[over.match]

# 12 Overloading [[over]](./#over)

## 12.2 Overload resolution [over.match]

### [12.2.1](#general) General [[over.match.general]](over.match.general)

[1](#general-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L49)

Overload resolution is a mechanism for selecting the best
function to call given a list of expressions that are to be the
arguments of the call and a set of[*candidate functions*](#def:candidate "12.2.1 General [over.match.general]") that can
be called based on the context of the call[.](#general-1.sentence-1)

The selection
criteria for the best function are the number of arguments, how
well the arguments match the parameter-type-list of the
candidate function,
how well (for non-static member functions) the object
matches the object parameter,
and certain other properties of the candidate function[.](#general-1.sentence-2)

[*Note [1](#general-note-1)*:

The function selected by overload resolution is not
guaranteed to be appropriate for the context[.](#general-1.sentence-3)

Other restrictions,
such as the accessibility of the function, can make its use in
the calling context ill-formed[.](#general-1.sentence-4)

â *end note*]

[2](#general-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L71)

Overload resolution selects the function to call in seven distinct
contexts within the language:

- [(2.1)](#general-2.1)

invocation of a function named in the [function call syntax](#over.call.func "12.2.2.2.2 Call to designated function [over.call.func]");

- [(2.2)](#general-2.2)

invocation of a function call operator, a pointer-to-function
conversion function, a reference-to-pointer-to-function conversion
function, or a reference-to-function
conversion function on a class object named in the function
call syntax ([[over.call.object]](#over.call.object "12.2.2.2.3 Call to object of class type"));

- [(2.3)](#general-2.3)

invocation of the operator referenced in an expression ([[over.match.oper]](#oper "12.2.2.3 Operators in expressions"));

- [(2.4)](#general-2.4)

invocation of a constructor for default- or direct-initialization ([[dcl.init]](dcl.init "9.5 Initializers"))
of a class object ([[over.match.ctor]](#ctor "12.2.2.4 Initialization by constructor"));

- [(2.5)](#general-2.5)

invocation of a user-defined conversion for[copy-initialization](dcl.init#def:copy-initialization "9.5 Initializers [dcl.init]") of a class object ([[over.match.copy]](#copy "12.2.2.5 Copy-initialization of class by user-defined conversion"));

- [(2.6)](#general-2.6)

invocation of a conversion function for initialization of an object of a
non-class type from an expression of class type ([[over.match.conv]](#conv "12.2.2.6 Initialization by conversion function")); and

- [(2.7)](#general-2.7)

invocation of a conversion function for conversion
in which a reference ([[dcl.init.ref]](dcl.init.ref "9.5.4 References"))
will be [directly bound](#ref "12.2.2.7 Initialization by conversion function for direct reference binding [over.match.ref]")[.](#general-2.sentence-1)

Each of these contexts defines the set of candidate functions and
the list of arguments in its own unique way[.](#general-2.sentence-2)

But, once the
candidate functions and argument lists have been identified, the
selection of the best function is the same in all cases:

- [(2.8)](#general-2.8)

  First, a subset of the candidate functions (those that have
the proper number of arguments and meet certain other
conditions) is selected to form a set ofviable functions ([[over.match.viable]](#viable "12.2.3 Viable functions"))[.](#general-2.8.sentence-1)

- [(2.9)](#general-2.9)

  Then the best viable function is selected based on the[implicit conversion sequences](#over.best.ics "12.2.4.2 Implicit conversion sequences [over.best.ics]") needed to
match each argument to the corresponding parameter of each
viable function[.](#general-2.9.sentence-1)

[3](#general-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L120)

If a best viable function exists and is unique, overload
resolution succeeds and produces it as the result[.](#general-3.sentence-1)

Otherwise
overload resolution fails and the invocation is ill-formed[.](#general-3.sentence-2)

When overload resolution succeeds,
and the best viable function is not [accessible](class.access "11.8 Member access control [class.access]") in the context
in which it is used,
the program is ill-formed[.](#general-3.sentence-3)

[4](#general-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L130)

Overload resolution results in a [*usable candidate*](#def:candidate,usable "12.2.1 General [over.match.general]") if overload resolution succeeds and
the selected candidate
is either not a function ([[over.built]](over.built "12.5 Built-in operators")), or
is a function that is not deleted and
is accessible from the context
in which overload resolution was performed[.](#general-4.sentence-1)

### [12.2.2](#funcs) Candidate functions and argument lists [[over.match.funcs]](over.match.funcs)

#### [12.2.2.1](#funcs.general) General [[over.match.funcs.general]](over.match.funcs.general)

[1](#funcs.general-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L145)

The subclauses of [[over.match.funcs]](#funcs "12.2.2 Candidate functions and argument lists") describe
the set of candidate functions and the argument list submitted to
overload resolution in each context in which
overload resolution is used[.](#funcs.general-1.sentence-1)

The source transformations and constructions defined
in these subclauses are only for the purpose of describing the
overload resolution process[.](#funcs.general-1.sentence-2)

An implementation is not required
to use such transformations and constructions[.](#funcs.general-1.sentence-3)

[2](#funcs.general-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L156)

The set of candidate functions can contain both member and non-member
functions to be resolved against the same argument list[.](#funcs.general-2.sentence-1)

If a member function is

- [(2.1)](#funcs.general-2.1)

an implicit object member function that is not a constructor, or

- [(2.2)](#funcs.general-2.2)

a static member function and
the argument list includes an implied object argument,

it is considered to have an extra first parameter,
called the [*implicit object parameter*](#def:object_parameter,implicit "12.2.2.1 General [over.match.funcs.general]"),
which represents the object for which the member function has been called[.](#funcs.general-2.sentence-2)

[3](#funcs.general-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L173)

Similarly, when appropriate, the context can construct an
argument list that contains an[*implied object argument*](#def:implied_object_argument "12.2.2.1 General [over.match.funcs.general]") as the first argument in the list to denote
the object to be operated on[.](#funcs.general-3.sentence-1)

[4](#funcs.general-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L180)

For implicit object member functions, the type of the implicit object
parameter is

- [(4.1)](#funcs.general-4.1)

âlvalue reference to cv Xâ for functions declared
without a [*ref-qualifier*](dcl.decl.general#nt:ref-qualifier "9.3.1 General [dcl.decl.general]") or with the& [*ref-qualifier*](dcl.decl.general#nt:ref-qualifier "9.3.1 General [dcl.decl.general]")

- [(4.2)](#funcs.general-4.2)

ârvalue reference to cv Xâ for functions declared with the&& [*ref-qualifier*](dcl.decl.general#nt:ref-qualifier "9.3.1 General [dcl.decl.general]")

whereX is the class of which the function is a direct member andcv is the cv-qualification on the
member function declaration[.](#funcs.general-4.sentence-1)

[*Example [1](#funcs.general-example-1)*:

For aconst member
function of classX,
the extra parameter is assumed to have type
âlvalue reference toconst Xâ[.](#funcs.general-4.sentence-2)

â *end example*]

For conversion functions that are implicit object member functions,
the function is considered to be a member of the
class of the implied object argument for the purpose of defining the
type of the implicit object parameter[.](#funcs.general-4.sentence-3)

For non-conversion functions that are implicit object member functions
nominated by a [*using-declaration*](namespace.udecl#nt:using-declaration "9.10 The using declaration [namespace.udecl]") in a derived class, the function is
considered to be a member of the derived class for the purpose of defining
the type of the implicit object parameter[.](#funcs.general-4.sentence-4)

For static member functions, the implicit object parameter is considered
to match any object (since if the function is selected, the object is
discarded)[.](#funcs.general-4.sentence-5)

[*Note [1](#funcs.general-note-1)*:

No actual type is established for the implicit object parameter
of a static member function, and no attempt will be made to determine a
conversion sequence for that parameter ([[over.match.best]](#best "12.2.4 Best viable function"))[.](#funcs.general-4.sentence-6)

â *end note*]

[5](#funcs.general-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L224)

During overload resolution, the implied object argument is
indistinguishable from other arguments[.](#funcs.general-5.sentence-1)

The implicit object
parameter, however, retains its identity since
no user-defined conversions can be applied to achieve a type
match with it[.](#funcs.general-5.sentence-2)

For implicit object member functions declared without a [*ref-qualifier*](dcl.decl.general#nt:ref-qualifier "9.3.1 General [dcl.decl.general]"),
even if the implicit object parameter is not const-qualified,
an rvalue can be bound to the parameter
as long as in all other respects the argument can be
converted to the type of the implicit object parameter[.](#funcs.general-5.sentence-3)

[*Note [2](#funcs.general-note-2)*:

