9 Declarations [dcl]

9.9 Namespaces [basic.namespace]

9.9.3 Namespace alias [namespace.alias]

A namespace-alias-definition declares a namespace alias according to the following grammar:
The splice-specifier (if any) shall designate a namespace that is not the global namespace.
The underlying entity ([basic.pre]) of the namespace alias is the namespace either denoted by the qualified-namespace-specifier or designated by the splice-specifier.
[Note 1: 
When looking up a namespace-name in a namespace-alias-definition, only namespace names are considered, see [basic.lookup.udir].
— end note]