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 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.
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, or as the simple-type-specifier in an explicit type conversion (functional notation) ([expr.type.conv]). A placeholder for a deduced class type shall not appear in any other context.
[ 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 ]