13 Templates [temp]

13.8 Name resolution [temp.res]

13.8.2 Locally declared names [temp.local]

Like normal (non-template) classes, class templates have an injected-class-name ([class.pre]).
The injected-class-name can be used as a template-name or a type-name.
When it is used with a template-argument-list, as a template-argument for a template template-parameter, or as the final identifier in the elaborated-type-specifier of a friend class template declaration, it is a template-name that refers to the class template itself.
Otherwise, it is a type-name equivalent to the template-name followed by the template-parameters of the class template enclosed in <>.
When the inject-class-name of a class template specialization or partial specialization is used as a type-name, it is equivalent to the template-name followed by the template-arguments of the class template specialization or partial specialization enclosed in <>.
[Example 1: template<template<class> class T> class A { }; template<class T> class Y; template<> class Y<int> { Y* p; // meaning Y<int> Y<char>* q; // meaning Y<char> A<Y>* a; // meaning A<​::​Y> class B { template<class> friend class Y; // meaning ​::​Y }; }; — end example]
The injected-class-name of a class template or class template specialization can be used as either a template-name or a type-name wherever it is named.
[Example 2: template <class T> struct Base { Base* p; }; template <class T> struct Derived: public Base<T> { typename Derived::Base* p; // meaning Derived​::​Base<T> }; template<class T, template<class> class U = T::Base> struct Third { }; Third<Derived<int> > t; // OK: default argument uses injected-class-name as a template — end example]
A lookup that finds an injected-class-name ([class.member.lookup]) can result in an ambiguity in certain cases (for example, if it is found in more than one base class).
If all of the injected-class-names that are found refer to specializations of the same class template, and if the name is used as a template-name, the reference refers to the class template itself and not a specialization thereof, and is not ambiguous.
[Example 3: template <class T> struct Base { }; template <class T> struct Derived: Base<int>, Base<char> { typename Derived::Base b; // error: ambiguous typename Derived::Base<double> d; // OK }; — end example]
When the normal name of the template (i.e., the name from the enclosing scope, not the injected-class-name) is used, it always refers to the class template itself and not a specialization of the template.
[Example 4: template<class T> class X { X* p; // meaning X<T> X<T>* p2; X<int>* p3; ::X* p4; // error: missing template argument list // ​::​X does not refer to the injected-class-name }; — end example]
The name of a template-parameter shall not be bound to any following declaration contained by the scope to which the template-parameter belongs.
[Example 5: template<class T, int i> class Y { int T; // error: template-parameter hidden void f() { char T; // error: template-parameter hidden } friend void T(); // OK: no name bound }; template<class X> class X; // error: hidden by template-parameter — end example]
Unqualified name lookup considers the template parameter scope of a template-declaration immediately after the outermost scope associated with the template declared (even if its parent scope does not contain the template-parameter-list).
[Note 1:
The scope of a class template, including its non-dependent base classes ([temp.dep.type], [class.member.lookup]), is searched before its template parameter scope.
— end note]
[Example 6: struct B { }; namespace N { typedef void V; template<class T> struct A : B { typedef void C; void f(); template<class U> void g(U); }; } template<class V> void N::A<V>::f() { // N​::​V not considered here V v; // V is still the template parameter, not N​::​V } template<class B> template<class C> void N::A<B>::g(C) { B b; // B is the base class, not the template parameter C c; // C is the template parameter, not A's C } — end example]