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[namespace.constraints]
16 Library introduction [library]
16.4 Library-wide requirements [requirements]
16.4.5 Constraints on programs [constraints]
16.4.5.2 Namespace use [namespace.constraints]
16.4.5.2.1 Namespace std [namespace.std]
Unless otherwise specified, the behavior of a C++ program is undefined if it adds declarations or definitions to namespacestd or to a namespace within namespacestd.
Unless explicitly prohibited, a program may add a template specialization for any standard library class template to namespacestd provided that
the added declaration depends on at least one program-defined type, and
the specialization meets the standard library requirements for the original template.151
The behavior of a C++ program is undefined if it declares an explicit or partial specialization of any standard library variable template, except where explicitly permitted by the specification of that variable template.
[Note 1:
The requirements on an explicit or partial specialization are stated by each variable template that grants such permission.
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The behavior of a C++ program is undefined if it declares
an explicit specialization of any member function of a standard library class template, or
an explicit specialization of any member function template of a standard library class or class template, or
an explicit or partial specialization of any member class template of a standard library class or class template, or
a deduction guide for any standard library class template.
A program may explicitly instantiate a class template defined in the standard library only if the declaration
depends on the name of at least one program-defined type, and
the instantiation meets the standard library requirements for the original template.
Let F denote a standard library function ([global.functions]), a standard library static member function, or an instantiation of a standard library function template.
Unless F is designated an addressable function, the behavior of a C++ program is unspecified (possibly ill-formed) if it explicitly or implicitly attempts to form a pointer to F.
[Note 2:
Possible means of forming such pointers include application of the unary & operator ([expr.unary.op]),addressof ([specialized.addressof]), or a function-to-pointer standard conversion ([conv.func]).
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Moreover, the behavior of a C++ program is unspecified (possibly ill-formed) if it attempts to form a reference to F or if it attempts to form a pointer-to-member designating either a standard library non-static member function ([member.functions]) or an instantiation of a standard library member function template.
Let F denote a standard library function or function template.
Unless F is designated an addressable function, it is unspecified if or how a reflection value designating the associated entity can be formed.
[Note 3:
For example, it is possible that std::meta::members_of will not return reflections of standard library functions that an implementation handles through an extra-linguistic mechanism.
â end note]
Let C denote a standard library class or class template specialization.
It is unspecified if or how a reflection value can be formed to any private member of C, or what the names of such members may be.
A translation unit shall not declare namespace std to be an inline namespace ([namespace.def]).
Any library code that instantiates other library templates must be prepared to work adequately with any user-supplied specialization that meets the minimum requirements of this document.
16.4.5.2.2 Namespace posix [namespace.posix]
The behavior of a C++ program is undefined if it adds declarations or definitions to namespaceposix or to a namespace within namespaceposix unless otherwise specified.
The namespace posix is reserved for use by ISO/IEC/IEEE 9945 and other POSIX standards.
16.4.5.2.3 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.
â end example]