[basic.namespace] - C++17 → C++20

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@@ -4,10 +4,27 @@ A namespace is an optionally-named declarative region. The name of a
 namespace can be used to access entities declared in that namespace;
 that is, the members of the namespace. Unlike other declarative regions,
 the definition of a namespace can be split over several parts of one or
 more translation units.
 The outermost declarative region of a translation unit is a namespace;
 see  [[basic.scope.namespace]].
 ### Namespace definition <a id="namespace.def">[[namespace.def]]</a>
@@ -24,46 +41,46 @@ namespace-definition:
         nested-namespace-definition
 ```
 ``` bnf
 named-namespace-definition:
- 'inline'ₒₚₜ 'namespace' attribute-specifier-seqₒₚₜ identifier '{' namespace-body '}'
 ```
 ``` bnf
 unnamed-namespace-definition:
- 'inline'ₒₚₜ 'namespace' attribute-specifier-seqₒₚₜ '{' namespace-body '}'
 ```
 ``` bnf
 nested-namespace-definition:
- 'namespace' enclosing-namespace-specifier '::' identifier '{' namespace-body '}'
 ```
 ``` bnf
 enclosing-namespace-specifier:
         identifier
-        enclosing-namespace-specifier '::' identifier
 ```
 ``` bnf
 namespace-body:
         declaration-seqₒₚₜ
 ```
-Every *namespace-definition* shall appear in the global scope or in a
-namespace scope ([[basic.scope.namespace]]).
 In a *named-namespace-definition*, the *identifier* is the name of the
-namespace. If the *identifier*, when looked up (
-[[basic.lookup.unqual]]), refers to a *namespace-name* (but not a
-*namespace-alias*) that was introduced in the namespace in which the
-*named-namespace-definition* appears or that was introduced in a member
-of the inline namespace set of that namespace, the
-*namespace-definition* *extends* the previously-declared namespace.
-Otherwise, the *identifier* is introduced as a *namespace-name* into the
-declarative region in which the *named-namespace-definition* appears.
 Because a *namespace-definition* contains *declaration*s in its
 *namespace-body* and a *namespace-definition* is itself a *declaration*,
 it follows that *namespace-definition*s can be nested.
@@ -117,22 +134,22 @@ The optional *attribute-specifier-seq* in a *named-namespace-definition*
 appertains to the namespace being defined or extended.
 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 partially specialized (
-[[temp.class.spec]]), 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
@@ -143,26 +160,29 @@ namespaces.
 A *nested-namespace-definition* with an *enclosing-namespace-specifier*
 `E`, *identifier* `I` and *namespace-body* `B` is equivalent to
 ``` cpp
-namespace E { namespace I { B } }
 ```
 [*Example 3*:
 ``` cpp
-namespace A::B::C {
   int i;
 }
 ```
 The above has the same effect as:
 ``` cpp
 namespace A {
-  namespace B {
     namespace C {
       int i;
     }
   }
 }
@@ -173,21 +193,21 @@ namespace A {
 #### Unnamed namespaces <a id="namespace.unnamed">[[namespace.unnamed]]</a>
 An *unnamed-namespace-definition* behaves as if it were replaced by
 ``` bnf
-'inline'ₒₚₜ '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* and all occurrences of `unique ` in a
 translation unit are replaced by the same identifier, and this
 identifier differs from all other identifiers in the translation unit.
 The optional *attribute-specifier-seq* in the
-*unnamed-namespace-definition* appertains to `unique `.
 [*Example 1*:
 ``` cpp
 namespace { int i; }            // unique::i
@@ -212,19 +232,19 @@ void h() {
 — *end example*]
 #### Namespace member definitions <a id="namespace.memdef">[[namespace.memdef]]</a>
 A declaration in a namespace `N` (excluding declarations in nested
-scopes) whose *declarator-id* is an *unqualified-id* ([[dcl.meaning]]),
-whose *class-head-name* (Clause [[class]]) or *enum-head-name* (
-[[dcl.enum]]) is an *identifier*, or whose *elaborated-type-specifier*
-is of the form *class-key* *attribute-specifier-seq*ₒₚₜ  *identifier* (
-[[dcl.type.elab]]), or that is an *opaque-enum-declaration*, declares
-(or redeclares) its *unqualified-id* or *identifier* as a member of `N`.
