9 Declarations [dcl.dcl]

9.8 Namespaces [basic.namespace]

9.8.2 Namespace definition [namespace.def]

9.8.2.2 Unnamed namespaces [namespace.unnamed]

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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 1: 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]