When resolving a placeholder for a deduced class type ([dcl.type.class.deduct])
where the *template-name* names a primary class template C,
a set of functions and function templates, called the guides of C,
is formed comprising:

- If C is defined, for each constructor of C, a function template with the following properties:
- The template parameters are the template parameters of C followed by the template parameters (including default template arguments) of the constructor, if any.
- The associated constraints ([temp.constr.decl]) are the conjunction of the associated constraints of C and the associated constraints of the constructor, if any.[
*Note 1*:A—*constraint-expression*in the*template-head*of C is checked for satisfaction before any constraints from the*template-head*or trailing*requires-clause*of the constructor.*end note*] - The
*parameter-declaration-clause*is that of the constructor. - The return type is the class template specialization designated by C and template arguments corresponding to the template parameters of C.

- If C is not defined or does not declare any constructors, an additional function template derived as above from a hypothetical constructor C().
- An additional function template derived as above from a hypothetical constructor C(C), called the
*copy deduction candidate*. - For each
*deduction-guide*, a function or function template with the following properties:

In addition, if C is defined
and its definition satisfies the conditions for
an aggregate class ([dcl.init.aggr])
with the assumption that any dependent base class has
no virtual functions and no virtual base classes, and
the initializer is a non-empty *braced-init-list* or
parenthesized *expression-list*, and
there are no *deduction-guide**s* for C,
the set contains an additional function template,
called the *aggregate deduction candidate*, defined as follows.

Let be the elements
of the *initializer-list* or
*designated-initializer-list*
of the *braced-init-list*, or
of the *expression-list*.

For each , let be the corresponding aggregate element
of C or of one of its (possibly recursive) subaggregates
that would be initialized by ([dcl.init.aggr]) if

- brace elision is not considered for any aggregate element
that has
- a dependent non-array type,
- an array type with a value-dependent bound, or
- an array type with a dependent array element type and is a string literal; and

- each non-trailing aggregate element that is a pack expansion is assumed to correspond to no elements of the initializer list, and
- a trailing aggregate element that is a pack expansion is assumed to correspond to all remaining elements of the initializer list (if any).

If there is no such aggregate element for any ,
the aggregate deduction candidate is not added to the set.

The aggregate deduction candidate is derived as above
from a hypothetical constructor ,
where

- if is of array type and
is a
*braced-init-list*, is an rvalue reference to the declared type of , and - if is of array type and
is a
*string-literal*, is an lvalue reference to the const-qualified declared type of , and - otherwise, is the declared type of ,

In addition,
if C is defined and
inherits constructors ([namespace.udecl])
from a direct base class denoted in the *base-specifier-list*
by a *class-or-decltype* B,
let A be an alias template
whose template parameter list is that of C and
whose *defining-type-id* is B.

If A is a deducible template ([dcl.type.simple]),
the set contains the guides of A
with the return type R of each guide
replaced with typename CC<R>::type given a class template
template <typename> class CC;
whose primary template is not defined and
with a single partial specialization
whose template parameter list is that of A and
whose template argument list is a specialization of A with
the template argument list of A ([temp.dep.type])
having a member typedef type designating a template specialization with
the template argument list of A but
with C as the template.

[*Example 1*: template <typename T> struct B {
B(T);
};
template <typename T> struct C : public B<T> {
using B<T>::B;
};
template <typename T> struct D : public B<T> {};
C c(42); // OK, deduces C<int>
D d(42); // error: deduction failed, no inherited deduction guides
B(int) -> B<char>;
C c2(42); // OK, deduces C<char>
template <typename T> struct E : public B<int> {
using B<int>::B;
};
E e(42); // error: deduction failed, arguments of E cannot be deduced from introduced guides
template <typename T, typename U, typename V> struct F {
F(T, U, V);
};
template <typename T, typename U> struct G : F<U, T, int> {
using G::F::F;
}
G g(true, 'a', 1); // OK, deduces G<char, bool>
template<class T, std::size_t N>
struct H {
T array[N];
};
template<class T, std::size_t N>
struct I {
volatile T array[N];
};
template<std::size_t N>
struct J {
unsigned char array[N];
};
H h = { "abc" }; // OK, deduces H<char, 4> (not T = const char)
I i = { "def" }; // OK, deduces I<char, 4>
J j = { "ghi" }; // error: cannot bind reference to array of unsigned char to array of char in deduction
— *end example*]