-[*Note 1*: An explicit instantiation ([[temp.explicit]]) or explicit
-specialization ([[temp.expl.spec]]) of a template does not introduce a
 name and thus may be declared using an *unqualified-id* in a member of
 the enclosing namespace set, if the primary template is declared in an
 inline namespace. — *end note*]
 [*Example 1*:
@@ -242,14 +262,14 @@ namespace X {
 ```
 — *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 2*:
 ``` cpp
 namespace Q {
@@ -268,32 +288,32 @@ namespace R {
 }
 ```
 — *end example*]
-If a `friend` declaration in a non-local class first declares a class,
-function, class template or function template[^5] 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 2*: 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 3*: 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 3*:
 ``` cpp
 // Assume f and g have not yet been declared.
@@ -339,11 +359,11 @@ namespace-alias:
         identifier
 ```
 ``` bnf
 namespace-alias-definition:
- 'namespace' identifier '=' qualified-namespace-specifier ';'
 ```
 ``` bnf
 qualified-namespace-specifier:
     nested-name-specifierₒₚₜ namespace-name
@@ -372,485 +392,15 @@ namespace CWVLN = Company_with_very_long_name;  // OK: duplicate
 namespace CWVLN = CWVLN;
 ```
 — *end example*]
-### The `using` declaration <a id="namespace.udecl">[[namespace.udecl]]</a>
-``` bnf
-using-declaration:
-    'using' using-declarator-list ';'
-```
-``` bnf
-using-declarator-list:
-    using-declarator '...'ₒₚₜ
-    using-declarator-list ',' using-declarator '...'ₒₚₜ
-```
-``` bnf
-using-declarator:
-    'typename'ₒₚₜ nested-name-specifier unqualified-id
-```
-Each *using-declarator* in a *using-declaration* [^6] introduces a set
-of declarations into the declarative region in which the
-*using-declaration* appears. The set of declarations introduced by the
-*using-declarator* is found by performing qualified name lookup (
-[[basic.lookup.qual]], [[class.member.lookup]]) for the name in the
-*using-declarator*, excluding functions that are hidden as described
-below. If the *using-declarator* does not name a constructor, the
-*unqualified-id* is declared in the declarative region in which the
-*using-declaration* appears as a synonym for each declaration introduced
-by the *using-declarator*.
-[*Note 1*: Only the specified name is so declared; specifying an
-enumeration name in a *using-declaration* does not declare its
-enumerators in the *using-declaration*'s declarative
-region. — *end note*]
-If the *using-declarator* names a constructor, it declares that the
-class *inherits* the set of constructor declarations introduced by the
-*using-declarator* from the nominated base class.
-Every *using-declaration* is a *declaration* and a *member-declaration*
-and can therefore be used in a class definition.
-[*Example 1*:
-``` cpp
-struct B {
-  void f(char);
-  void g(char);
-  enum E { e };
-  union { int x; };
-};
-struct D : B {
-  using B::f;
-  void f(int) { f('c'); }       // calls B::f(char)
-  void g(int) { g('c'); }       // recursively calls D::g(int)
-};
-```
-— *end example*]
-In a *using-declaration* used as a *member-declaration*, each
-*using-declarator*'s *nested-name-specifier* shall name a base class of
-the class being defined. If a *using-declarator* names a constructor,
-its *nested-name-specifier* shall name a direct base class of the class
-being defined.