The fact that such an argument is an rvalue does not
affect the [ranking](#over.ics.rank "12.2.4.3 Ranking implicit conversion sequences [over.ics.rank]") of implicit conversion sequences[.](#funcs.general-5.sentence-4)

â *end note*]

[6](#funcs.general-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L243)

Because other than in list-initialization only one user-defined conversion
is allowed
in an
implicit conversion sequence, special rules apply when selecting
the best user-defined conversion ([[over.match.best]](#best "12.2.4 Best viable function"), [[over.best.ics]](#over.best.ics "12.2.4.2 Implicit conversion sequences"))[.](#funcs.general-6.sentence-1)

[*Example [2](#funcs.general-example-2)*: class T {public: T();};

class C : T {public: C(int);};
T a = 1; // error: no viable conversion (T(C(1)) not considered) â *end example*]

[7](#funcs.general-7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L264)

In each case where conversion functions of a class S are considered
for initializing an object or reference of type T,
the candidate functions include the result of a search
for the [*conversion-function-id*](class.conv.fct#nt:conversion-function-id "11.4.8.3 Conversion functions [class.conv.fct]") operator T in S[.](#funcs.general-7.sentence-1)

[*Note [3](#funcs.general-note-3)*:

This search can find a specialization of
a conversion function template ([[basic.lookup]](basic.lookup "6.5 Name lookup"))[.](#funcs.general-7.sentence-2)

â *end note*]

Each such case also defines sets of [*permissible types*](#def:types,permissible "12.2.2.1 General [over.match.funcs.general]") for explicit and non-explicit conversion functions;
each (non-template) conversion function
that

- [(7.1)](#funcs.general-7.1)

is a non-hidden member of S,

- [(7.2)](#funcs.general-7.2)

yields a permissible type, and,

- [(7.3)](#funcs.general-7.3)

for the former set, is non-explicit

is also a candidate function[.](#funcs.general-7.sentence-3)

If initializing an object, for any permissible type cv U, any*cv2* U, *cv2* U&, or *cv2* U&& is also a permissible type[.](#funcs.general-7.sentence-4)

If the set of permissible types for explicit conversion functions is empty,
any candidates that are explicit are discarded[.](#funcs.general-7.sentence-5)

[8](#funcs.general-8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L290)

In each case where a candidate is a function template, candidate
function template specializations
are generated using template argument deduction ([[temp.over]](temp.over "13.10.4 Overload resolution"), [[temp.deduct]](temp.deduct "13.10.3 Template argument deduction"))[.](#funcs.general-8.sentence-1)

If a constructor template or conversion function template
has an [*explicit-specifier*](dcl.fct.spec#nt:explicit-specifier "9.2.3 Function specifiers [dcl.fct.spec]") whose [*constant-expression*](expr.const#nt:constant-expression "7.7 Constant expressions [expr.const]") is value-dependent ([[temp.dep]](temp.dep "13.8.3 Dependent names")),
template argument deduction is performed first and then,
if the context admits only candidates that
are not explicit and the generated specialization is explicit ([[dcl.fct.spec]](dcl.fct.spec "9.2.3 Function specifiers")),
it will be removed from the candidate set[.](#funcs.general-8.sentence-2)

Those candidates are then handled as candidate
functions in the usual way[.](#funcs.general-8.sentence-3)[99](#footnote-99 "The process of argument deduction fully determines the parameter types of the function template specializations, i.e., the parameters of function template specializations contain no template parameter types. Therefore, except where specified otherwise, function template specializations and non-template functions ([dcl.fct]) are treated equivalently for the remainder of overload resolution.")

A given name can refer to, or a conversion can consider,
one or more function templates as well as a set of non-template functions[.](#funcs.general-8.sentence-4)

In such a case, the
candidate functions generated from each function template are combined
with the set of non-template candidate functions[.](#funcs.general-8.sentence-5)

[9](#funcs.general-9)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L323)

A
defaulted move special member function ([[class.copy.ctor]](class.copy.ctor "11.4.5.3 Copy/move constructors"), [[class.copy.assign]](class.copy.assign "11.4.6 Copy/move assignment operator"))
that is defined as deleted
is excluded from the set of candidate functions in all contexts[.](#funcs.general-9.sentence-1)

A constructor inherited from class type C ([[class.inhctor.init]](class.inhctor.init "11.9.4 Initialization by inherited constructor"))
that has a first parameter of type âreference to *cv1* Pâ
(including such a constructor instantiated from a template)
is excluded from the set of candidate functions
when constructing an object of type *cv2* D if the argument list has exactly one argument andC is reference-related to P andP is reference-related to D[.](#funcs.general-9.sentence-2)

[*Example [3](#funcs.general-example-3)*: struct A { A(); // #1 A(A &&); // #2template<typename T> A(T &&); // #3};struct B : A {using A::A;
 B(const B &); // #4 B(B &&) = default; // #5, implicitly deletedstruct X { X(X &&) = delete; } x;};extern B b1;
B b2 = static_cast<B&&>(b1); // calls #4: #1 is not viable, #2, #3, and #5 are not candidatesstruct C { operator B&&(); };
B b3 = C(); // calls #4 â *end example*]

[99)](#footnote-99)[99)](#footnoteref-99)

The process of argument deduction fully
determines the parameter types of
the
function template specializations,
i.e., the parameters of
function template specializations
contain
no template parameter types[.](#footnote-99.sentence-1)

Therefore, except where specified otherwise,
function template specializations
and non-template functions ([[dcl.fct]](dcl.fct "9.3.4.6 Functions")) are treated equivalently
for the remainder of overload resolution[.](#footnote-99.sentence-2)

#### [12.2.2.2](#call) Function call syntax [[over.match.call]](over.match.call)

#### [12.2.2.2.1](#call.general) General [[over.match.call.general]](over.match.call.general)

[1](#call.general-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L362)

In a [function call](expr.call "7.6.1.3 Function call [expr.call]")

[*postfix-expression*](expr.post.general#nt:postfix-expression "7.6.1.1 General [expr.post.general]") ( [*expression-list*](expr.post.general#nt:expression-list "7.6.1.1 General [expr.post.general]")opt )

if the [*postfix-expression*](expr.post.general#nt:postfix-expression "7.6.1.1 General [expr.post.general]") names at least one function or
function template,
overload resolution is applied as specified in [[over.call.func]](#over.call.func "12.2.2.2.2 Call to designated function")[.](#call.general-1.sentence-1)

If the [*postfix-expression*](expr.post.general#nt:postfix-expression "7.6.1.1 General [expr.post.general]") denotes an object of class type, overload
resolution is applied as specified in [[over.call.object]](#over.call.object "12.2.2.2.3 Call to object of class type")[.](#call.general-1.sentence-2)

[2](#call.general-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L373)

If the [*postfix-expression*](expr.post.general#nt:postfix-expression "7.6.1.1 General [expr.post.general]") is the address of an overload set,
overload resolution is applied using that set as described above[.](#call.general-2.sentence-1)

[*Note [1](#call.general-note-1)*:

No implied object argument is added in this case[.](#call.general-2.sentence-2)

â *end note*]

If the function selected by overload resolution is
an implicit object member function,
the program is ill-formed[.](#call.general-2.sentence-3)

[*Note [2](#call.general-note-2)*:

The resolution of the address of an
overload set in other contexts is described in [[over.over]](over.over "12.3 Address of an overload set")[.](#call.general-2.sentence-4)

â *end note*]

#### [12.2.2.2.2](#over.call.func) Call to designated function [[over.call.func]](over.call.func)

[1](#over.call.func-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L389)

Of interest in [[over.call.func]](#over.call.func "12.2.2.2.2 Call to designated function") are only those function calls
in which the [*postfix-expression*](expr.post.general#nt:postfix-expression "7.6.1.1 General [expr.post.general]") ultimately contains
an [*id-expression*](expr.prim.id.general#nt:id-expression "7.5.5.1 General [expr.prim.id.general]") or [*splice-expression*](expr.prim.splice#nt:splice-expression "7.5.9 Expression splicing [expr.prim.splice]") that designates one or more functions[.](#over.call.func-1.sentence-1)

Such a[*postfix-expression*](expr.post.general#nt:postfix-expression "7.6.1.1 General [expr.post.general]"),
perhaps nested arbitrarily deep in
parentheses, has one of the following forms:

