Proposed resolution:

This wording is relative to N4944.

1. Modify [namespace.std] as indicated:

   -?- The restrictions on the use of namespace std apply only to a top-level namespace named std. Namespaces named std nested in program-defined namespaces are not restricted this way or otherwise reserved.

   -1- Unless otherwise specified, the behavior of a C++ program is undefined if it adds declarations or definitions to namespace std or to a namespace within namespace std.

[ 2023-05-24; Reflector poll ]

Set priority to 4 after reflector poll.

[ 2023-04-24; Ville provides wording ]

[namespace.std] p1 says

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

Which std namespace does the rule intend to refer to? Does the text refer to any namespace named std? Consider this case:

namespace A{
   namespace std{  // UB or not? 
      void show(){}
      namespace B{
         void fun(){}  // UB or not? 
     }
  }
}
int main(){  
}

A resemble usage like the above example can be seen in a libcudacxx github code, for example.

Suggested resolution:

We may want to say the namespace std only refers to the namespace whose declaration inhabits the global scope.

[namespace.std] p2, p7 that refers to namespace std have a similar issue.