-[*Example 2*:
-``` cpp
-template <typename... bases>
-struct X : bases... {
-  using bases::g...;
-};
-X<B, D> x;                      // OK: B::g and D::g introduced
-```
-— *end example*]
-[*Example 3*:
-``` cpp
-class C {
-  int g();
-};
-class D2 : public B {
-  using B::f;                   // OK: B is a base of D2
-  using B::e;                   // OK: e is an enumerator of base B
-  using B::x;                   // OK: x is a union member of base B
-  using C::g;                   // error: C isn't a base of D2
-};
-```
-— *end example*]
-[*Note 2*: Since destructors do not have names, a *using-declaration*
-cannot refer to a destructor for a base class. Since specializations of
-member templates for conversion functions are not found by name lookup,
-they are not considered when a *using-declaration* specifies a
-conversion function ([[temp.mem]]). — *end note*]
-If a constructor or assignment operator brought from a base class into a
-derived class has the signature of a copy/move constructor or assignment
-operator for the derived class ([[class.copy]]), the
-*using-declaration* does not by itself suppress the implicit declaration
-of the derived class member; the member from the base class is hidden or
-overridden by the implicitly-declared copy/move constructor or
-assignment operator of the derived class, as described below.
-A *using-declaration* shall not name a *template-id*.
-[*Example 4*:
-``` cpp
-struct A {
-  template <class T> void f(T);
-  template <class T> struct X { };
-};
-struct B : A {
-  using A::f<double>;           // ill-formed
-  using A::X<int>;              // ill-formed
-};
-```
-— *end example*]
-A *using-declaration* shall not name a namespace.
-A *using-declaration* shall not name a scoped enumerator.
-A *using-declaration* that names a class member shall be a
-*member-declaration*.
-[*Example 5*:
-``` cpp
-struct X {
-  int i;
-  static int s;
-};
-void f() {
-  using X::i;                   // error: X::i is a class member and this is not a member declaration.
-  using X::s;                   // error: X::s is a class member and this is not a member declaration.
-}
-```
-— *end example*]
-Members declared by a *using-declaration* can be referred to by explicit
-qualification just like other member names ([[namespace.qual]]).
-[*Example 6*:
-``` cpp
-void f();
-namespace A {
-  void g();
-}
-namespace X {
-  using ::f;                    // global f
-  using A::g;                   // A's g
-}
-void h()
-{
-  X::f();                       // calls ::f
-  X::g();                       // calls A::g
-}
-```
-— *end example*]
-A *using-declaration* is a *declaration* and can therefore be used
-repeatedly where (and only where) multiple declarations are allowed.
-[*Example 7*:
-``` cpp
-namespace A {
-  int i;
-}
-namespace A1 {
-  using A::i, A::i;             // OK: double declaration
-}
-struct B {
-  int i;
-};
-struct X : B {
-  using B::i, B::i;             // error: double member declaration
-};
-```
-— *end example*]
-[*Note 3*: For a *using-declaration* whose *nested-name-specifier*
-names a namespace, members added to the namespace after the
-*using-declaration* are not in the set of introduced declarations, so
-they are not considered when a use of the name is made. Thus, additional
-overloads added after the *using-declaration* are ignored, but default
-function arguments ([[dcl.fct.default]]), default template arguments (
-[[temp.param]]), and template specializations ([[temp.class.spec]],
-[[temp.expl.spec]]) are considered. — *end note*]
-[*Example 8*:
-``` cpp
-namespace A {
-  void f(int);
-}
-using A::f;         // f is a synonym for A::f; that is, for A::f(int).
-namespace A {
-  void f(char);
-}
-void foo() {
-  f('a');           // calls f(int), even though f(char) exists.
-}
-void bar() {
-  using A::f;       // f is a synonym for A::f; that is, for A::f(int) and A::f(char).
-  f('a');           // calls f(char)
-}
-```
-— *end example*]
-[*Note 4*: Partial specializations of class templates are found by
-looking up the primary class template and then considering all partial
-specializations of that template. If a *using-declaration* names a class
-template, partial specializations introduced after the
-*using-declaration* are effectively visible because the primary template
-is visible ([[temp.class.spec]]). — *end note*]
-Since a *using-declaration* is a declaration, the restrictions on
-declarations of the same name in the same declarative region (
-[[basic.scope]]) also apply to *using-declaration*s.
