13 Templates [temp]

13.7 Template declarations [temp.decls]

13.7.1 Class templates [temp.class]

13.7.1.2 Deduction guides [temp.deduct.guide]

1
#
Deduction guides are used when a template-name appears as a type specifier for a deduced class type ([dcl.type.class.deduct]).
Deduction guides are not found by name lookup.
Instead, when performing class template argument deduction ([over.match.class.deduct]), any deduction guides declared for the class template are considered.
2
#
[Example
: 
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
]
3
#
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.