6 Basic concepts [basic]

6.4 Name lookup [basic.lookup]

1
#
The name lookup rules apply uniformly to all names (including typedef-names ([dcl.typedef]), namespace-names ([basic.namespace]), and class-names ([class.name])) wherever the grammar allows such names in the context discussed by a particular rule.
Name lookup associates the use of a name with a set of declarations ([basic.def]) of that name.
The declarations found by name lookup shall either all declare the same entity or shall all declare functions; in the latter case, the declarations are said to form a set of overloaded functions ([over.load]).
Overload resolution ([over.match]) takes place after name lookup has succeeded.
The access rules (Clause [class.access]) are considered only once name lookup and function overload resolution (if applicable) have succeeded.
Only after name lookup, function overload resolution (if applicable) and access checking have succeeded are the attributes introduced by the name's declaration used further in expression processing (Clause [expr]).
2
#
A name “looked up in the context of an expression” is looked up as an unqualified name in the scope where the expression is found.
3
#
The injected-class-name of a class (Clause [class]) is also considered to be a member of that class for the purposes of name hiding and lookup.
4
#
[ Note
:
[basic.link] discusses linkage issues.
The notions of scope, point of declaration and name hiding are discussed in [basic.scope].
— end note
 ]

6.4.1 Unqualified name lookup [basic.lookup.unqual]

1
#
In all the cases listed in [basic.lookup.unqual], the scopes are searched for a declaration in the order listed in each of the respective categories; name lookup ends as soon as a declaration is found for the name.
If no declaration is found, the program is ill-formed.
2
#
The declarations from the namespace nominated by a using-directive become visible in a namespace enclosing the using-directive; see [namespace.udir].
For the purpose of the unqualified name lookup rules described in [basic.lookup.unqual], the declarations from the namespace nominated by the using-directive are considered members of that enclosing namespace.
3
#
The lookup for an unqualified name used as the postfix-expression of a function call is described in [basic.lookup.argdep].
[ Note
:
For purposes of determining (during parsing) whether an expression is a postfix-expression for a function call, the usual name lookup rules apply.
The rules in [basic.lookup.argdep] have no effect on the syntactic interpretation of an expression.
For example,
typedef int f;
namespace N {
  struct A {
    friend void f(A &);
    operator int();
    void g(A a) {
      int i = f(a);  // f is the typedef, not the friend function: equivalent to int(a)
    }
  };
}
Because the expression is not a function call, the argument-dependent name lookup ([basic.lookup.argdep]) does not apply and the friend function f is not found.
— end note
 ]
4
#
A name used in global scope, outside of any function, class or user-declared namespace, shall be declared before its use in global scope.
5
#
A name used in a user-declared namespace outside of the definition of any function or class shall be declared before its use in that namespace or before its use in a namespace enclosing its namespace.
6
#
In the definition of a function that is a member of namespace N, a name used after the function's declarator-id29 shall be declared before its use in the block in which it is used or in one of its enclosing blocks ([stmt.block]) or shall be declared before its use in namespace N or, if N is a nested namespace, shall be declared before its use in one of N's enclosing namespaces.
[ Example
: 
namespace A {
  namespace N {
    void f();
  }
}
void A::N::f() {
  i = 5;
  // The following scopes are searched for a declaration of i:
  // 1) outermost block scope of A​::​N​::​f, before the use of i
  // 2) scope of namespace N
  // 3) scope of namespace A
  // 4) global scope, before the definition of A​::​N​::​f
}
— end example
 ]
7
#
A name used in the definition of a class X outside of a member function body, default argument, noexcept-specifier, brace-or-equal-initializer of a non-static data member, or nested class definition30 shall be declared in one of the following ways:
  • (7.1)
    before its use in class X or be a member of a base class of X ([class.member.lookup]), or
  • (7.2)
    if X is a nested class of class Y ([class.nest]), before the definition of X in Y, or shall be a member of a base class of Y (this lookup applies in turn to Y's enclosing classes, starting with the innermost enclosing class),31 or
  • (7.3)
    if X is a local class ([class.local]) or is a nested class of a local class, before the definition of class X in a block enclosing the definition of class X, or
  • (7.4)
    if X is a member of namespace N, or is a nested class of a class that is a member of N, or is a local class or a nested class within a local class of a function that is a member of N, before the definition of class X in namespace N or in one of N's enclosing namespaces.
[ Example
: 
namespace M {
  class B { };
}
namespace N {
  class Y : public M::B {
    class X {
      int a[i];
    };
  };
}

