9 Declarations [dcl.dcl]

9.8 Namespaces [basic.namespace]

9.8.1 Namespace definition [namespace.def]

9.8.1.1 Unnamed namespaces [namespace.unnamed]

1
#
An unnamed-namespace-definition behaves as if it were replaced by 
inline namespace unique { /* empty body */ }
using namespace unique ;
namespace unique { namespace-body }
where inline appears if and only if it appears in the unnamed-namespace-definition and all occurrences of unique in a translation unit are replaced by the same identifier, and this identifier differs from all other identifiers in the translation unit.
The optional attribute-specifier-seq in the unnamed-namespace-definition appertains to unique.
[ Example
: 
namespace { int i; }            // unique​::​i
void f() { i++; }               // unique​::​i++

namespace A {
  namespace {
    int i;                      // A​::​unique​::​i
    int j;                      // A​::​unique​::​j
  }
  void g() { i++; }             // A​::​unique​::​i++
}

using namespace A;
void h() {
  i++;                          // error: unique​::​i or A​::​unique​::​i
  A::i++;                       // A​::​unique​::​i
  j++;                          // A​::​unique​::​j
}
— end example
 ]