9 Declarations [dcl.dcl]

9.8 Namespaces [basic.namespace]

9.8.3 Namespace alias [namespace.alias]

A namespace-alias-definition declares an alternate name for a namespace according to the following grammar:
The identifier in a namespace-alias-definition becomes a namespace-alias and denotes the namespace denoted by the qualified-namespace-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]