// The following scopes are searched for a declaration of i:
// 1) scope of class N​::​Y​::​X, before the use of i
// 2) scope of class N​::​Y, before the definition of N​::​Y​::​X
// 3) scope of N​::​Y's base class M​::​B
// 4) scope of namespace N, before the definition of N​::​Y
// 5) global scope, before the definition of N
— end example
 ]
[ Note
:
When looking for a prior declaration of a class or function introduced by a friend declaration, scopes outside of the innermost enclosing namespace scope are not considered; see [namespace.memdef].
— end note
 ]
[ Note
:
[basic.scope.class] further describes the restrictions on the use of names in a class definition.
[class.nest] further describes the restrictions on the use of names in nested class definitions.
[class.local] further describes the restrictions on the use of names in local class definitions.
— end note
 ]
8
#
For the members of a class X, a name used in a member function body, in a default argument, in a noexcept-specifier, in the brace-or-equal-initializer of a non-static data member ([class.mem]), or in the definition of a class member outside of the definition of X, following the member's declarator-id32, shall be declared in one of the following ways:
  • (8.1)
    before its use in the block in which it is used or in an enclosing block ([stmt.block]), or
  • (8.2)
    shall be a member of class X or be a member of a base class of X ([class.member.lookup]), or
  • (8.3)
    if X is a nested class of class Y ([class.nest]), shall be a member of Y, or shall be a member of a base class of Y (this lookup applies in turn to Y's enclosing classes, starting with the innermost enclosing class),33 or
  • (8.4)
    if X is a local class ([class.local]) or is a nested class of a local class, before the definition of class X in a block enclosing the definition of class X, or
  • (8.5)
    if X is a member of namespace N, or is a nested class of a class that is a member of N, or is a local class or a nested class within a local class of a function that is a member of N, before the use of the name, in namespace N or in one of N's enclosing namespaces.
[ Example
: 
class B { };
namespace M {
  namespace N {
    class X : public B {
      void f();
    };
  }
}
void M::N::X::f() {
  i = 16;
}

// The following scopes are searched for a declaration of i:
// 1) outermost block scope of M​::​N​::​X​::​f, before the use of i
// 2) scope of class M​::​N​::​X
// 3) scope of M​::​N​::​X's base class B
// 4) scope of namespace M​::​N
// 5) scope of namespace M
// 6) global scope, before the definition of M​::​N​::​X​::​f
— end example
 ]
[ Note
:
[class.mfct] and [class.static] further describe the restrictions on the use of names in member function definitions.
[class.nest] further describes the restrictions on the use of names in the scope of nested classes.
[class.local] further describes the restrictions on the use of names in local class definitions.
— end note
 ]
9
#
Name lookup for a name used in the definition of a friend function ([class.friend]) defined inline in the class granting friendship shall proceed as described for lookup in member function definitions.
If the friend function is not defined in the class granting friendship, name lookup in the friend function definition shall proceed as described for lookup in namespace member function definitions.
10
#
In a friend declaration naming a member function, a name used in the function declarator and not part of a template-argument in the declarator-id is first looked up in the scope of the member function's class ([class.member.lookup]).
If it is not found, or if the name is part of a template-argument in the declarator-id, the look up is as described for unqualified names in the definition of the class granting friendship.
[ Example
: 
struct A {
  typedef int AT;
  void f1(AT);
  void f2(float);
  template <class T> void f3();
};
struct B {
  typedef char AT;
  typedef float BT;
  friend void A::f1(AT);      // parameter type is A​::​AT
  friend void A::f2(BT);      // parameter type is B​::​BT
  friend void A::f3<AT>();    // template argument is B​::​AT
};
— end example
 ]
11
#
During the lookup for a name used as a default argument ([dcl.fct.default]) in a function parameter-declaration-clause or used in the expression of a mem-initializer for a constructor ([class.base.init]), the function parameter names are visible and hide the names of entities declared in the block, class or namespace scopes containing the function declaration.
[ Note
:
[dcl.fct.default] further describes the restrictions on the use of names in default arguments.
[class.base.init] further describes the restrictions on the use of names in a ctor-initializer.
— end note
 ]
12
#
During the lookup of a name used in the constant-expression of an enumerator-definition, previously declared enumerators of the enumeration are visible and hide the names of entities declared in the block, class, or namespace scopes containing the enum-specifier.
13
#
A name used in the definition of a static data member of class X ([class.static.data]) (after the qualified-id of the static member) is looked up as if the name was used in a member function of X.
[ Note
:
[class.static.data] further describes the restrictions on the use of names in the definition of a static data member.
— end note
 ]
14
#
If a variable member of a namespace is defined outside of the scope of its namespace then any name that appears in the definition of the member (after the declarator-id) is looked up as if the definition of the member occurred in its namespace.
[ Example
: 
namespace N {
  int i = 4;
  extern int j;
}