[*postfix-expression*](expr.post.general#nt:postfix-expression "7.6.1.1 General [expr.post.general]"):  
[*postfix-expression*](expr.post.general#nt:postfix-expression "7.6.1.1 General [expr.post.general]") . [*id-expression*](expr.prim.id.general#nt:id-expression "7.5.5.1 General [expr.prim.id.general]")  
[*postfix-expression*](expr.post.general#nt:postfix-expression "7.6.1.1 General [expr.post.general]") . [*splice-expression*](expr.prim.splice#nt:splice-expression "7.5.9 Expression splicing [expr.prim.splice]")  
[*postfix-expression*](expr.post.general#nt:postfix-expression "7.6.1.1 General [expr.post.general]") -> [*id-expression*](expr.prim.id.general#nt:id-expression "7.5.5.1 General [expr.prim.id.general]")  
[*postfix-expression*](expr.post.general#nt:postfix-expression "7.6.1.1 General [expr.post.general]") -> [*splice-expression*](expr.prim.splice#nt:splice-expression "7.5.9 Expression splicing [expr.prim.splice]")  
[*id-expression*](expr.prim.id.general#nt:id-expression "7.5.5.1 General [expr.prim.id.general]")  
[*splice-expression*](expr.prim.splice#nt:splice-expression "7.5.9 Expression splicing [expr.prim.splice]")

These represent two syntactic subcategories of function calls:
qualified function calls and unqualified function calls[.](#over.call.func-1.sentence-3)

[2](#over.call.func-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L411)

In qualified function calls,
the function is designated by
an [*id-expression*](expr.prim.id.general#nt:id-expression "7.5.5.1 General [expr.prim.id.general]") or [*splice-expression*](expr.prim.splice#nt:splice-expression "7.5.9 Expression splicing [expr.prim.splice]") E preceded by an -> or . operator[.](#over.call.func-2.sentence-1)

Since the
constructA->B is generally equivalent to(*A).B,
the rest of[[over]](over "12 Overloading") assumes, without loss of generality, that all member
function calls have been normalized to the form that uses an
object and the. operator[.](#over.call.func-2.sentence-2)

Furthermore, [[over]](over "12 Overloading") assumes that
the[*postfix-expression*](expr.post.general#nt:postfix-expression "7.6.1.1 General [expr.post.general]") that is the left operand of the. operator
has type âcv Tâ
whereT denotes a class[.](#over.call.func-2.sentence-3)[100](#footnote-100 "Note that cv-qualifiers on the type of objects are significant in overload resolution for both glvalue and class prvalue objects.")

The set of candidate functions either
is the set found by name lookup ([[class.member.lookup]](class.member.lookup "6.5.2 Member name lookup"))
if E is an [*id-expression*](expr.prim.id.general#nt:id-expression "7.5.5.1 General [expr.prim.id.general]") or
is the set determined as specified in [[expr.prim.splice]](expr.prim.splice "7.5.9 Expression splicing") if E is a [*splice-expression*](expr.prim.splice#nt:splice-expression "7.5.9 Expression splicing [expr.prim.splice]")[.](#over.call.func-2.sentence-4)

The argument list is the[*expression-list*](expr.post.general#nt:expression-list "7.6.1.1 General [expr.post.general]") in the call augmented by the addition of the left operand of
the. operator in the normalized member function call as the
implied object argument ([[over.match.funcs]](#funcs "12.2.2 Candidate functions and argument lists"))[.](#over.call.func-2.sentence-5)

[3](#over.call.func-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L456)

In unqualified function calls, the function is designated by
an [*id-expression*](expr.prim.id.general#nt:id-expression "7.5.5.1 General [expr.prim.id.general]") or a [*splice-expression*](expr.prim.splice#nt:splice-expression "7.5.9 Expression splicing [expr.prim.splice]") E[.](#over.call.func-3.sentence-1)

The set of candidate functions either
is the set found by name lookup ([[basic.lookup]](basic.lookup "6.5 Name lookup"))
if E is an [*id-expression*](expr.prim.id.general#nt:id-expression "7.5.5.1 General [expr.prim.id.general]") or
is the set determined as specified in [[expr.prim.splice]](expr.prim.splice "7.5.9 Expression splicing") if E is a [*splice-expression*](expr.prim.splice#nt:splice-expression "7.5.9 Expression splicing [expr.prim.splice]")[.](#over.call.func-3.sentence-2)

The set of candidate functions
consists either entirely of non-member functions or entirely of
member functions of some classT[.](#over.call.func-3.sentence-3)

In the former case or
if E is either a [*splice-expression*](expr.prim.splice#nt:splice-expression "7.5.9 Expression splicing [expr.prim.splice]") or
the address of an overload set,
the argument list is
the same as the[*expression-list*](expr.post.general#nt:expression-list "7.6.1.1 General [expr.post.general]") in the call[.](#over.call.func-3.sentence-4)

Otherwise, the argument list is the[*expression-list*](expr.post.general#nt:expression-list "7.6.1.1 General [expr.post.general]") in the call augmented by the addition of an implied object
argument as in a qualified function call[.](#over.call.func-3.sentence-5)

If the current class is, or is derived from, T, and the keywordthis ([[expr.prim.this]](expr.prim.this "7.5.3 This")) refers to it,

- [(3.1)](#over.call.func-3.1)

if the unqualified function call
appears in a precondition assertion of a constructor
or a postcondition assertion of a destructor
and overload resolution selects a non-static member function,
the call is ill-formed;

- [(3.2)](#over.call.func-3.2)

otherwise,
the implied object argument is(*this)[.](#over.call.func-3.sentence-6)

Otherwise,

- [(3.3)](#over.call.func-3.3)

if overload resolution selects a non-static member function,
the call is ill-formed;

- [(3.4)](#over.call.func-3.4)

otherwise,
a contrived object of typeT becomes the implied object argument[.](#over.call.func-3.sentence-7)[101](#footnote-101 "An implied object argument is contrived to correspond to the implicit object parameter attributed to member functions during overload resolution. It is not used in the call to the selected function. Since the member functions all have the same implicit object parameter, the contrived object will not be the cause to select or reject a function.")

[*Example [1](#over.call.func-example-1)*: struct C {bool a(); void b() { a(); // OK, (*this).a()}void c(this const C&); // #1void c() &; // #2static void c(int = 0); // #3void d() { c(); // error: ambiguous between #2 and #3(C::c)(); // error: as above(&(C::c))(); // error: cannot resolve address of overloaded this->C​::​c ([[over.over]](over.over "12.3 Address of an overload set"))(&C::c)(C{}); // selects #1(&C::c)(*this); // error: selects #2, and is ill-formed ([[over.match.call.general]](#call.general "12.2.2.2.1 General"))(&C::c)(); // selects #3}void f(this const C&); void g() const { f(); // OK, (*this).f() f(*this); // error: no viable candidate for (*this).f(*this)this->f(); // OK}static void h() { f(); // error: contrived object argument, but overload resolution// picked a non-static member function f(C{}); // error: no viable candidate C{}.f(); // OK}void k(this int); operator int() const; void m(this const C& c) { c.k(); // OK} C() pre(a()) // error: implied this in constructor precondition pre(this->a()) // OK post(a()); // OK~C() pre(a()) // OK post(a()) // error: implied this in destructor postcondition post(this->a()); // OK}; â *end example*]

[100)](#footnote-100)[100)](#footnoteref-100)

Note that cv-qualifiers on the type of objects are
significant in overload
resolution for
both glvalue and class prvalue objects[.](#footnote-100.sentence-1)

[101)](#footnote-101)[101)](#footnoteref-101)

An implied object argument is contrived to
correspond to the implicit object
parameter attributed to member functions during overload resolution[.](#footnote-101.sentence-1)

It is not
used in
the call to the selected function[.](#footnote-101.sentence-2)

Since the member functions all have the
same implicit
object parameter, the contrived object will not be the cause to select or
reject a
function[.](#footnote-101.sentence-3)

#### [12.2.2.2.3](#over.call.object) Call to object of class type [[over.call.object]](over.call.object)

[1](#over.call.object-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L573)

If the [*postfix-expression*](expr.post.general#nt:postfix-expression "7.6.1.1 General [expr.post.general]") E in the function call syntax evaluates
to a class object of type âcv Tâ,
then the set of candidate functions
includes at least the function call operators of T[.](#over.call.object-1.sentence-1)

The function call operators of T are the results of a search for the name operator() in the scope of T[.](#over.call.object-1.sentence-2)

[2](#over.call.object-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L583)

