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Showing content from https://timsong-cpp.github.io/cppwp/n4140/namespace.def below:

[namespace.def]

The enclosing namespaces of a declaration are those namespaces in which the declaration lexically appears, except for a redeclaration of a namespace member outside its original namespace (e.g., a definition as specified in [namespace.memdef]). Such a redeclaration has the same enclosing namespaces as the original declaration. [ Example:

namespace Q {
  namespace V {
    void f();       class C { void m(); };
  }
  void V::f() {     extern void h();    }
  void V::C::m() {   }
}

 — end example ]

Members of an inline namespace can be used in most respects as though they were members of the enclosing namespace. Specifically, the inline namespace and its enclosing namespace are both added to the set of associated namespaces used in argument-dependent lookup ([basic.lookup.argdep]) whenever one of them is, and a using-directive ([namespace.udir]) that names the inline namespace is implicitly inserted into the enclosing namespace as for an unnamed namespace ([namespace.unnamed]). Furthermore, each member of the inline namespace can subsequently be explicitly instantiated ([temp.explicit]) or explicitly specialized ([temp.expl.spec]) as though it were a member of the enclosing namespace. Finally, looking up a name in the enclosing namespace via explicit qualification ([namespace.qual]) will include members of the inline namespace brought in by the using-directive even if there are declarations of that name in the enclosing namespace.

These properties are transitive: if a namespace N contains an inline namespace M, which in turn contains an inline namespace O, then the members of O can be used as though they were members of M or N. The inline namespace set of N is the transitive closure of all inline namespaces in N. The enclosing namespace set of O is the set of namespaces consisting of the innermost non-inline namespace enclosing an inline namespace O, together with any intervening inline namespaces.

An unnamed-namespace-definition behaves as if it were replaced by

inlineopt namespace unique  { /* empty body */ }
using namespace unique  ;
namespace unique  { namespace-body }

where inline appears if and only if it appears in the unnamed-namespace-definition, all occurrences of unique in a translation unit are replaced by the same identifier, and this identifier differs from all other identifiers in the entire program.96 [ Example:

namespace { int i; }            void f() { i++; }               
namespace A {
  namespace {
    int i;                          int j;                        }
  void g() { i++; }             }

using namespace A;
void h() {
  i++;                            A::i++;                         j++;                          }

 — end example ]

Members (including explicit specializations of templates ([temp.expl.spec])) of a namespace can be defined within that namespace. [ Example:

namespace X {
  void f() {  }
}

 — end example ]

Members of a named namespace can also be defined outside that namespace by explicit qualification ([namespace.qual]) of the name being defined, provided that the entity being defined was already declared in the namespace and the definition appears after the point of declaration in a namespace that encloses the declaration's namespace. [ Example:

namespace Q {
  namespace V {
    void f();
  }
  void V::f() {  }       void V::g() {  }       namespace V {
    void g();
  }
}

namespace R {
  void Q::V::g() {  }  }

 — end example ]

Every name first declared in a namespace is a member of that namespace. If a friend declaration in a non-local class first declares a class, function, class template or function template97 the friend is a member of the innermost enclosing namespace. The friend declaration does not by itself make the name visible to unqualified lookup ([basic.lookup.unqual]) or qualified lookup ([basic.lookup.qual]). [ Note: The name of the friend will be visible in its namespace if a matching declaration is provided at namespace scope (either before or after the class definition granting friendship).  — end note ] If a friend function or function template is called, its name may be found by the name lookup that considers functions from namespaces and classes associated with the types of the function arguments ([basic.lookup.argdep]). If the name in a friend declaration is neither qualified nor a template-id and the declaration is a function or an elaborated-type-specifier, the lookup to determine whether the entity has been previously declared shall not consider any scopes outside the innermost enclosing namespace. [ Note: The other forms of friend declarations cannot declare a new member of the innermost enclosing namespace and thus follow the usual lookup rules.  — end note ] [ Example:

void h(int);
template <class T> void f2(T);
namespace A {
  class X {
    friend void f(X);               class Y {
      friend void g();                friend void h(int);                                             friend void f2<>(int);        };
  };

    X x;
  void g() { f(x); }              void f(X) { /* ... */}         void h(int) { /* ... */ }      }

using A::x;

void h() {
  A::f(x);
  A::X::f(x);                     A::X::Y::g();                 }

 — end example ]

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