int i = 2;

int N::j = i;       // N​::​j == 4
— end example
 ]
15
#
A name used in the handler for a function-try-block (Clause [except]) is looked up as if the name was used in the outermost block of the function definition.
In particular, the function parameter names shall not be redeclared in the exception-declaration nor in the outermost block of a handler for the function-try-block.
Names declared in the outermost block of the function definition are not found when looked up in the scope of a handler for the function-try-block.
[ Note
:
But function parameter names are found.
— end note
 ]
16
#
[ Note
:
The rules for name lookup in template definitions are described in [temp.res].
— end note
 ]
29)
This refers to unqualified names that occur, for instance, in a type or default argument in the parameter-declaration-clause or used in the function body.
30)
This refers to unqualified names following the class name; such a name may be used in the base-clause or may be used in the class definition.
31)
This lookup applies whether the definition of X is nested within Y's definition or whether X's definition appears in a namespace scope enclosing Y's definition ([class.nest]).
32)
That is, an unqualified name that occurs, for instance, in a type in the parameter-declaration-clause or in the noexcept-specifier.
33)
This lookup applies whether the member function is defined within the definition of class X or whether the member function is defined in a namespace scope enclosing X's definition.

6.4.2 Argument-dependent name lookup [basic.lookup.argdep]

1
#
When the postfix-expression in a function call ([expr.call]) is an unqualified-id, other namespaces not considered during the usual unqualified lookup ([basic.lookup.unqual]) may be searched, and in those namespaces, namespace-scope friend function or function template declarations ([class.friend]) not otherwise visible may be found.
These modifications to the search depend on the types of the arguments (and for template template arguments, the namespace of the template argument).
[ Example
: 
namespace N {
  struct S { };
  void f(S);
}

