Name lookup in using-declaration via using-directive

Question

Is the following program well-formed or ill-formed according to the c++ standard?

namespace N { int i; }
using namespace N;
using ::i;
int main() {}

I get different results with different compilers:

• Clang (http://melpon.org/wandbox/permlink/c8vl7XbumyyS6vsw): No errors.
• GCC (http://melpon.org/wandbox/permlink/immhNeWFCMcCA800): Error: 'i' not declared.

Is this program well-formed or ill-formed according to the c++ standard? References to the c++ standard needed.

I'm trying to figure out for which compiler I should file a bug.

Answer 1

Well-formed.

The using-directive doesn't introduce the name i in the global namespace, but it is used during lookup. The using-declaration uses qualified lookup to find i; qualified lookup in the presence of using-directives is specified in [3.4.3.2 p1, p2] (quotes from N4527, the current working draft):

If the nested-name-specifier of a qualified-id nominates a namespace (including the case where the nested-name-specifier is ::, i.e., nominating the global namespace), the name specified after the nested-name-specifier is looked up in the scope of the namespace. [...]

For a namespace X and name m, the namespace-qualified lookup set S(X,m) is defined as follows: Let S'(X,m) be the set of all declarations of m in X and the inline namespace set of X (7.3.1). If S'(X,m) is not empty, S(X,m) is S'(X,m); otherwise, S(X,m) is the union of S(N_i,m) for all namespaces N_i nominated by using-directives in X and its inline namespace set.

So, for qualified lookup, the first step is to look for declarations of i made directly in the namespace indicated by the nested-name-specifier (:: in this case). There are no such declarations, so lookup then proceeds to the second step, which is to form the set of all declarations of i found by qualified lookup in all namespaces nominated by using-directives in the global namespace. That set is comprised of N::i, which is the result of name lookup, and is introduced as a name in global namespace by the using declaration.

I find it worth noting (although pretty obvious) that this definition of qualified lookup is recursive: using the notation in the quote, qualified lookup in each namespace N_i will first look for declarations made directly in N_i, then, if none is found, will in turn proceed to look in the namespaces nominated by using-directives in N_i, and so on.

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For what it's worth, MSVC accepts the code as well.