In addition, for each non-explicit conversion function declared in T of the
form

operator [*conversion-type-id*](class.conv.fct#nt:conversion-type-id "11.4.8.3 Conversion functions [class.conv.fct]") ( ) [*cv-qualifier-seq*](dcl.decl.general#nt:cv-qualifier-seq "9.3.1 General [dcl.decl.general]")opt [*ref-qualifier*](dcl.decl.general#nt:ref-qualifier "9.3.1 General [dcl.decl.general]")opt [*noexcept-specifier*](except.spec#nt:noexcept-specifier "14.5 Exception specifications [except.spec]")opt [*attribute-specifier-seq*](dcl.attr.grammar#nt:attribute-specifier-seq "9.13.1 Attribute syntax and semantics [dcl.attr.grammar]")opt ;

where the optional[*cv-qualifier-seq*](dcl.decl.general#nt:cv-qualifier-seq "9.3.1 General [dcl.decl.general]") is the same cv-qualification as, or a greater cv-qualification than,cv,
and where[*conversion-type-id*](class.conv.fct#nt:conversion-type-id "11.4.8.3 Conversion functions [class.conv.fct]") denotes the type âpointer to function
of (P1,â¦,Pn) returning Râ,
or the type âreference to pointer to function
of (P1,â¦,Pn) returning Râ,
or the type
âreference to function of (P1,â¦,Pn)
returning Râ, a [*surrogate call function*](#def:surrogate_call_function "12.2.2.2.3 Call to object of class type [over.call.object]") with the unique name*call-function* and having the form

R *call-function* ( [*conversion-type-id*](class.conv.fct#nt:conversion-type-id "11.4.8.3 Conversion functions [class.conv.fct]") F, P1 a1, …, Pn an) { return F (a1, …, an); }

is also considered as a candidate function[.](#over.call.object-2.sentence-1)

Similarly, surrogate
call functions are added to the set of candidate functions for
each non-explicit conversion function declared in a base class ofT provided the function is not hidden withinT by another
intervening declaration[.](#over.call.object-2.sentence-2)[102](#footnote-102 "Note that this construction can yield candidate call functions that cannot be differentiated one from the other by overload resolution because they have identical declarations or differ only in their return type. The call will be ambiguous if overload resolution cannot select a match to the call that is uniquely better than such undifferentiable functions.")

[3](#over.call.object-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L629)

The argument list submitted to overload resolution consists of
the argument expressions present in the function call syntax
preceded by the implied object argument(E)[.](#over.call.object-3.sentence-1)

[*Note [1](#over.call.object-note-1)*:

When comparing the
call against the function call operators, the implied object
argument is compared against the object parameter of the
function call operator[.](#over.call.object-3.sentence-2)

When comparing the call against a
surrogate call function, the implied object argument is compared
against the first parameter of the surrogate call function[.](#over.call.object-3.sentence-3)

â *end note*]

[*Example [1](#over.call.object-example-1)*: int f1(int);int f2(float);typedef int (*fp1)(int);typedef int (*fp2)(float);struct A {operator fp1() { return f1; }operator fp2() { return f2; }} a;int i = a(1); // calls f1 via pointer returned from conversion function â *end example*]

[102)](#footnote-102)[102)](#footnoteref-102)

Note that this construction can yield
candidate call functions that cannot be
differentiated one from the other by overload resolution because they have
identical
declarations or differ only in their return type[.](#footnote-102.sentence-1)

The call will be ambiguous
if overload
resolution cannot select a match to the call that is uniquely better than such
undifferentiable functions[.](#footnote-102.sentence-2)

#### [12.2.2.3](#oper) Operators in expressions [[over.match.oper]](over.match.oper)

[1](#oper-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L661)

If no operand of an operator in an expression has a type that is a class
or an enumeration, the operator is assumed to be a built-in operator
and interpreted according to [[expr.compound]](expr.compound "7.6 Compound expressions")[.](#oper-1.sentence-1)

[*Note [1](#oper-note-1)*:

Because.,.*,
and​::​ cannot be overloaded,
these operators are always built-in operators interpreted according to[[expr.compound]](expr.compound "7.6 Compound expressions")[.](#oper-1.sentence-2)

?: cannot be overloaded, but the rules in this subclause are used to determine
the conversions to be applied to the second and third operands when they
have class or enumeration type ([[expr.cond]](expr.cond "7.6.16 Conditional operator"))[.](#oper-1.sentence-3)

â *end note*]

[*Example [1](#oper-example-1)*: struct String { String (const String&);
 String (const char*); operator const char* ();};
String operator + (const String&, const String&);

void f() {const char* p= "one" + "two"; // error: cannot add two pointers; overloaded operator+ not considered// because neither operand has class or enumeration typeint I = 1 + 1; // always evaluates to 2 even if class or enumeration types exist// that would perform the operation.} â *end example*]

[2](#oper-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L697)

If either operand has a type that is a class or an enumeration, a
user-defined operator function can be declared that implements
this operator or a user-defined conversion can be necessary to
convert the operand to a type that is appropriate for a built-in
operator[.](#oper-2.sentence-1)

In this case, overload resolution is used to determine
which operator function or built-in operator is to be invoked to implement the
operator[.](#oper-2.sentence-2)

Therefore, the operator notation is first transformed
to the equivalent function-call notation as summarized in
Table [18](#tab:over.match.oper "Table 18: Relationship between operator and function call notation") (where @ denotes one of the operators covered in the specified subclause)[.](#oper-2.sentence-3)

However, the operands are sequenced in the order prescribed
for the built-in operator ([[expr.compound]](expr.compound "7.6 Compound expressions"))[.](#oper-2.sentence-4)

Table [18](#tab:over.match.oper) — Relationship between operator and function call notation [[tab:over.match.oper]](./tab:over.match.oper)  

| [ð](#tab:over.match.oper-row-1)<br>**Subclause** | **Expression** | **As member function** | **As non-member function** |
| --- | --- | --- | --- |
| [ð](#tab:over.match.oper-row-2)<br>[[over.unary]](over.unary "12.4.2 Unary operators") | @a | (a).operator@ ( ) | operator@(a) |
| [ð](#tab:over.match.oper-row-3)<br>[[over.binary]](over.binary "12.4.3 Binary operators") | a@b | (a).operator@ (b) | operator@(a, b) |
| [ð](#tab:over.match.oper-row-4)<br>[[over.assign]](over.assign "12.4.3.2 Simple assignment") | a=b | (a).operator= (b) |  |
| [ð](#tab:over.match.oper-row-5)<br>[[over.sub]](over.sub "12.4.5 Subscripting") | a[b] | (a).operator[](b) |  |
| [ð](#tab:over.match.oper-row-6)<br>[[over.ref]](over.ref "12.4.6 Class member access") | a-> | (a).operator->( ) |  |
| [ð](#tab:over.match.oper-row-7)<br>[[over.inc]](over.inc "12.4.7 Increment and decrement") | a@ | (a).operator@ (0) | operator@(a, 0) |

[3](#oper-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L725)

For a unary operator @ with an operand of type *cv1* T1,
and for a binary operator @ with a left operand of type *cv1* T1 and a right operand of type *cv2* T2,
four sets of candidate functions, designated[*member candidates*](#def:member_candidate "12.2.2.3 Operators in expressions [over.match.oper]"),[*non-member candidates*](#def:non-member_candidate "12.2.2.3 Operators in expressions [over.match.oper]"),[*built-in candidates*](#def:built-in_candidate "12.2.2.3 Operators in expressions [over.match.oper]"),
and[*rewritten candidates*](#def:rewritten_candidate "12.2.2.3 Operators in expressions [over.match.oper]"),
are constructed as follows:

- [(3.1)](#oper-3.1)

  If T1 is a complete class type or a class currently being defined,
the set of member candidates is the result of a search foroperator@ in the scope of T1;
otherwise, the set of member candidates is empty[.](#oper-3.1.sentence-1)

- [(3.2)](#oper-3.2)

  For the operators =, [], or ->,
the set of non-member candidates is empty;
otherwise, it includes the result of unqualified lookup foroperator@ in the rewritten function call ([[basic.lookup.unqual]](basic.lookup.unqual "6.5.3 Unqualified name lookup"), [[basic.lookup.argdep]](basic.lookup.argdep "6.5.4 Argument-dependent name lookup")),
ignoring all member functions[.](#oper-3.2.sentence-1)
  However, if no operand has a class type, only those non-member
functions in the lookup set that have a first parameter of typeT1 or âreference to cv T1â,
whenT1 is an enumeration type,
or (if there is a right operand) a second parameter of typeT2 or âreference to cv T2â,
whenT2 is an enumeration type,
are candidate functions[.](#oper-3.2.sentence-2)

- [(3.3)](#oper-3.3)

  For the operator,,
the unary operator&,
or the operator->,
the built-in candidates set is empty[.](#oper-3.3.sentence-1)
  For all other operators, the built-in candidates include all
of the candidate operator functions defined in [[over.built]](over.built "12.5 Built-in operators") that,
compared to the given operator,
  * [(3.3.1)](#oper-3.3.1)

have the same operator name, and

  * [(3.3.2)](#oper-3.3.2)

accept the same number of operands, and

  * [(3.3.3)](#oper-3.3.3)

accept operand types to which the given operand or
operands can be converted according to [[over.best.ics]](#over.best.ics "12.2.4.2 Implicit conversion sequences"), and

