13 Templates [temp]

13.7 Template declarations [temp.decls]

13.7.7 Alias templates [temp.alias]

1
#
An alias template is a name for a family of types.
The name of the alias template is a template-name.
2
#
When a template-id refers to the specialization of an alias template, it is equivalent to the associated type obtained by substitution of its template-arguments for the template-parameters in the defining-type-id of the alias template.
[Note
:
An alias template name is never deduced.
— end note
]
[Example
: 
template<class T> struct Alloc { /* ... */ };
template<class T> using Vec = vector<T, Alloc<T>>;
Vec<int> v;         // same as vector<int, Alloc<int>> v;

template<class T>
  void process(Vec<T>& v)
  { /* ... */ }

template<class T>
  void process(vector<T, Alloc<T>>& w)
  { /* ... */ }     // error: redefinition

template<template<class> class TT>
  void f(TT<int>);

f(v);               // error: Vec not deduced

template<template<class,class> class TT>
  void g(TT<int, Alloc<int>>);
g(v);               // OK: TT = vector
— end example
]
3
#
However, if the template-id is dependent, subsequent template argument substitution still applies to the template-id.
[Example
: 
template<typename...> using void_t = void;
template<typename T> void_t<typename T::foo> f();
f<int>();           // error: int does not have a nested type foo
— end example
]
4
#
The defining-type-id in an alias template declaration shall not refer to the alias template being declared.
The type produced by an alias template specialization shall not directly or indirectly make use of that specialization.
[Example
: 
template <class T> struct A;
template <class T> using B = typename A<T>::U;
template <class T> struct A {
  typedef B<T> U;
};
B<short> b;         // error: instantiation of B<short> uses own type via A<short>​::​U
— end example
]
5
#
The type of a lambda-expression appearing in an alias template declaration is different between instantiations of that template, even when the lambda-expression is not dependent.
[Example
: 
template <class T>
  using A = decltype([] { });   // A<int> and A<char> refer to different closure types
— end example
]