[Note 6:

If name lookup finds a declaration for a name in two different namespaces, and the declarations do not declare the same entity and do not declare functions or function templates, the use of the name is ill-formed ([basic.lookup]).

In particular, the name of a variable, function or enumerator does not hide the name of a class or enumeration declared in a different namespace.

For example, namespace A { class X { }; extern "C" int g(); extern "C++" int h(); } namespace B { void X(int); extern "C" int g(); extern "C++" int h(int); } using namespace A; using namespace B; void f() { X(1); g(); h(); }

— end note]