  * [(3.3.4)](#oper-3.3.4)

do not have the same parameter-type-list as any non-member candidate
or rewritten non-member candidate
that is not a function template specialization[.](#oper-3.3.sentence-2)

- [(3.4)](#oper-3.4)

  The rewritten candidate set is determined as follows:
  * [(3.4.1)](#oper-3.4.1)

      For the relational ([[expr.rel]](expr.rel "7.6.9 Relational operators")) operators,
the rewritten candidates include
all non-rewritten candidates
for the expression x <=> y[.](#oper-3.4.1.sentence-1)

  * [(3.4.2)](#oper-3.4.2)

      For the
relational ([[expr.rel]](expr.rel "7.6.9 Relational operators")) and
three-way comparison ([[expr.spaceship]](expr.spaceship "7.6.8 Three-way comparison operator"))
operators,
the rewritten candidates also include
a synthesized candidate,
with the order of the two parameters reversed,
for each non-rewritten candidate
for the expressiony <=> x[.](#oper-3.4.2.sentence-1)

  * [(3.4.3)](#oper-3.4.3)

      For the != operator ([[expr.eq]](expr.eq "7.6.10 Equality operators")),
the rewritten candidates
include all non-rewritten candidates
for the expression x == y that are rewrite targets with first operand x (see below)[.](#oper-3.4.3.sentence-1)

  * [(3.4.4)](#oper-3.4.4)

      For the equality operators,
the rewritten candidates also include a synthesized candidate,
with the order of the two parameters reversed,
for each non-rewritten candidate
for the expression y == x that is a rewrite target with first operand y[.](#oper-3.4.4.sentence-1)

  * [(3.4.5)](#oper-3.4.5)

      For all other operators, the rewritten candidate set is empty[.](#oper-3.4.5.sentence-1)

  [*Note [2](#oper-note-2)*:
  A candidate synthesized from a member candidate has its
object parameter as the second parameter, thus implicit conversions
are considered for the first, but not for the second, parameter[.](#oper-3.4.sentence-2)
 â *end note*]

[4](#oper-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L832)

A non-template function or function template F named operator== is a rewrite target with first operand o unless a search for the name operator!= in the scope S from the instantiation context of the operator expression
finds a function or function template
that would correspond ([[basic.scope.scope]](basic.scope.scope "6.4.1 General")) to F if its name were operator==,
where S is the scope of the class type of o if F is a class member, and
the namespace scope of which F is a member otherwise[.](#oper-4.sentence-1)

A function template specialization named operator== is a rewrite target
if its function template is a rewrite target[.](#oper-4.sentence-2)

[*Example [2](#oper-example-2)*: struct A {};template<typename T> bool operator==(A, T); // #1bool a1 = 0 == A(); // OK, calls reversed #1template<typename T> bool operator!=(A, T);bool a2 = 0 == A(); // error, #1 is not a rewrite targetstruct B {bool operator==(const B&); // #2};struct C : B { C();
 C(B); bool operator!=(const B&); // #3};bool c1 = B() == C(); // OK, calls #2; reversed #2 is not a candidate// because search for operator!= in C finds #3bool c2 = C() == B(); // error: ambiguous between #2 found when searching C and// reversed #2 found when searching Bstruct D {};template<typename T> bool operator==(D, T); // #4inline namespace N {template<typename T> bool operator!=(D, T); // #5}bool d1 = 0 == D(); // OK, calls reversed #4; #5 does not forbid #4 as a rewrite target â *end example*]

[5](#oper-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L875)

For the first parameter of the built-in assignment operators,
only standard conversion sequences ([[over.ics.scs]](#over.ics.scs "12.2.4.2.2 Standard conversion sequences")) are considered[.](#oper-5.sentence-1)

[6](#oper-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L879)

For all other operators, no such restrictions apply[.](#oper-6.sentence-1)

[7](#oper-7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L882)

The set of candidate functions for overload resolution
for some operator @ is the
union of
the member candidates,
the non-member candidates,
the built-in candidates,
and the rewritten candidates
for that operator @[.](#oper-7.sentence-1)

[8](#oper-8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L893)

The argument list contains all of the
operands of the operator[.](#oper-8.sentence-1)

The best function from the set of candidate functions is selected
according to [[over.match.viable]](#viable "12.2.3 Viable functions") and [[over.match.best]](#best "12.2.4 Best viable function")[.](#oper-8.sentence-2)[103](#footnote-103 "If the set of candidate functions is empty, overload resolution is unsuccessful.")

[*Example [3](#oper-example-3)*: struct A {operator int();};
A operator+(const A&, const A&);void m() { A a, b;
 a + b; // operator+(a, b) chosen over int(a) + int(b)} â *end example*]

[9](#oper-9)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L916)

If a rewritten operator<=> candidate
is selected by overload resolution
for an operator @,x @ y is interpreted as0 @ (y <=> x) if the selected candidate is a synthesized candidate
with reversed order of parameters,
or (x <=> y) @ 0 otherwise,
using the selected rewritten operator<=> candidate[.](#oper-9.sentence-1)

Rewritten candidates for the operator @ are not considered in the context of the resulting expression[.](#oper-9.sentence-2)

[10](#oper-10)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L930)

If a rewritten operator== candidate
is selected by overload resolution
for an operator @,
its return type shall be cv bool, andx @ y is interpreted as:

- [(10.1)](#oper-10.1)

if @ is != and the selected candidate is a synthesized candidate
with reversed order of parameters,!(y == x),

- [(10.2)](#oper-10.2)

otherwise, if @ is !=,!(x == y),

- [(10.3)](#oper-10.3)

otherwise (when @ is ==),y == x,

in each case using the selected rewritten operator== candidate[.](#oper-10.sentence-1)

[11](#oper-11)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L951)

If a built-in candidate is selected by overload resolution, the
operands of class type are converted to the types of the corresponding parameters
of the selected operation function, except that the second standard conversion
sequence of a [user-defined conversion sequence](#over.ics.user "12.2.4.2.3 User-defined conversion sequences [over.ics.user]") is not applied[.](#oper-11.sentence-1)

Then the operator is treated as the corresponding
built-in operator and interpreted according to [[expr.compound]](expr.compound "7.6 Compound expressions")[.](#oper-11.sentence-2)

[*Example [4](#oper-example-4)*: struct X {operator double();};

struct Y {operator int*();};

int *a = Y() + 100.0; // error: pointer arithmetic requires integral operandint *b = Y() + X(); // error: pointer arithmetic requires integral operand â *end example*]

[12](#oper-12)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L973)

The second operand of operator-> is ignored in selecting anoperator-> function, and is not an argument when theoperator-> function is called[.](#oper-12.sentence-1)

Whenoperator-> returns, the operator-> is applied to the value returned, with the original second
operand[.](#oper-12.sentence-2)[104](#footnote-104 "If the value returned by the operator-> function has class type, this can result in selecting and calling another operator-> function. The process repeats until an operator-> function returns a value of non-class type.")

[13](#oper-13)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L998)

If the operator is the operator,,
the unary operator&,
or the operator->,
and there are no viable functions, then the operator is
assumed to be the built-in operator and interpreted according to[[expr.compound]](expr.compound "7.6 Compound expressions")[.](#oper-13.sentence-1)

[14](#oper-14)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L1009)

[*Note [3](#oper-note-3)*:

The lookup rules for operators in expressions are different than
the lookup
rules for operator function names in a function call, as shown in the following
example:struct A { };void operator + (A, A);

struct B {void operator + (B); void f ();};

A a;

void B::f() {operator+ (a,a); // error: global operator hidden by member a + a; // OK, calls global operator+}

â *end note*]

[103)](#footnote-103)[103)](#footnoteref-103)

If the set of candidate functions is empty,
overload resolution is unsuccessful[.](#footnote-103.sentence-1)

[104)](#footnote-104)[104)](#footnoteref-104)

If the value returned by theoperator-> function has class type, this can result in selecting and calling anotheroperator-> function[.](#footnote-104.sentence-1)

The process repeats until anoperator-> function returns a value of non-class type[.](#footnote-104.sentence-2)

#### [12.2.2.4](#ctor) Initialization by constructor [[over.match.ctor]](over.match.ctor)

[1](#ctor-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L1036)

When objects of class type are [direct-initialized](dcl.init#def:direct-initialization "9.5 Initializers [dcl.init]"),
copy-initialized from an expression of the same or a
derived class type ([[dcl.init]](dcl.init "9.5 Initializers")),
or [default-initialized](dcl.init#def:default-initialization "9.5 Initializers [dcl.init]"),
overload resolution selects the constructor[.](#ctor-1.sentence-1)

