12 Overloading [over]

12.3 Overload resolution [over.match]

12.3.1 Candidate functions and argument lists [over.match.funcs]

12.3.1.8 Class template argument deduction [over.match.class.deduct]

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:
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, 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 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 element that has a dependent type.
If there is no such element , the program is ill-formed.
The aggregate deduction candidate is derived as above from a hypothetical constructor , where is the declared type of the element .
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-id
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.
Let g denote the result of substituting these deductions into f.
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.
As an exception, 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.
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
:
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>
— end example
 ]
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>>;
— end example
 ]