void g() {
  N::S s;
  f(s);             // OK: calls N​::​f
  (f)(s);           // error: N​::​f not considered; parentheses prevent argument-dependent lookup
}
— end example
 ]
2
#
For each argument type T in the function call, there is a set of zero or more associated namespaces and a set of zero or more associated classes to be considered.
The sets of namespaces and classes are determined entirely by the types of the function arguments (and the namespace of any template template argument).
Typedef names and using-declarations used to specify the types do not contribute to this set.
The sets of namespaces and classes are determined in the following way:
  • (2.1)
    If T is a fundamental type, its associated sets of namespaces and classes are both empty.
  • (2.2)
    If T is a class type (including unions), its associated classes are: the class itself; the class of which it is a member, if any; and its direct and indirect base classes.
    Its associated namespaces are the innermost enclosing namespaces of its associated classes.
    Furthermore, if T is a class template specialization, its associated namespaces and classes also include: the namespaces and classes associated with the types of the template arguments provided for template type parameters (excluding template template parameters); the namespaces of which any template template arguments are members; and the classes of which any member templates used as template template arguments are members.
    [ Note
    :
    Non-type template arguments do not contribute to the set of associated namespaces.
    — end note
     ]
  • (2.3)
    If T is an enumeration type, its associated namespace is the innermost enclosing namespace of its declaration.
    If it is a class member, its associated class is the member's class; else it has no associated class.
  • (2.4)
    If T is a pointer to U or an array of U, its associated namespaces and classes are those associated with U.
  • (2.5)
    If T is a function type, its associated namespaces and classes are those associated with the function parameter types and those associated with the return type.
  • (2.6)
    If T is a pointer to a member function of a class X, its associated namespaces and classes are those associated with the function parameter types and return type, together with those associated with X.
  • (2.7)
    If T is a pointer to a data member of class X, its associated namespaces and classes are those associated with the member type together with those associated with X.
If an associated namespace is an inline namespace ([namespace.def]), its enclosing namespace is also included in the set.
If an associated namespace directly contains inline namespaces, those inline namespaces are also included in the set.
In addition, if the argument is the name or address of a set of overloaded functions and/or function templates, its associated classes and namespaces are the union of those associated with each of the members of the set, i.e., the classes and namespaces associated with its parameter types and return type. Additionally, if the aforementioned set of overloaded functions is named with a template-id, its associated classes and namespaces also include those of its type template-arguments and its template template-arguments.
3
#
Let X be the lookup set produced by unqualified lookup ([basic.lookup.unqual]) and let Y be the lookup set produced by argument dependent lookup (defined as follows).
If X contains
  • (3.1)
    a declaration of a class member, or
  • (3.2)
    a block-scope function declaration that is not a using-declaration, or
  • (3.3)
    a declaration that is neither a function nor a function template
then Y is empty.
Otherwise Y is the set of declarations found in the namespaces associated with the argument types as described below.
The set of declarations found by the lookup of the name is the union of X and Y.
[ Note
:
The namespaces and classes associated with the argument types can include namespaces and classes already considered by the ordinary unqualified lookup.
— end note
 ]
[ Example
: 
namespace NS {
  class T { };
  void f(T);
  void g(T, int);
}
NS::T parm;
void g(NS::T, float);
int main() {
  f(parm);                      // OK: calls NS​::​f
  extern void g(NS::T, float);
  g(parm, 1);                   // OK: calls g(NS​::​T, float)
}
— end example
 ]
4
#
When considering an associated namespace, the lookup is the same as the lookup performed when the associated namespace is used as a qualifier ([namespace.qual]) except that:
  • (4.1)
    Any using-directives in the associated namespace are ignored.
  • (4.2)
    Any namespace-scope friend functions or friend function templates declared in associated classes are visible within their respective namespaces even if they are not visible during an ordinary lookup ([class.friend]).
  • (4.3)
    All names except those of (possibly overloaded) functions and function templates are ignored.

6.4.3 Qualified name lookup [basic.lookup.qual]

1
#
The name of a class or namespace member or enumerator can be referred to after the ​::​ scope resolution operator ([expr.prim]) applied to a nested-name-specifier that denotes its class, namespace, or enumeration.
If a ​::​ scope resolution operator in a nested-name-specifier is not preceded by a decltype-specifier, lookup of the name preceding that ​::​ considers only namespaces, types, and templates whose specializations are types.
If the name found does not designate a namespace or a class, enumeration, or dependent type, the program is ill-formed.
[ Example
: 
class A {
public:
  static int n;
};
int main() {
  int A;
  A::n = 42;        // OK
  A b;              // ill-formed: A does not name a type
}
— end example
 ]
2
#
[ Note
:
Multiply qualified names, such as N1​::​N2​::​N3​::​n, can be used to refer to members of nested classes ([class.nest]) or members of nested namespaces.
— end note
 ]
3
#
In a declaration in which the declarator-id is a qualified-id, names used before the qualified-id being declared are looked up in the defining namespace scope; names following the qualified-id are looked up in the scope of the member's class or namespace.
[ Example
: 
class X { };
class C {
  class X { };
  static const int number = 50;
  static X arr[number];
};
X C::arr[number];   // ill-formed:
                    // equivalent to ​::​X C​::​arr[C​::​number];
                    // and not to C​::​X C​::​arr[C​::​number];
— end example
 ]
4
#
A name prefixed by the unary scope operator ​::​ ([expr.prim]) is looked up in global scope, in the translation unit where it is used.
The name shall be declared in global namespace scope or shall be a name whose declaration is visible in global scope because of a using-directive ([namespace.qual]).
The use of ​::​ allows a global name to be referred to even if its identifier has been hidden ([basic.scope.hiding]).
5
#
A name prefixed by a nested-name-specifier that nominates an enumeration type shall represent an enumerator of that enumeration.
6
#
If a pseudo-destructor-name ([expr.pseudo]) contains a nested-name-specifier, the type-names are looked up as types in the scope designated by the nested-name-specifier.
Similarly, in a qualified-id of the form:
nested-name-specifier class-name :: ~ class-name
the second class-name is looked up in the same scope as the first.
[ Example
: 
struct C {
  typedef int I;
};
typedef int I1, I2;
extern int* p;
extern int* q;
p->C::I::~I();      // I is looked up in the scope of C
q->I1::~I2();       // I2 is looked up in the scope of the postfix-expression

