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 types of the function parameters are those 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:
- The template parameters, if any, and function parameters are those of the deduction-guide.
- The return type is the simple-template-id of the deduction-guide.

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-guides 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 or an array type with a value-dependent bound, 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 or string-literal, is an rvalue reference to the declared type of , and
- otherwise, is the declared type of ,

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

typename nested-name-specifier template simple-template-idas 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 substitution succeeds,
form a function or function template f'
with the following properties and add it to the set
of guides of A:

- The function type of f' is the function type of g.
- 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 is a copy deduction candidate ([over.match.class.deduct]), then f' is considered to be so as well.
- If f was generated from a deduction-guide ([over.match.class.deduct]), then f' is considered to be so as well.
- The explicit-specifier of f' is the explicit-specifier of g (if any).

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

: *end example*

]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—

[ Example

: *end example*

]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 *>

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<T, 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>>;