17 Templates [temp]

17.9 Deduction guides [temp.deduct.guide]

Deduction guides are used when a template-name appears as a type specifier for a deduced class type. Deduction guides are not found by name lookup. Instead, when performing class template argument deduction, any deduction guides declared for the class template are considered.

	explicit template-name ( parameter-declaration-clause ) -> simple-template-id ;


template<class T, class D = int>
struct S {
  T data;
template<class U>
S(U) -> S<typename U::type>;

struct A {
  using type = short;
  operator type();
S x{A()};           // x is of type S<short, int>

end example]

The same restrictions apply to the parameter-declaration-clause of a deduction guide as in a function declaration ([dcl.fct]). The simple-template-id shall name a class template specialization. The template-name shall be the same identifier as the template-name of the simple-template-id. A deduction-guide shall be declared in the same scope as the corresponding class template and, for a member class template, with the same access. Two deduction guide declarations in the same translation unit for the same class template shall not have equivalent parameter-declaration-clauses.