struct A {
  ~A();
};
typedef A AB;
int main() {
  AB* p;
  p->AB::~AB();     // explicitly calls the destructor for A
}
— end example
 ]
[ Note
:
[basic.lookup.classref] describes how name lookup proceeds after the . and -> operators.
— end note
 ]

6.4.3.1 Class members [class.qual]

1
#
If the nested-name-specifier of a qualified-id nominates a class, the name specified after the nested-name-specifier is looked up in the scope of the class ([class.member.lookup]), except for the cases listed below.
The name shall represent one or more members of that class or of one of its base classes (Clause [class.derived]).
[ Note
:
A class member can be referred to using a qualified-id at any point in its potential scope ([basic.scope.class]).
— end note
 ]
The exceptions to the name lookup rule above are the following:
2
#
In a lookup in which function names are not ignored34 and the nested-name-specifier nominates a class C: the name is instead considered to name the constructor of class C.
[ Note
:
For example, the constructor is not an acceptable lookup result in an elaborated-type-specifier so the constructor would not be used in place of the injected-class-name.
— end note
 ]
Such a constructor name shall be used only in the declarator-id of a declaration that names a constructor or in a using-declaration.
[ Example
: 
struct A { A(); };
struct B: public A { B(); };

A::A() { }
B::B() { }

B::A ba;            // object of type A
A::A a;             // error, A​::​A is not a type name
struct A::A a2;     // object of type A
— end example
 ]
3
#
A class member name hidden by a name in a nested declarative region or by the name of a derived class member can still be found if qualified by the name of its class followed by the ​::​ operator.
34)
Lookups in which function names are ignored include names appearing in a nested-name-specifier, an elaborated-type-specifier, or a base-specifier.

6.4.3.2 Namespace members [namespace.qual]

1
#
If the nested-name-specifier of a qualified-id nominates a namespace (including the case where the nested-name-specifier is ​::​, i.e., nominating the global namespace), the name specified after the nested-name-specifier is looked up in the scope of the namespace.
The names in a template-argument of a template-id are looked up in the context in which the entire postfix-expression occurs.
2
#
For a namespace X and name m, the namespace-qualified lookup set is defined as follows: Let be the set of all declarations of m in X and the inline namespace set of X ([namespace.def]).
If is not empty, is ; otherwise, is the union of for all namespaces nominated by using-directives in X and its inline namespace set.
3
#
Given X​::​m (where X is a user-declared namespace), or given ​::​m (where X is the global namespace), if is the empty set, the program is ill-formed.
Otherwise, if has exactly one member, or if the context of the reference is a using-declaration ([namespace.udecl]), is the required set of declarations of m.
Otherwise if the use of m is not one that allows a unique declaration to be chosen from , the program is ill-formed.
[ Example
: 
int x;
namespace Y {
  void f(float);
  void h(int);
}

namespace Z {
  void h(double);
}

namespace A {
  using namespace Y;
  void f(int);
  void g(int);
  int i;
}

namespace B {
  using namespace Z;
  void f(char);
  int i;
}

namespace AB {
  using namespace A;
  using namespace B;
  void g();
}

void h()
{
  AB::g();          // g is declared directly in AB, therefore S is { AB​::​g() } and AB​::​g() is chosen

  AB::f(1);         // f is not declared directly in AB so the rules are applied recursively to A and B;
                    // namespace Y is not searched and Y​::​f(float) is not considered;
                    // S is  and overload resolution chooses A​::​f(int)

  AB::f('c');       // as above but resolution chooses B​::​f(char)