-[*Example 9*:
-``` cpp
-namespace A {
-  int x;
-}
-namespace B {
-  int i;
-  struct g { };
-  struct x { };
-  void f(int);
-  void f(double);
-  void g(char);     // OK: hides struct g
-}
-void func() {
-  int i;
-  using B::i;       // error: i declared twice
-  void f(char);
-  using B::f;       // OK: each f is a function
-  f(3.5);           // calls B::f(double)
-  using B::g;
-  g('a');           // calls B::g(char)
-  struct g g1;      // g1 has class type B::g
-  using B::x;
-  using A::x;       // OK: hides struct B::x
-  x = 99;           // assigns to A::x
-  struct x x1;      // x1 has class type B::x
-}
-```
-— *end example*]
-If a function declaration in namespace scope or block scope has the same
-name and the same parameter-type-list ([[dcl.fct]]) as a function
-introduced by a *using-declaration*, and the declarations do not declare
-the same function, the program is ill-formed. If a function template
-declaration in namespace scope has the same name, parameter-type-list,
-return type, and template parameter list as a function template
-introduced by a *using-declaration*, the program is ill-formed.
-[*Note 5*:
-Two *using-declaration*s may introduce functions with the same name and
-the same parameter-type-list. If, for a call to an unqualified function
-name, function overload resolution selects the functions introduced by
-such *using-declaration*s, the function call is ill-formed.
-[*Example 10*:
-``` cpp
-namespace B {
-  void f(int);
-  void f(double);
-}
-namespace C {
-  void f(int);
-  void f(double);
-  void f(char);
-}
-void h() {
-  using B::f;       // B::f(int) and B::f(double)
-  using C::f;       // C::f(int), C::f(double), and C::f(char)
-  f('h');           // calls C::f(char)
-  f(1);             // error: ambiguous: B::f(int) or C::f(int)?
-  void f(int);      // error: f(int) conflicts with C::f(int) and B::f(int)
-}
-```
-— *end example*]
-— *end note*]
-When a *using-declarator* brings declarations from a base class into a
-derived class, member functions and member function templates in the
-derived class override and/or hide member functions and member function
-templates with the same name, parameter-type-list ([[dcl.fct]]),
-cv-qualification, and *ref-qualifier* (if any) in a base class (rather
-than conflicting). Such hidden or overridden declarations are excluded
-from the set of declarations introduced by the *using-declarator*.
-[*Example 11*:
-``` cpp
-struct B {
-  virtual void f(int);
-  virtual void f(char);
-  void g(int);
-  void h(int);
-};
-struct D : B {
-  using B::f;
-  void f(int);      // OK: D::f(int) overrides B::f(int);
-  using B::g;
-  void g(char);     // OK
-  using B::h;
-  void h(int);      // OK: D::h(int) hides B::h(int)
-};
-void k(D* p)
-{
-  p->f(1);          // calls D::f(int)
-  p->f('a');        // calls B::f(char)
-  p->g(1);          // calls B::g(int)
-  p->g('a');        // calls D::g(char)
-}
-struct B1 {
-  B1(int);
-};
-struct B2 {
-  B2(int);
-};
-struct D1 : B1, B2 {
-  using B1::B1;
-  using B2::B2;
-};
-D1 d1(0);           // ill-formed: ambiguous
-struct D2 : B1, B2 {
-  using B1::B1;
-  using B2::B2;
-  D2(int);          // OK: D2::D2(int) hides B1::B1(int) and B2::B2(int)
-};
-D2 d2(0);           // calls D2::D2(int)
-```
-— *end example*]
-For the purpose of overload resolution, the functions that are
-introduced by a *using-declaration* into a derived class are treated as
-though they were members of the derived class. In particular, the
-implicit `this` parameter shall be treated as if it were a pointer to
-the derived class rather than to the base class. This has no effect on
-the type of the function, and in all other respects the function remains
-a member of the base class. Likewise, constructors that are introduced
-by a *using-declaration* are treated as though they were constructors of
-the derived class when looking up the constructors of the derived
-class ([[class.qual]]) or forming a set of overload candidates (
-[[over.match.ctor]], [[over.match.copy]], [[over.match.list]]). If such
-a constructor is selected to perform the initialization of an object of
-class type, all subobjects other than the base class from which the
-constructor originated are implicitly initialized (
-[[class.inhctor.init]]).