For direct-initialization or default-initialization
(including default-initialization in the context of copy-list-initialization),
the candidate functions are
all the constructors of the class of the object being
initialized[.](#ctor-1.sentence-2)

Otherwise, the candidate functions are all
the non-explicit constructors ([[class.conv.ctor]](class.conv.ctor "11.4.8.2 Conversion by constructor")) of that
class[.](#ctor-1.sentence-3)

The argument list is the[*expression-list*](expr.post.general#nt:expression-list "7.6.1.1 General [expr.post.general]") or [*assignment-expression*](expr.assign#nt:assignment-expression "7.6.19 Assignment and compound assignment operators [expr.assign]") of the [*initializer*](dcl.init.general#nt:initializer "9.5.1 General [dcl.init.general]")[.](#ctor-1.sentence-4)

For default-initialization in the context of copy-list-initialization,
if an explicit constructor is chosen, the initialization is ill-formed[.](#ctor-1.sentence-5)

#### [12.2.2.5](#copy) Copy-initialization of class by user-defined conversion [[over.match.copy]](over.match.copy)

[1](#copy-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L1059)

Under the conditions specified in [[dcl.init]](dcl.init "9.5 Initializers"), as
part of a copy-initialization of an object of class type, a user-defined
conversion can be invoked to convert an initializer expression to the
type of the object being initialized[.](#copy-1.sentence-1)

Overload resolution is used
to select the user-defined conversion to be invoked[.](#copy-1.sentence-2)

[*Note [1](#copy-note-1)*:

The conversion performed for indirect binding to a reference to a possibly
cv-qualified class type is determined in terms of a corresponding non-reference
copy-initialization[.](#copy-1.sentence-3)

â *end note*]

Assuming that
â*cv1* Tâ is the type of the object being initialized, withT a class type,
the candidate functions are selected as follows:

- [(1.1)](#copy-1.1)

  The non-explicit constructors ([[class.conv.ctor]](class.conv.ctor "11.4.8.2 Conversion by constructor")) ofT are candidate functions[.](#copy-1.1.sentence-1)

- [(1.2)](#copy-1.2)

  When the type of the initializer expression is a class type
âcv Sâ,
conversion functions are considered[.](#copy-1.2.sentence-1)
  The permissible types for non-explicit conversion functions areT and any class derived from T[.](#copy-1.2.sentence-2)
  When initializing a temporary object ([[class.mem]](class.mem "11.4 Class members"))
to be bound to the first parameter of a constructor
where the parameter is of type
âreference to *cv2* Tâ
and the constructor is
called with a single argument in the context of
direct-initialization of an object of type â*cv3* Tâ,
the permissible types for explicit conversion functions are the same;
otherwise there are none[.](#copy-1.2.sentence-3)

[2](#copy-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L1098)

In both cases, the argument list has one argument, which is the initializer
expression[.](#copy-2.sentence-1)

[*Note [2](#copy-note-2)*:

This argument will be compared against
the first parameter of the constructors and against the
object parameter of the conversion functions[.](#copy-2.sentence-2)

â *end note*]

#### [12.2.2.6](#conv) Initialization by conversion function [[over.match.conv]](over.match.conv)

[1](#conv-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L1110)

Under the conditions specified in [[dcl.init]](dcl.init "9.5 Initializers"), as
part of an initialization of an object of non-class type,
a conversion function can be invoked to convert an initializer
expression of class type to the type of the object
being initialized[.](#conv-1.sentence-1)

Overload resolution is used to select the
conversion function to be invoked[.](#conv-1.sentence-2)

Assuming that âcv Tâ is the
type of the object being initialized,
the candidate functions are selected as follows:

- [(1.1)](#conv-1.1)

The permissible types for non-explicit conversion functions are
those that can be converted to type T via a standard conversion sequence ([[over.ics.scs]](#over.ics.scs "12.2.4.2.2 Standard conversion sequences"))[.](#conv-1.sentence-3)
  For direct-initialization,
the permissible types for explicit conversion functions are
those that can be converted to type T with a (possibly trivial) qualification conversion ([[conv.qual]](conv.qual "7.3.6 Qualification conversions"));
otherwise there are none[.](#conv-1.1.sentence-2)

[2](#conv-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L1133)

The argument list has one argument, which is the initializer expression[.](#conv-2.sentence-1)

[*Note [1](#conv-note-1)*:

This argument will be compared against
the object parameter of the conversion functions[.](#conv-2.sentence-2)

â *end note*]

#### [12.2.2.7](#ref) Initialization by conversion function for direct reference binding [[over.match.ref]](over.match.ref)

[1](#ref-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L1143)

Under the conditions specified in [[dcl.init.ref]](dcl.init.ref "9.5.4 References"), a reference can be bound directly
to the result of applying a conversion
function to an initializer expression[.](#ref-1.sentence-1)

Overload resolution is used to select the
conversion function to be invoked[.](#ref-1.sentence-2)

Assuming that âreference to *cv1* Tâ is the
type of the reference being initialized,
the candidate functions are selected as follows:

- [(1.1)](#ref-1.1)

Let R be a set of types including
  * [(1.1.1)](#ref-1.1.1)

âlvalue reference to *cv2* T2â
(when converting to an lvalue) and

  * [(1.1.2)](#ref-1.1.2)

â*cv2* T2â
and ârvalue reference to *cv2* T2â
(when converting to an rvalue or an lvalue of function type)

for any T2[.](#ref-1.sentence-3)
  The permissible types for non-explicit conversion functions are
the members of R where â*cv1* Tâ is reference-compatible ([[dcl.init.ref]](dcl.init.ref "9.5.4 References"))
with â*cv2* T2â[.](#ref-1.1.sentence-2)
  For direct-initialization, the permissible types for explicit
conversion functions are the members of R where T2 can be converted to type T with a (possibly trivial) qualification conversion ([[conv.qual]](conv.qual "7.3.6 Qualification conversions"));
otherwise there are none[.](#ref-1.1.sentence-3)

[2](#ref-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L1176)

The argument list has one argument, which is the initializer expression[.](#ref-2.sentence-1)

[*Note [1](#ref-note-1)*:

This argument will be compared against
the object parameter of the conversion functions[.](#ref-2.sentence-2)

â *end note*]

#### [12.2.2.8](#list) Initialization by list-initialization [[over.match.list]](over.match.list)

[1](#list-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L1186)

When objects of non-aggregate class type T are
list-initialized such that [[dcl.init.list]](dcl.init.list "9.5.5 List-initialization") specifies that overload resolution
is performed according to the rules in this subclause
or when forming a list-initialization sequence according to [[over.ics.list]](#over.ics.list "12.2.4.2.6 List-initialization sequence"),
overload resolution selects the constructor in two phases:

- [(1.1)](#list-1.1)

  If the initializer list is not empty or T has no default constructor,
overload resolution is first performed
where the candidate functions are the initializer-list constructors ([[dcl.init.list]](dcl.init.list "9.5.5 List-initialization"))
of the class T and
the argument list consists of the initializer list as a single argument[.](#list-1.1.sentence-1)

- [(1.2)](#list-1.2)

  Otherwise, or if no viable initializer-list constructor is found,
overload resolution is
performed again, where the candidate functions are all the constructors of
the class T and
the argument list consists of the elements of the initializer list[.](#list-1.2.sentence-1)

In copy-list-initialization, if an explicit constructor is
chosen, the initialization is ill-formed[.](#list-1.sentence-2)

[*Note [1](#list-note-1)*:

This differs from other situations ([[over.match.ctor]](#ctor "12.2.2.4 Initialization by constructor"), [[over.match.copy]](#copy "12.2.2.5 Copy-initialization of class by user-defined conversion")),
where only non-explicit constructors are considered for copy-initialization[.](#list-1.sentence-3)

This restriction only
applies if this initialization is part of the final result of overload
resolution[.](#list-1.sentence-4)

â *end note*]

#### [12.2.2.9](#class.deduct) Class template argument deduction [[over.match.class.deduct]](over.match.class.deduct)

[1](#class.deduct-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L1221)

When resolving a placeholder for a deduced class type ([[dcl.type.class.deduct]](dcl.type.class.deduct "9.2.9.8 Deduced class template specialization types"))
where the [*template-name*](temp.names#nt:template-name "13.3 Names of template specializations [temp.names]") or [*splice-type-specifier*](dcl.type.splice#nt:splice-type-specifier "9.2.9.9 Type splicing [dcl.type.splice]") designates a primary class template C,
a set of functions and function templates, called the guides of C,
is formed comprising:

- [(1.1)](#class.deduct-1.1)

