20 Library introduction [library]

The behavior of a C++ program is undefined if it adds declarations or definitions to namespace std or to a namespace within namespace std unless otherwise specified.

A program may add a template specialization for any standard library template to namespace std only if the declaration depends on a user-defined type and the specialization meets the standard library requirements for the original template and is not explicitly prohibited.176

The behavior of a C++ program is undefined if it declares

• (2.1)
  an explicit specialization of any member function of a standard library class template, or
• (2.2)
  an explicit specialization of any member function template of a standard library class or class template, or
• (2.3)
  an explicit or partial specialization of any member class template of a standard library class or class template, or
• (2.4)
  a deduction guide for any standard library class template.

A program may explicitly instantiate a template defined in the standard library only if the declaration depends on the name of a user-defined type and the instantiation meets the standard library requirements for the original template.

A translation unit shall not declare namespace std to be an inline namespace ([namespace.def]).

The behavior of a C++ program is undefined if it adds declarations or definitions to namespace posix or to a namespace within namespace posix unless otherwise specified.

The namespace posix is reserved for use by ISO/IEC 9945 and other POSIX standards.

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.