10 Declarations [dcl.dcl]

10.3 Namespaces [basic.namespace]

10.3.1 Namespace definition [namespace.def]

10.3.1.1 Unnamed namespaces [namespace.unnamed]

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An unnamed-namespace-definition behaves as if it were replaced by

inlineopt 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 ]