7 Declarations [dcl.dcl]

7.3 Namespaces [basic.namespace]

7.3.2 Namespace alias [namespace.alias]

The identifier in a namespace-alias-definition is a synonym for the name of the namespace denoted by the qualified-namespace-specifier and becomes a namespace-alias. [ Note: When looking up a namespace-name in a namespace-alias-definition, only namespace names are considered, see [basic.lookup.udir].  — end note ]

In a declarative region, a namespace-alias-definition can be used to redefine a namespace-alias declared in that declarative region to refer only to the namespace to which it already refers. [ Example: the following declarations are well-formed:

namespace Company_with_very_long_name { /* ... */ }
namespace CWVLN = Company_with_very_long_name;
namespace CWVLN = Company_with_very_long_name;          // OK: duplicate
namespace CWVLN = CWVLN;

 — end example ]

A namespace-name or namespace-alias shall not be declared as the name of any other entity in the same declarative region. A namespace-name defined at global scope shall not be declared as the name of any other entity in any global scope of the program. No diagnostic is required for a violation of this rule by declarations in different translation units.