When resolving a placeholder for a deduced class type ([dcl.type.simple])
where the *template-name* names an alias template A,
the *defining-type-id* of A must be of the form
as specified in [dcl.type.simple].

The guides of A are the set of functions or function templates
formed as follows.

For each function or function template f in the guides of
the template named by the *simple-template-id*
of the *defining-type-id*,
the template arguments of the return type of f
are deduced
from the *defining-type-id* of A
according to the process in [temp.deduct.type]
with the exception that deduction does not fail
if not all template arguments are deduced.

If deduction fails for another reason,
proceed with an empty set of deduced template arguments.

If substitution succeeds,
form a function or function template f'
with the following properties and add it to the set
of guides of A:

- If f is a function template, f' is a function template whose template parameter list consists of all the template parameters of A (including their default template arguments) that appear in the above deductions or (recursively) in their default template arguments, followed by the template parameters of f that were not deduced (including their default template arguments), otherwise f' is not a function template.
- The associated constraints ([temp.constr.decl]) are the conjunction of the associated constraints of g and a constraint that is satisfied if and only if the arguments of A are deducible (see below) from the return type.
- If f was generated from a
*deduction-guide*([temp.deduct.guide]), then f' is considered to be so as well.

The arguments of a template A are said to be
deducible from a type T if, given a class template
template <typename> class AA;
with a single partial specialization
whose template parameter list is that of A and
whose template argument list is a specialization of A
with the template argument list of A ([temp.dep.type]),
AA<T> matches the partial specialization.

Initialization and overload resolution are performed as described
in [dcl.init] and [over.match.ctor], [over.match.copy],
or [over.match.list] (as appropriate for the type of initialization
performed) for an object of a hypothetical class type, where
the guides of the template named by the placeholder are considered to be the
constructors of that class type for the purpose of forming an overload
set, and the initializer is provided by the context in which class
template argument deduction was performed.

The following exceptions apply:

- The first phase in [over.match.list] (considering initializer-list constructors) is omitted if the initializer list consists of a single expression of type cv U, where U is, or is derived from, a specialization of the class template directly or indirectly named by the placeholder.
- During template argument deduction for the aggregate deduction candidate, the number of elements in a trailing parameter pack is only deduced from the number of remaining function arguments if it is not otherwise deduced.

If the function or function template was generated from
a constructor or *deduction-guide*
that had an *explicit-specifier*,
each such notional constructor is considered to have
that same *explicit-specifier*.

All such notional constructors are considered to be
public members of the hypothetical class type.

