20 Library introduction [library]

20.5 Library-wide requirements [requirements]

20.5.4 Constraints on programs [constraints]

20.5.4.2 Namespace use [namespace.constraints]

20.5.4.2.3 Namespaces for future standardization [namespace.future]

1
#
Top level namespaces with a name starting with std and followed by a non-empty sequence of digits are reserved for future standardization.
The behavior of a C++ program is undefined if it adds declarations or definitions to such a namespace.
[ Example
:
The top level namespace std2 is reserved for use by future revisions of this International Standard.
— end example
 ]