9 Declarations [dcl.dcl]

9.2 Specifiers [dcl.spec]

9.2.8 Type specifiers [dcl.type]

9.2.8.6 Deduced class template specialization types [dcl.type.class.deduct]

1
#
If a placeholder for a deduced class type appears as a decl-specifier in the decl-specifier-seq of an initializing declaration ([dcl.init]) of a variable, the declared type of the variable shall be cv T, where T is the placeholder.
[Example
: 
template <class ...T> struct A {
  A(T...) {}
};
A x[29]{};          // error: no declarator operators allowed
const A& y{};       // error: no declarator operators allowed
— end example
]
The placeholder is replaced by the return type of the function selected by overload resolution for class template deduction ([over.match.class.deduct]).
If the decl-specifier-seq is followed by an init-declarator-list or member-declarator-list containing more than one declarator, the type that replaces the placeholder shall be the same in each deduction.
2
#
A placeholder for a deduced class type can also be used in the type-specifier-seq in the new-type-id or type-id of a new-expression, as the simple-type-specifier in an explicit type conversion (functional notation), or as the type-specifier in the parameter-declaration of a template-parameter.
A placeholder for a deduced class type shall not appear in any other context.
3
#
[Example
: 
template<class T> struct container {
    container(T t) {}
    template<class Iter> container(Iter beg, Iter end);
};
template<class Iter>
container(Iter b, Iter e) -> container<typename std::iterator_traits<Iter>::value_type>;
std::vector<double> v = { /* ... */ };

container c(7);                         // OK, deduces int for T
auto d = container(v.begin(), v.end()); // OK, deduces double for T
container e{5, 6};                      // error: int is not an iterator
— end example
]