6 Basics [basic]

6.5 Name lookup [basic.lookup]

6.5.5 Qualified name lookup [basic.lookup.qual] General [basic.lookup.qual.general]

Lookup of an identifier followed by a ​::​ scope resolution operator considers only namespaces, types, and templates whose specializations are types.
If a name, template-id, or computed-type-specifier is followed by a ​::​, it shall designate a namespace, class, enumeration, or dependent type, and the ​::​ is never interpreted as a complete nested-name-specifier.
[Example 1: class A { public: static int n; }; int main() { int A; A::n = 42; // OK A b; // error: A does not name a type } template<int> struct B : A {}; namespace N { template<int> void B(); int f() { return B<0>::n; // error: N​::​B<0> is not a type } } — end example]
A member-qualified name is the (unique) component name ([expr.prim.id.unqual]), if any, of in the id-expression of a class member access expression ([expr.ref]).
The lookup context of a member-qualified name is the type of its associated object expression (considered dependent if the object expression is type-dependent).
The lookup context of any other qualified name is the type, template, or namespace nominated by the preceding nested-name-specifier.
[Note 1: 
When parsing a class member access, the name following the -> or . is a qualified name even though it is not yet known of which kind.
— end note]
[Example 2: 
In N::C::m.Base::f() Base is a member-qualified name; the other qualified names are C, m, and f.
— end example]
Qualified name lookup in a class, namespace, or enumeration performs a search of the scope associated with it ([class.member.lookup]) except as specified below.
Unless otherwise specified, a qualified name undergoes qualified name lookup in its lookup context from the point where it appears unless the lookup context either is dependent and is not the current instantiation ([temp.dep.type]) or is not a class or class template.
If nothing is found by qualified lookup for a member-qualified name that is the terminal name ([expr.prim.id.unqual]) of a nested-name-specifier and is not dependent, it undergoes unqualified lookup.
[Note 2: 
During lookup for a template specialization, no names are dependent.
— end note]
[Example 3: int f(); struct A { int B, C; template<int> using D = void; using T = void; void f(); }; using B = A; template<int> using C = A; template<int> using D = A; template<int> using X = A; template<class T> void g(T *p) { // as instantiated for g<A>: p->X<0>::f(); // error: A​::​X not found in ((p->X) < 0) > ​::​f() p->template X<0>::f(); // OK, ​::​X found in definition context p->B::f(); // OK, non-type A​::​B ignored p->template C<0>::f(); // error: A​::​C is not a template p->template D<0>::f(); // error: A​::​D<0> is not a class type p->T::f(); // error: A​::​T is not a class type } template void g(A*); — end example]
If a qualified name Q follows a ~:
  • If Q is a member-qualified name, it undergoes unqualified lookup as well as qualified lookup.
  • Otherwise, its nested-name-specifier N shall nominate a type.
    If N has another nested-name-specifier S, Q is looked up as if its lookup context were that nominated by S.
  • Otherwise, if the terminal name of N is a member-qualified name M, Q is looked up as if ~Q appeared in place of M (as above).
  • Otherwise, Q undergoes unqualified lookup.
  • Each lookup for Q considers only types (if Q is not followed by a <) and templates whose specializations are types.
    If it finds nothing or is ambiguous, it is discarded.
  • The type-name that is or contains Q shall refer to its (original) lookup context (ignoring cv-qualification) under the interpretation established by at least one (successful) lookup performed.
[Example 4: struct C { typedef int I; }; typedef int I1, I2; extern int* p; extern int* q; void f() { p->C::I::~I(); // I is looked up in the scope of C q->I1::~I2(); // I2 is found by unqualified lookup } struct A { ~A(); }; typedef A AB; int main() { AB* p; p->AB::~AB(); // explicitly calls the destructor for A } — end example]