  AB::x++;          // x is not declared directly in AB, and is not declared in A or B, so the rules
                    // are applied recursively to Y and Z, S is { } so the program is ill-formed

  AB::i++;          // i is not declared directly in AB so the rules are applied recursively to A and B,
                    // S is  so the use is ambiguous and the program is ill-formed

  AB::h(16.8);      // h is not declared directly in AB and not declared directly in A or B so the rules
                    // are applied recursively to Y and Z, S is  and
                    // overload resolution chooses Z​::​h(double)
}
— end example
 ]
4
#
[ Note
:
The same declaration found more than once is not an ambiguity (because it is still a unique declaration).
[ Example
: 
namespace A {
  int a;
}

namespace B {
  using namespace A;
}

namespace C {
  using namespace A;
}

namespace BC {
  using namespace B;
  using namespace C;
}

void f()
{
  BC::a++;          // OK: S is 
}

namespace D {
  using A::a;
}

namespace BD {
  using namespace B;
  using namespace D;
}

void g()
{
  BD::a++;          // OK: S is 
}
— end example
 ]
— end note
 ]
5
#
[ Example
:
Because each referenced namespace is searched at most once, the following is well-defined:
namespace B {
  int b;
}

namespace A {
  using namespace B;
  int a;
}

namespace B {
  using namespace A;
}

void f()
{
  A::a++;           // OK: a declared directly in A, S is { A​::​a }
  B::a++;           // OK: both A and B searched (once), S is { A​::​a }
  A::b++;           // OK: both A and B searched (once), S is { B​::​b }
  B::b++;           // OK: b declared directly in B, S is { B​::​b }
}
— end example
 ]
6
#
During the lookup of a qualified namespace member name, if the lookup finds more than one declaration of the member, and if one declaration introduces a class name or enumeration name and the other declarations either introduce the same variable, the same enumerator or a set of functions, the non-type name hides the class or enumeration name if and only if the declarations are from the same namespace; otherwise (the declarations are from different namespaces), the program is ill-formed.
[ Example
: 
namespace A {
  struct x { };
  int x;
  int y;
}

namespace B {
  struct y { };
}

namespace C {
  using namespace A;
  using namespace B;
  int i = C::x;     // OK, A​::​x (of type int)
  int j = C::y;     // ambiguous, A​::​y or B​::​y
}
— end example
 ]
7
#
In a declaration for a namespace member in which the declarator-id is a qualified-id, given that the qualified-id for the namespace member has the form
nested-name-specifier unqualified-id
the unqualified-id shall name a member of the namespace designated by the nested-name-specifier or of an element of the inline namespace set ([namespace.def]) of that namespace.
[ Example
: 
namespace A {
  namespace B {
    void f1(int);
  }
  using namespace B;
}
void A::f1(int){ }  // ill-formed, f1 is not a member of A
— end example
 ]
However, in such namespace member declarations, the nested-name-specifier may rely on using-directives to implicitly provide the initial part of the nested-name-specifier.
[ Example
: 
namespace A {
  namespace B {
    void f1(int);
  }
}

namespace C {
  namespace D {
    void f1(int);
  }
}

using namespace A;
using namespace C::D;
void B::f1(int){ }  // OK, defines A​::​B​::​f1(int)
— end example
 ]

6.4.4 Elaborated type specifiers [basic.lookup.elab]

1
#
An elaborated-type-specifier ([dcl.type.elab]) may be used to refer to a previously declared class-name or enum-name even though the name has been hidden by a non-type declaration ([basic.scope.hiding]).
2
#
If the elaborated-type-specifier has no nested-name-specifier, and unless the elaborated-type-specifier appears in a declaration with the following form:
class-key attribute-specifier-seq identifier ;
the identifier is looked up according to [basic.lookup.unqual] but ignoring any non-type names that have been declared.
If the elaborated-type-specifier is introduced by the enum keyword and this lookup does not find a previously declared type-name, the elaborated-type-specifier is ill-formed.
If the elaborated-type-specifier is introduced by the class-key and this lookup does not find a previously declared type-name, or if the elaborated-type-specifier appears in a declaration with the form:
class-key attribute-specifier-seq identifier ;
the elaborated-type-specifier is a declaration that introduces the class-name as described in [basic.scope.pdecl].
3
#
If the elaborated-type-specifier has a nested-name-specifier, qualified name lookup is performed, as described in [basic.lookup.qual], but ignoring any non-type names that have been declared.
If the name lookup does not find a previously declared type-name, the elaborated-type-specifier is ill-formed.
[ Example
: 
struct Node {
  struct Node* Next;            // OK: Refers to Node at global scope
  struct Data* Data;            // OK: Declares type Data
                                // at global scope and member Data
};

