12 Overloading [over]

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:

• (1.1)
  If C is defined, for each constructor of C, a function template with the following properties:

  • (1.1.1)
    The template parameters are the template parameters of C followed by the template parameters (including default template arguments) of the constructor, if any.
  • (1.1.2)
    The types of the function parameters are those of the constructor.
  • (1.1.3)
    The return type is the class template specialization designated by C and template arguments corresponding to the template parameters of C.
• (1.2)
  If C is not defined or does not declare any constructors, an additional function template derived as above from a hypothetical constructor C().
• (1.3)
  An additional function template derived as above from a hypothetical constructor C(C), called the copy deduction candidate.
• (1.4)
  For each deduction-guide, a function or function template with the following properties:

  • (1.4.1)
    The template parameters, if any, and function parameters are those of the deduction-guide.
  • (1.4.2)
    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 $x_1,\dots ,x_n$ be the elements of the initializer-list or designated-initializer-list of the braced-init-list, or of the expression-list.

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

• (1.5)
  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
• (1.6)
  each non-trailing aggregate element that is a pack expansion is assumed to correspond to no elements of the initializer list, and
• (1.7)
  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 $e_i$ for any $x_i$, the aggregate deduction candidate is not added to the set.

The aggregate deduction candidate is derived as above from a hypothetical constructor $\text{C}({\text{T}}_1,\dots ,{\text{T}}_n)$, where

• (1.8)
  if $e_i$ is of array type and $x_i$ is a braced-init-list or string-literal, ${\text{T}}_i$ is an rvalue reference to the declared type of $e_i$, and
• (1.9)
  otherwise, ${\text{T}}_i$ is the declared type of $e_i$,

except that additional parameter packs of the form ${\text{P}}_j\text{...}$ are inserted into the parameter list in their original aggregate element position corresponding to each non-trailing aggregate element of type ${\text{P}}_j$ that was skipped because it was a parameter pack, and the trailing sequence of parameters corresponding to a trailing aggregate element that is a pack expansion (if any) is replaced by a single parameter of the form ${\text{T}}_n\text{...}$.

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.

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:

• (2.1)
  The function type of f' is the function type of g.
• (2.2)
  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.
• (2.3)
  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.
• (2.4)
  If f is a copy deduction candidate, then f' is considered to be so as well.
• (2.5)
  If f was generated from a deduction-guide ([temp.deduct.guide]), then f' is considered to be so as well.
• (2.6)
  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:

• (4.1)
  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.
• (4.2)
  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.