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.

Egotism

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.

---

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

Psychophysics

I reported the bug to GCC now: gcc.gnu.org/bugzilla/show_bug.cgi?id=67008 – Egotism

Answer 2

GCC is wrong. Qualified name lookup does consider N::i; §3.4.3.2/2 & /3:

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.

Given X::m (where X is a user-declared namespace), or given ::m (where X is the global namespace), […] if S(X, m) has exactly one member, or if the context of the reference is a using-declaration (7.3.3), S(X, m) is the required set of declarations of m.

There is only one namespace nominated by a using-directive in your program: N. It's therefore included in the union and ::i is resolved to N::i.

Note that GCC is inconsistent with its lookup: Using ::i in another context is fine.

namespace N { int i; }
using namespace N;

int main() {
    ::i = 5;
}

This compiles. The only difference that a using-declaration makes as a context is shown in the above quote and does not affect the established conclusion.

Pearl

@Psychophysics Well, I saw this question really late (like a few minutes ago only), so I was lucky to get first ;-) – Pearl