  If C is defined,
for each constructor of C,
a function template with the following properties:
  * [(1.1.1)](#class.deduct-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[.](#class.deduct-1.1.1.sentence-1)

  * [(1.1.2)](#class.deduct-1.1.2)

      The associated constraints ([[temp.constr.decl]](temp.constr.decl "13.5.3 Constrained declarations")) are the conjunction of
the associated constraints of C and
the associated constraints of the constructor, if any[.](#class.deduct-1.1.2.sentence-1)
      [*Note [1](#class.deduct-note-1)*:
      A [*constraint-expression*](temp.constr.decl#nt:constraint-expression "13.5.3 Constrained declarations [temp.constr.decl]") in
the [*template-head*](temp.pre#nt:template-head "13.1 Preamble [temp.pre]") of C is checked for satisfaction before any constraints from
the [*template-head*](temp.pre#nt:template-head "13.1 Preamble [temp.pre]") or trailing [*requires-clause*](temp.pre#nt:requires-clause "13.1 Preamble [temp.pre]") of the constructor[.](#class.deduct-1.1.2.sentence-2)
 â *end note*]

  * [(1.1.3)](#class.deduct-1.1.3)

      The [*parameter-declaration-clause*](dcl.fct#nt:parameter-declaration-clause "9.3.4.6 Functions [dcl.fct]") is that of the constructor[.](#class.deduct-1.1.3.sentence-1)

  * [(1.1.4)](#class.deduct-1.1.4)

      The return type is the class template specialization
designated by C and template arguments
corresponding to the template parameters of C[.](#class.deduct-1.1.4.sentence-1)

- [(1.2)](#class.deduct-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()[.](#class.deduct-1.2.sentence-1)

- [(1.3)](#class.deduct-1.3)

  An additional function template derived as above
from a hypothetical constructor C(C),
called the [*copy deduction candidate*](#def:copy_deduction_candidate "12.2.2.9 Class template argument deduction [over.match.class.deduct]")[.](#class.deduct-1.3.sentence-1)

- [(1.4)](#class.deduct-1.4)

  For each [*deduction-guide*](temp.deduct.guide#nt:deduction-guide "13.7.2.3 Deduction guides [temp.deduct.guide]"),
a function or function template
with the following properties:
  * [(1.4.1)](#class.deduct-1.4.1)

      The [*template-head*](temp.pre#nt:template-head "13.1 Preamble [temp.pre]"), if any,
and [*parameter-declaration-clause*](dcl.fct#nt:parameter-declaration-clause "9.3.4.6 Functions [dcl.fct]") are those of the [*deduction-guide*](temp.deduct.guide#nt:deduction-guide "13.7.2.3 Deduction guides [temp.deduct.guide]")[.](#class.deduct-1.4.1.sentence-1)

  * [(1.4.2)](#class.deduct-1.4.2)

      The return type
is the [*simple-template-id*](temp.names#nt:simple-template-id "13.3 Names of template specializations [temp.names]") of the [*deduction-guide*](temp.deduct.guide#nt:deduction-guide "13.7.2.3 Deduction guides [temp.deduct.guide]")[.](#class.deduct-1.4.2.sentence-1)

In addition, if C is defined
and its definition satisfies the conditions for
an aggregate class ([[dcl.init.aggr]](dcl.init.aggr "9.5.2 Aggregates"))
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*](dcl.init.general#nt:braced-init-list "9.5.1 General [dcl.init.general]") or
parenthesized [*expression-list*](expr.post.general#nt:expression-list "7.6.1.1 General [expr.post.general]"), and
there are no [*deduction-guide*](temp.deduct.guide#nt:deduction-guide "13.7.2.3 Deduction guides [temp.deduct.guide]")*s* for C,
the set contains an additional function template,
called the [*aggregate deduction candidate*](#def:candidate,aggregate_deduction "12.2.2.9 Class template argument deduction [over.match.class.deduct]"), defined as follows[.](#class.deduct-1.sentence-2)

Let x1,â¦,xn be the elements
of the [*initializer-list*](dcl.init.general#nt:initializer-list "9.5.1 General [dcl.init.general]") or[*designated-initializer-list*](dcl.init.general#nt:designated-initializer-list "9.5.1 General [dcl.init.general]") of the [*braced-init-list*](dcl.init.general#nt:braced-init-list "9.5.1 General [dcl.init.general]"), or
of the [*expression-list*](expr.post.general#nt:expression-list "7.6.1.1 General [expr.post.general]")[.](#class.deduct-1.sentence-3)

For each xi, let ei be the corresponding aggregate element
of C or of one of its (possibly recursive) subaggregates
that would be initialized by xi ([[dcl.init.aggr]](dcl.init.aggr "9.5.2 Aggregates")) if

- [(1.5)](#class.deduct-1.5)

brace elision is not considered for any aggregate element
that has
  * [(1.5.1)](#class.deduct-1.5.1)

a dependent non-array type,

  * [(1.5.2)](#class.deduct-1.5.2)

an array type with a value-dependent bound, or

  * [(1.5.3)](#class.deduct-1.5.3)

an array type with a dependent array element type and xi is a string literal; and

- [(1.6)](#class.deduct-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)](#class.deduct-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)[.](#class.deduct-1.sentence-4)

If there is no such aggregate element ei for any xi,
the aggregate deduction candidate is not added to the set[.](#class.deduct-1.sentence-5)

The aggregate deduction candidate is derived as above
from a hypothetical constructor C(T1,â¦,Tn),
where

- [(1.8)](#class.deduct-1.8)

if ei is of array type andxi is a [*braced-init-list*](dcl.init.general#nt:braced-init-list "9.5.1 General [dcl.init.general]"),Ti is an rvalue reference to the declared type of ei, and

- [(1.9)](#class.deduct-1.9)

if ei is of array type andxi is a [*string-literal*](lex.string#nt:string-literal "5.13.5 String literals [lex.string]"),Ti is an lvalue reference to
the const-qualified declared type of ei, and

- [(1.10)](#class.deduct-1.10)

otherwise, Ti is the declared type of ei,

except that additional parameter packs of the form Pj... are inserted into the parameter list in their original aggregate element position corresponding to each non-trailing aggregate element of type Pj 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 Tn...[.](#class.deduct-1.sentence-6)

In addition,
if C is defined and
inherits constructors ([[namespace.udecl]](namespace.udecl "9.10 The using declaration"))
from a direct base class denoted in the [*base-specifier-list*](class.derived.general#nt:base-specifier-list "11.7.1 General [class.derived.general]") by a [*class-or-decltype*](class.derived.general#nt:class-or-decltype "11.7.1 General [class.derived.general]") B,
let A be an alias template
whose template parameter list is that of C and
whose [*defining-type-id*](dcl.name#nt:defining-type-id "9.3.2 Type names [dcl.name]") is B[.](#class.deduct-1.sentence-7)

If A is a deducible template ([[dcl.type.simple]](dcl.type.simple "9.2.9.3 Simple type specifiers")),
the set contains the guides of A with the return type R of each guide
replaced with typename CC<R>​::​type given a class templatetemplate <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]](temp.dep.type "13.8.3.2 Dependent types"))
having a member typedef type designating a template specialization with
the template argument list of A but
with C as the template[.](#class.deduct-1.sentence-8)

[*Note [2](#class.deduct-note-2)*:

Equivalently,
the template parameter list of the specialization is that of C,
the template argument list of the specialization is B, and
the member typedef names C with the template argument list of C[.](#class.deduct-1.sentence-9)

â *end note*]

[2](#class.deduct-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L1373)

[*Example [1](#class.deduct-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 guidestemplate <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 â *end example*]

[3](#class.deduct-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L1423)

When resolving a placeholder for a deduced class type ([[dcl.type.simple]](dcl.type.simple "9.2.9.3 Simple type specifiers"))
where
the [*template-name*](temp.names#nt:template-name "13.3 Names of template specializations [temp.names]") or [*splice-type-specifier*](dcl.type.splice#nt:splice-type-specifier "9.2.9.9 Type splicing [dcl.type.splice]") designates an alias template A,
the [*defining-type-id*](dcl.name#nt:defining-type-id "9.3.2 Type names [dcl.name]") of A must be of the form

typenameopt [*nested-name-specifier*](expr.prim.id.qual#nt:nested-name-specifier "7.5.5.3 Qualified names [expr.prim.id.qual]")opt templateopt [*simple-template-id*](temp.names#nt:simple-template-id "13.3 Names of template specializations [temp.names]")

as specified in [[dcl.type.simple]](dcl.type.simple "9.2.9.3 Simple type specifiers")[.](#class.deduct-3.sentence-1)