struct Data {
  struct Node* Node;            // OK: Refers to Node at global scope
  friend struct ::Glob;         // error: Glob is not declared, cannot introduce a qualified type ([dcl.type.elab])
  friend struct Glob;           // OK: Refers to (as yet) undeclared Glob at global scope.
  /* ... */
};

struct Base {
  struct Data;                  // OK: Declares nested Data
  struct ::Data*     thatData;  // OK: Refers to ​::​Data
  struct Base::Data* thisData;  // OK: Refers to nested Data
  friend class ::Data;          // OK: global Data is a friend
  friend class Data;            // OK: nested Data is a friend
  struct Data { /* ... */ };    // Defines nested Data
};

struct Data;                    // OK: Redeclares Data at global scope
struct ::Data;                  // error: cannot introduce a qualified type ([dcl.type.elab])
struct Base::Data;              // error: cannot introduce a qualified type ([dcl.type.elab])
struct Base::Datum;             // error: Datum undefined
struct Base::Data* pBase;       // OK: refers to nested Data
— end example
 ]

6.4.5 Class member access [basic.lookup.classref]

1
#
In a class member access expression ([expr.ref]), if the . or -> token is immediately followed by an identifier followed by a <, the identifier must be looked up to determine whether the < is the beginning of a template argument list ([temp.names]) or a less-than operator.
The identifier is first looked up in the class of the object expression.
If the identifier is not found, it is then looked up in the context of the entire postfix-expression and shall name a class template.
2
#
If the id-expression in a class member access ([expr.ref]) is an unqualified-id, and the type of the object expression is of a class type C, the unqualified-id is looked up in the scope of class C.
For a pseudo-destructor call ([expr.pseudo]), the unqualified-id is looked up in the context of the complete postfix-expression.
3
#
If the unqualified-id is ~type-name, the type-name is looked up in the context of the entire postfix-expression.
If the type T of the object expression is of a class type C, the type-name is also looked up in the scope of class C.
At least one of the lookups shall find a name that refers to cv T.
[ Example
: 
struct A { };

struct B {
  struct A { };
  void f(::A* a);
};

void B::f(::A* a) {
  a->~A();                      // OK: lookup in *a finds the injected-class-name
}
— end example
 ]
4
#
If the id-expression in a class member access is a qualified-id of the form 
class-name-or-namespace-name::...
the class-name-or-namespace-name following the . or -> operator is first looked up in the class of the object expression and the name, if found, is used.
Otherwise it is looked up in the context of the entire postfix-expression.
[ Note
:
See [basic.lookup.qual], which describes the lookup of a name before ​::​, which will only find a type or namespace name.
— end note
 ]
5
#
If the qualified-id has the form 
::class-name-or-namespace-name::...
the class-name-or-namespace-name is looked up in global scope as a class-name or namespace-name.
6
#
If the nested-name-specifier contains a simple-template-id ([temp.names]), the names in its template-arguments are looked up in the context in which the entire postfix-expression occurs.
7
#
If the id-expression is a conversion-function-id, its conversion-type-id is first looked up in the class of the object expression and the name, if found, is used.
Otherwise it is looked up in the context of the entire postfix-expression.
In each of these lookups, only names that denote types or templates whose specializations are types are considered.
[ Example
: 
struct A { };
namespace N {
  struct A {
    void g() { }
    template <class T> operator T();
  };
}

int main() {
  N::A a;
  a.operator A();               // calls N​::​A​::​operator N​::​A
}
— end example
 ]

6.4.6 Using-directives and namespace aliases [basic.lookup.udir]

1
#
In a using-directive or namespace-alias-definition, during the lookup for a namespace-name or for a name in a nested-name-specifier only namespace names are considered.