3 Basic concepts [basic]

The declarative region of the name of a template parameter of a template template-parameter is the smallest template-parameter-list in which the name was introduced.

The declarative region of the name of a template parameter of a template is the smallest template-declaration in which the name was introduced. Only template parameter names belong to this declarative region; any other kind of name introduced by the declaration of a template-declaration is instead introduced into the same declarative region where it would be introduced as a result of a non-template declaration of the same name. [ Example:

namespace N {
  template<class T> struct A { };               // #1
  template<class U> void f(U) { }               // #2
  struct B {
    template<class V> friend int g(struct C*);  // #3
  };
}

The declarative regions of T, U and V are the template-declarations on lines #1, #2 and #3, respectively. But the names A, f, g and C all belong to the same declarative region — namely, the namespace-body of N. (g is still considered to belong to this declarative region in spite of its being hidden during qualified and unqualified name lookup.)  — end example ]

The potential scope of a template parameter name begins at its point of declaration ([basic.scope.pdecl]) and ends at the end of its declarative region. [ Note: This implies that a template-parameter can be used in the declaration of subsequent template-parameters and their default arguments but cannot be used in preceding template-parameters or their default arguments. For example,

template<class T, T* p, class U = T> class X { /* ... */ };
template<class T> void f(T* p = new T);

This also implies that a template-parameter can be used in the specification of base classes. For example,

template<class T> class X : public Array<T> { /* ... */ };
template<class T> class Y : public T { /* ... */ };

The use of a template parameter as a base class implies that a class used as a template argument must be defined and not just declared when the class template is instantiated.  — end note ]

The declarative region of the name of a template parameter is nested within the immediately-enclosing declarative region. [ Note: As a result, a template-parameter hides any entity with the same name in an enclosing scope ([basic.scope.hiding]). [ Example:

typedef int N;
template<N X, typename N, template<N Y> class T> struct A;

Here, X is a non-type template parameter of type int and Y is a non-type template parameter of the same type as the second template parameter of A.  — end example ] — end note ]

[ Note: Because the name of a template parameter cannot be redeclared within its potential scope ([temp.local]), a template parameter's scope is often its potential scope. However, it is still possible for a template parameter name to be hidden; see [temp.local].  — end note ]