16 Library introduction [library]

16.4 Library-wide requirements [requirements]

16.4.5 Constraints on programs [constraints] Namespace use [namespace.constraints] Namespaces for future standardization [namespace.future]

Top-level namespaces whose namespace-name consists of std followed by one or more digits ([lex.name]) are reserved for future standardization.
The behavior of a C++ program is undefined if it adds declarations or definitions to such a namespace.
[Example 1:
The top-level namespace std2 is reserved for use by future revisions of this International Standard.
— end example]