-In a *using-declarator* that does not name a constructor, all members of
-the set of introduced declarations shall be accessible. In a
-*using-declarator* that names a constructor, no access check is
-performed. In particular, if a derived class uses a *using-declarator*
-to access a member of a base class, the member name shall be accessible.
-If the name is that of an overloaded member function, then all functions
-named shall be accessible. The base class members mentioned by a
-*using-declarator* shall be visible in the scope of at least one of the
-direct base classes of the class where the *using-declarator* is
-specified.
-[*Note 6*:
-Because a *using-declarator* designates a base class member (and not a
-member subobject or a member function of a base class subobject), a
-*using-declarator* cannot be used to resolve inherited member
-ambiguities.
-[*Example 12*:
-``` cpp
-struct A { int x(); };
-struct B : A { };
-struct C : A {
-  using A::x;
-  int x(int);
-};
-struct D : B, C {
-  using C::x;
-  int x(double);
-};
-int f(D* d) {
-  return d->x();    // error: overload resolution selects A::x, but A is an ambiguous base class
-}
-```
-— *end example*]
-— *end note*]
-A synonym created by a *using-declaration* has the usual accessibility
-for a *member-declaration*. A *using-declarator* that names a
-constructor does not create a synonym; instead, the additional
-constructors are accessible if they would be accessible when used to
-construct an object of the corresponding base class, and the
-accessibility of the *using-declaration* is ignored.
-[*Example 13*:
-``` cpp
-class A {
-private:
-    void f(char);
-public:
-    void f(int);
-protected:
-    void g();
-};
-class B : public A {
-  using A::f;       // error: A::f(char) is inaccessible
-public:
-  using A::g;       // B::g is a public synonym for A::g
-};
-```
-— *end example*]
-If a *using-declarator* uses the keyword `typename` and specifies a
-dependent name ([[temp.dep]]), the name introduced by the
-*using-declaration* is treated as a *typedef-name* ([[dcl.typedef]]).
-### Using directive <a id="namespace.udir">[[namespace.udir]]</a>
 ``` bnf
 using-directive:
-    attribute-specifier-seqₒₚₜ 'using namespace' nested-name-specifierₒₚₜ namespace-name ';'
 ```
 A *using-directive* shall not appear in class scope, but may appear in
 namespace scope or in block scope.
@@ -861,12 +411,12 @@ only namespace names are considered, see
 The optional *attribute-specifier-seq* appertains to the
 *using-directive*.
 A *using-directive* specifies that the names in the nominated namespace
 can be used in the scope in which the *using-directive* appears after
-the *using-directive*. During unqualified name lookup (
-[[basic.lookup.unqual]]), the names appear as if they were declared in
 the nearest enclosing namespace which contains both the
 *using-directive* and the nominated namespace.
 [*Note 2*: In this context, “contains” means “contains directly or
 indirectly”. — *end note*]
@@ -898,18 +448,18 @@ namespace A {
   void f3() {
     i = 5;          // uses A::i
   }
 }
 void f4() {
-  i = 5;            // ill-formed; neither i is visible
 }
 ```
 — *end example*]
-For unqualified lookup ([[basic.lookup.unqual]]), the *using-directive*
-is transitive: if a scope contains a *using-directive* that nominates a
 second namespace that itself contains *using-directive*s, the effect is
 as if the *using-directive*s from the second namespace also appeared in
 the first.
 [*Note 3*: For qualified lookup, see
@@ -958,25 +508,24 @@ namespace B {
 }
 ```
 — *end example*]
-If a namespace is extended ([[namespace.def]]) after a
-*using-directive* for that namespace is given, the additional members of
-the extended namespace and the members of namespaces nominated by
-*using-directive*s in the extending *namespace-definition* can be used
-after the extending *namespace-definition*.
 If name lookup finds a declaration for a name in two different
 namespaces, and the declarations do not declare the same entity and do
-not declare functions, the use of the name is ill-formed.