[*Example 2*: template <class T> struct A {
explicit A(const T&, ...) noexcept; // #1
A(T&&, ...); // #2
};
int i;
A a1 = { i, i }; // error: explicit constructor #1 selected in copy-list-initialization during deduction,
// cannot deduce from non-forwarding rvalue reference in #2
A a2{i, i}; // OK, #1 deduces to A<int> and also initializes
A a3{0, i}; // OK, #2 deduces to A<int> and also initializes
A a4 = {0, i}; // OK, #2 deduces to A<int> and also initializes
template <class T> A(const T&, const T&) -> A<T&>; // #3
template <class T> explicit A(T&&, T&&) -> A<T>; // #4
A a5 = {0, 1}; // error: explicit deduction guide #4 selected in copy-list-initialization during deduction
A a6{0,1}; // OK, #4 deduces to A<int> and #2 initializes
A a7 = {0, i}; // error: #3 deduces to A<int&>, #1 and #2 declare same constructor
A a8{0,i}; // error: #3 deduces to A<int&>, #1 and #2 declare same constructor
template <class T> struct B {
template <class U> using TA = T;
template <class U> B(U, TA<U>);
};
B b{(int*)0, (char*)0}; // OK, deduces B<char*>
template <typename T>
struct S {
T x;
T y;
};
template <typename T>
struct C {
S<T> s;
T t;
};
template <typename T>
struct D {
S<int> s;
T t;
};
C c1 = {1, 2}; // error: deduction failed
C c2 = {1, 2, 3}; // error: deduction failed
C c3 = {{1u, 2u}, 3}; // OK, deduces C<int>
D d1 = {1, 2}; // error: deduction failed
D d2 = {1, 2, 3}; // OK, braces elided, deduces D<int>
template <typename T>
struct E {
T t;
decltype(t) t2;
};
E e1 = {1, 2}; // OK, deduces E<int>
template <typename... T>
struct Types {};
template <typename... T>
struct F : Types<T...>, T... {};
struct X {};
struct Y {};
struct Z {};
struct W { operator Y(); };
F f1 = {Types<X, Y, Z>{}, {}, {}}; // OK, F<X, Y, Z> deduced
F f2 = {Types<X, Y, Z>{}, X{}, Y{}}; // OK, F<X, Y, Z> deduced
F f3 = {Types<X, Y, Z>{}, X{}, W{}}; // error: conflicting types deduced; operator Y not considered
— *end example*]

[*Example 3*: template <class T, class U> struct C {
C(T, U); // #1
};
template<class T, class U>
C(T, U) -> C<T, std::type_identity_t<U>>; // #2
template<class V> using A = C<V *, V *>;
template<std::integral W> using B = A<W>;
int i{};
double d{};
A a1(&i, &i); // deduces A<int>
A a2(i, i); // error: cannot deduce V * from i
A a3(&i, &d); // error: #1: cannot deduce (V*, V*) from (int *, double *)
// #2: cannot deduce A<V> from C<int *, double *>
B b1(&i, &i); // deduces B<int>
B b2(&d, &d); // error: cannot deduce B<W> from C<double *, double *>
*end example*]

Possible exposition-only implementation of the above procedure:
// The following concept ensures a specialization of A is deduced.
template <class> class AA;
template <class V> class AA<A<V>> { };
template <class T> concept deduces_A = requires { sizeof(AA<T>); };
// f1 is formed from the constructor #1 of C, generating the following function template
template<class T, class U>
auto f1(T, U) -> C<T, U>;
// Deducing arguments for C<T, U> from C<V *, V*> deduces T as V * and U as V *;
// f1' is obtained by transforming f1 as described by the above procedure.
template<class V> requires deduces_A<C<V *, V *>>
auto f1_prime(V *, V*) -> C<V *, V *>;
// f2 is formed from the deduction-guide #2 of C
template<class T, class U> auto f2(T, U) -> C<T, std::type_identity_t<U>>;
// Deducing arguments for C<T, std::type_identity_t<U>> from C<V *, V*> deduces T as V *;
// f2' is obtained by transforming f2 as described by the above procedure.
template<class V, class U>
requires deduces_A<C<V *, std::type_identity_t<U>>>
auto f2_prime(V *, U) -> C<V *, std::type_identity_t<U>>;
// The following concept ensures a specialization of B is deduced.
template <class> class BB;
template <class V> class BB<B<V>> { };
template <class T> concept deduces_B = requires { sizeof(BB<T>); };
// The guides for B derived from the above f1' and f2' for A are as follows:
template<std::integral W>
requires deduces_A<C<W *, W *>> && deduces_B<C<W *, W *>>
auto f1_prime_for_B(W *, W *) -> C<W *, W *>;
template<std::integral W, class U>
requires deduces_A<C<W *, std::type_identity_t<U>>> &&
deduces_B<C<W *, std::type_identity_t<U>>>
auto f2_prime_for_B(W *, U) -> C<W *, std::type_identity_t<U>>;

—