoke a syntax error:

  struct A;
  struct ::A { };

However, all of EDG, GCC and Microsoft's compiler accept it without a qualm. In fact, I couldn't get any of them to even warn about it.