6 Basics [basic]

6.4 Scope [basic.scope]

6.4.9 Template parameter scope [basic.scope.temp]

1
#
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.
2
#
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
]
3
#
The potential scope of a template parameter name begins at its point of declaration 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
]
4
#
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
]
5
#
[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
]