-[*Note 4*:
-In particular, the name of a variable, function or enumerator does not
-hide the name of a class or enumeration declared in a different
-namespace. For example,
 ``` cpp
 namespace A {
   class X { };
   extern "C"   int g();
@@ -1008,11 +557,11 @@ and the relationships among the namespaces implied by the
 *using-directive*s do not cause preference to be given to any of the
 declarations found by the search. — *end note*]
 An ambiguity exists if the best match finds two functions with the same
 signature, even if one is in a namespace reachable through
-*using-directive*s in the namespace of the other.[^7]
 [*Example 3*:
 ``` cpp
 namespace D {

 namespace can be used to access entities declared in that namespace;
 that is, the members of the namespace. Unlike other declarative regions,
 the definition of a namespace can be split over several parts of one or
 more translation units.
+[*Note 1*: A namespace name with external linkage is exported if any of
+its *namespace-definition*s is exported, or if it contains any
+*export-declaration*s [[module.interface]]. A namespace is never
+attached to a module, and never has module linkage even if it is not
+exported. — *end note*]
+[*Example 1*:
+``` cpp
+export module M;
+namespace N1 {}                 // N1 is not exported
+export namespace N2 {}          // N2 is exported
+namespace N3 { export int n; }  // N3 is exported
+```
+— *end example*]
 The outermost declarative region of a translation unit is a namespace;
 see  [[basic.scope.namespace]].
 ### Namespace definition <a id="namespace.def">[[namespace.def]]</a>
         nested-namespace-definition
 ```
 ``` bnf
 named-namespace-definition:
+        inlineₒₚₜ namespace attribute-specifier-seqₒₚₜ identifier '{' namespace-body '}'
 ```
 ``` bnf
 unnamed-namespace-definition:
+        inlineₒₚₜ namespace attribute-specifier-seqₒₚₜ '{' namespace-body '}'
 ```
 ``` bnf
 nested-namespace-definition:
+        namespace enclosing-namespace-specifier '::' inlineₒₚₜ identifier '{' namespace-body '}'
 ```
 ``` bnf
 enclosing-namespace-specifier:
         identifier
+        enclosing-namespace-specifier '::' inlineₒₚₜ identifier
 ```
 ``` bnf
 namespace-body:
         declaration-seqₒₚₜ
 ```
+Every *namespace-definition* shall appear at namespace scope
+[[basic.scope.namespace]].
 In a *named-namespace-definition*, the *identifier* is the name of the
+namespace. If the *identifier*, when looked up [[basic.lookup.unqual]],
+refers to a *namespace-name* (but not a *namespace-alias*) that was
+introduced in the namespace in which the *named-namespace-definition*
+appears or that was introduced in a member of the inline namespace set
+of that namespace, the *namespace-definition* *extends* the
+previously-declared namespace. Otherwise, the *identifier* is introduced
+as a *namespace-name* into the declarative region in which the
+*named-namespace-definition* appears.
 Because a *namespace-definition* contains *declaration*s in its
 *namespace-body* and a *namespace-definition* is itself a *declaration*,
 it follows that *namespace-definition*s can be nested.
 appertains to the namespace being defined or extended.
 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 partially specialized [[temp.class.spec]],
+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
 A *nested-namespace-definition* with an *enclosing-namespace-specifier*
 `E`, *identifier* `I` and *namespace-body* `B` is equivalent to
 ``` cpp
+namespace E { \opt{inline} namespace I { B } }
 ```
+where the optional `inline` is present if and only if the *identifier*
+`I` is preceded by `inline`.
 [*Example 3*:
 ``` cpp
+namespace A::inline B::C {
   int i;
 }
 ```
 The above has the same effect as:
 ``` cpp
 namespace A {
+ inline namespace B {
     namespace C {
       int i;
     }
   }
 }
 #### Unnamed namespaces <a id="namespace.unnamed">[[namespace.unnamed]]</a>
 An *unnamed-namespace-definition* behaves as if it were replaced by
 ``` bnf
+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* and all occurrences of *`unique`* in a
 translation unit are replaced by the same identifier, and this
 identifier differs from all other identifiers in the translation unit.
 The optional *attribute-specifier-seq* in the
+*unnamed-namespace-definition* appertains to *`unique`*.
