Created attachment 10318 [details]
Testcase (bug.cpp)

I don't think this is a bug as what is happening is that :: is a qualified name and qualified namelookup (IIRC) does not find decls which are injected via using.

(In reply to comment #2)
> I don't think this is a bug as what is happening is that :: is a qualified name
> and qualified namelookup (IIRC) does not find decls which are injected via
> using.
> 

It does find. The following program (where `foo(long)` is commented out) is compiled fine.

===
namespace N {
  class C {};
  void foo(C) {};
}
using namespace N;

/* void foo(long) {}; */

class B {
public:
/*  void foo(long l) { ::foo(l); }; */
  void foo(C c) { ::foo(c); };
};

int main() {
  B b;
  return 0;
}
===

I don't have chapter and verse to justify this, but icc and gcc agree
that they should reject this code
--------------
namespace N {
  class C {};
  void foo(C) {};
}
using namespace N;

void foo(long) {};

void bar() { C c; ::foo(c); };
---------------------
while they both accept the code with ::foo(long) removed:
---------------------
namespace N {
  class C {};
  void foo(C) {};
}
using namespace N;

void bar() { C c; ::foo(c); };
---------------------

One may certainly be tempted to find this confusing.

W.

Closing.