 [*Example 1*:
 ``` cpp
 namespace { int i; }            // unique::i
 — *end example*]
 #### Namespace member definitions <a id="namespace.memdef">[[namespace.memdef]]</a>
 A declaration in a namespace `N` (excluding declarations in nested
+scopes) whose *declarator-id* is an *unqualified-id* [[dcl.meaning]],
+whose *class-head-name* [[class.pre]] or *enum-head-name* [[dcl.enum]]
+is an *identifier*, or whose *elaborated-type-specifier* is of the form
+*class-key* *attribute-specifier-seq*ₒₚₜ  *identifier*
+[[dcl.type.elab]], or that is an *opaque-enum-declaration*, declares (or
+redeclares) its *unqualified-id* or *identifier* as a member of `N`.
+[*Note 1*: An explicit instantiation [[temp.explicit]] or explicit
+specialization [[temp.expl.spec]] of a template does not introduce a
 name and thus may be declared using an *unqualified-id* in a member of
 the enclosing namespace set, if the primary template is declared in an
 inline namespace. — *end note*]
 [*Example 1*:
 ```
 — *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 2*:
 ``` cpp
 namespace Q {
 }
 ```
 — *end example*]
+If a friend declaration in a non-local class first declares a class,
+function, class template or function template[^10] 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 2*: 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 3*: 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 3*:
 ``` cpp
 // Assume f and g have not yet been declared.
         identifier
 ```
 ``` bnf
 namespace-alias-definition:
+        namespace identifier '=' qualified-namespace-specifier ';'
 ```
 ``` bnf
 qualified-namespace-specifier:
     nested-name-specifierₒₚₜ namespace-name
 namespace CWVLN = CWVLN;
 ```
 — *end example*]
+### Using namespace directive <a id="namespace.udir">[[namespace.udir]]</a>
 ``` bnf
 using-directive:
+    attribute-specifier-seqₒₚₜ using namespace nested-name-specifierₒₚₜ namespace-name ';'
 ```
 A *using-directive* shall not appear in class scope, but may appear in
 namespace scope or in block scope.
 The optional *attribute-specifier-seq* appertains to the
 *using-directive*.
 A *using-directive* specifies that the names in the nominated namespace
 can be used in the scope in which the *using-directive* appears after
+the *using-directive*. During unqualified name lookup
+[[basic.lookup.unqual]], the names appear as if they were declared in
 the nearest enclosing namespace which contains both the
 *using-directive* and the nominated namespace.
 [*Note 2*: In this context, “contains” means “contains directly or
 indirectly”. — *end note*]
   void f3() {
     i = 5;          // uses A::i
   }
 }
 void f4() {
+  i = 5;            // error: neither i is visible
 }
 ```
 — *end example*]
+For unqualified lookup [[basic.lookup.unqual]], the *using-directive* is
+transitive: if a scope contains a *using-directive* that nominates a
 second namespace that itself contains *using-directive*s, the effect is
 as if the *using-directive*s from the second namespace also appeared in
 the first.
 [*Note 3*: For qualified lookup, see
 }
 ```
 — *end example*]
+If a namespace is extended [[namespace.def]] after a *using-directive*
+for that namespace is given, the additional members of the extended
+namespace and the members of namespaces nominated by *using-directive*s
+in the extending *namespace-definition* can be used after the extending
+*namespace-definition*.
+[*Note 4*:
 If name lookup finds a declaration for a name in two different
 namespaces, and the declarations do not declare the same entity and do
+not declare functions or function templates, the use of the name is
+ill-formed [[basic.lookup]]. In particular, the name of a variable,
+function or enumerator does not hide the name of a class or enumeration
+declared in a different namespace. For example,
 ``` cpp
 namespace A {
   class X { };
   extern "C"   int g();
 *using-directive*s do not cause preference to be given to any of the
 declarations found by the search. — *end note*]
 An ambiguity exists if the best match finds two functions with the same
 signature, even if one is in a namespace reachable through
+*using-directive*s in the namespace of the other.[^11]
 [*Example 3*:
 ``` cpp
 namespace D {

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