The guides of A are the set of functions or function templates
formed as follows[.](#class.deduct-3.sentence-2)

For each function or function template f in the guides of
the template named by the [*simple-template-id*](temp.names#nt:simple-template-id "13.3 Names of template specializations [temp.names]") of the [*defining-type-id*](dcl.name#nt:defining-type-id "9.3.2 Type names [dcl.name]"),
the template arguments of the return type of f are deduced
from the [*defining-type-id*](dcl.name#nt:defining-type-id "9.3.2 Type names [dcl.name]") of A according to the process in [[temp.deduct.type]](temp.deduct.type "13.10.3.6 Deducing template arguments from a type") with the exception that deduction does not fail
if not all template arguments are deduced[.](#class.deduct-3.sentence-3)

If deduction fails for another reason,
proceed with an empty set of deduced template arguments[.](#class.deduct-3.sentence-4)

Let g denote the result of substituting
these deductions into f[.](#class.deduct-3.sentence-5)

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

- [(3.1)](#class.deduct-3.1)

  The function type of f' is the function type of g[.](#class.deduct-3.1.sentence-1)

- [(3.2)](#class.deduct-3.2)

  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[.](#class.deduct-3.2.sentence-1)

- [(3.3)](#class.deduct-3.3)

  The associated constraints ([[temp.constr.decl]](temp.constr.decl "13.5.3 Constrained declarations")) 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[.](#class.deduct-3.3.sentence-1)

- [(3.4)](#class.deduct-3.4)

  If f is a copy deduction candidate,
then f' is considered to be so as well[.](#class.deduct-3.4.sentence-1)

- [(3.5)](#class.deduct-3.5)

  If f was generated
from a [*deduction-guide*](temp.deduct.guide#nt:deduction-guide "13.7.2.3 Deduction guides [temp.deduct.guide]") ([[temp.deduct.guide]](temp.deduct.guide "13.7.2.3 Deduction guides")),
then f' is considered to be so as well[.](#class.deduct-3.5.sentence-1)

- [(3.6)](#class.deduct-3.6)

  The [*explicit-specifier*](dcl.fct.spec#nt:explicit-specifier "9.2.3 Function specifiers [dcl.fct.spec]") of f' is
the [*explicit-specifier*](dcl.fct.spec#nt:explicit-specifier "9.2.3 Function specifiers [dcl.fct.spec]") of g (if any)[.](#class.deduct-3.6.sentence-1)

[4](#class.deduct-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L1489)

The arguments of a template A are said to be
deducible from a type T if, given a class templatetemplate <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]](temp.dep.type "13.8.3.2 Dependent types")),AA<T> matches the partial specialization[.](#class.deduct-4.sentence-1)

[5](#class.deduct-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L1501)

Initialization and overload resolution are performed as described
in [[dcl.init]](dcl.init "9.5 Initializers") and [[over.match.ctor]](#ctor "12.2.2.4 Initialization by constructor"), [[over.match.copy]](#copy "12.2.2.5 Copy-initialization of class by user-defined conversion"),
or [[over.match.list]](#list "12.2.2.8 Initialization by list-initialization") (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[.](#class.deduct-5.sentence-1)

The following exceptions apply:

- [(5.1)](#class.deduct-5.1)

  The first phase in [[over.match.list]](#list "12.2.2.8 Initialization by list-initialization") (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[.](#class.deduct-5.1.sentence-1)

- [(5.2)](#class.deduct-5.2)

  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[.](#class.deduct-5.2.sentence-1)

If the function or function template was generated from
a constructor or [*deduction-guide*](temp.deduct.guide#nt:deduction-guide "13.7.2.3 Deduction guides [temp.deduct.guide]") that had an [*explicit-specifier*](dcl.fct.spec#nt:explicit-specifier "9.2.3 Function specifiers [dcl.fct.spec]"),
each such notional constructor is considered to have
that same [*explicit-specifier*](dcl.fct.spec#nt:explicit-specifier "9.2.3 Function specifiers [dcl.fct.spec]")[.](#class.deduct-5.sentence-3)

All such notional constructors are considered to be
public members of the hypothetical class type[.](#class.deduct-5.sentence-4)

[6](#class.deduct-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L1534)

[*Example [2](#class.deduct-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 initializestemplate <class T> A(const T&, const T&) -> A<T&>; // #3template <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 constructortemplate <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 â *end example*]

[7](#class.deduct-7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L1615)

[*Example [3](#class.deduct-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>>; // #2template<class V> using A = C<V *, V *>;template<std::[integral](concepts.arithmetic#concept:integral "18.4.7 Arithmetic concepts [concepts.arithmetic]") 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 templatetemplate<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 Ctemplate<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](concepts.arithmetic#concept:integral "18.4.7 Arithmetic concepts [concepts.arithmetic]") 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](concepts.arithmetic#concept:integral "18.4.7 Arithmetic concepts [concepts.arithmetic]") 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>>;

â *end example*]

### [12.2.3](#viable) Viable functions [[over.match.viable]](over.match.viable)

[1](#viable-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L1683)

From the set of candidate functions constructed for a given
context ([[over.match.funcs]](#funcs "12.2.2 Candidate functions and argument lists")), a set of viable functions is
chosen, from which the best function will be selected by
comparing argument conversion sequences
and associated constraints ([[temp.constr.decl]](temp.constr.decl "13.5.3 Constrained declarations"))
for the best fit ([[over.match.best]](#best "12.2.4 Best viable function"))[.](#viable-1.sentence-1)

The selection of viable functions considers
associated constraints, if any, and
relationships between arguments and function parameters other
than the ranking of conversion sequences[.](#viable-1.sentence-2)

[2](#viable-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L1695)

First, to be a viable function, a candidate function shall have
enough parameters to agree in number with the arguments in the
list[.](#viable-2.sentence-1)

- [(2.1)](#viable-2.1)

  If there are m arguments in the list,
all candidate functions having exactly m parameters are viable[.](#viable-2.1.sentence-1)

- [(2.2)](#viable-2.2)

  A candidate function having fewer than m parameters is viable
only if it has an ellipsis in its parameter list ([[dcl.fct]](dcl.fct "9.3.4.6 Functions"))[.](#viable-2.2.sentence-1)
  For the purposes of overload resolution,
any argument for which there is no corresponding parameter is
considered to âmatch the ellipsisâ ([[over.ics.ellipsis]](#over.ics.ellipsis "12.2.4.2.4 Ellipsis conversion sequences"))[.](#viable-2.2.sentence-2)

- [(2.3)](#viable-2.3)

  A candidate function C having more than m parameters is viable
only if the set of scopes G, as defined below, is not empty[.](#viable-2.3.sentence-1)
  G consists of every scope X that satisfies all of the following:
  * [(2.3.1)](#viable-2.3.1)

      There is a declaration of C, whose host scope is X,
considered by the overload resolution[.](#viable-2.3.1.sentence-1)

  * [(2.3.2)](#viable-2.3.2)

      For every kth parameter P where k > m,
there is a reachable declaration, whose host scope is X,
that specifies a default argument ([[dcl.fct.default]](dcl.fct.default "9.3.4.7 Default arguments")) for P[.](#viable-2.3.2.sentence-1)

   If C is selected as the best viable function ([[over.match.best]](#best "12.2.4 Best viable function")):
  * [(2.3.3)](#viable-2.3.3)

      G shall contain exactly one scope (call it S)[.](#viable-2.3.3.sentence-1)

  * [(2.3.4)](#viable-2.3.4)

      If the candidates are denoted by a [*splice-expression*](expr.prim.splice#nt:splice-expression "7.5.9 Expression splicing [expr.prim.splice]"),
then S shall not be a block scope[.](#viable-2.3.4.sentence-1)

  * [(2.3.5)](#viable-2.3.5)

      The default arguments used in the call to C are
the default arguments specified by
the reachable declarations whose host scope is S[.](#viable-2.3.5.sentence-1)

   For the purposes of overload resolution,
the parameter list is truncated on the right,
so that there are exactly m parameters[.](#viable-2.3.sentence-4)

[*Example [1](#viable-example-1)*: namespace A {extern "C" void f(int, int = 5); extern "C" void f(int = 6, int);}namespace B {extern "C" void f(int, int = 7);}void use() {[:^^A::f:](3, 4); // OK, default argument was not used for viability[:^^A::f:](3); // error: default argument provided by declarations from two scopes[:^^A::f:](); // OK, default arguments provided by declarations in the scope of Ausing A::f; using B::f;
 f(3, 4); // OK, default argument was not used for viability f(3); // error: default argument provided by declaration from two scopes f(); // OK, default arguments provided by declarations in the scope of Avoid g(int = 8);
 g(); // OK[:^^g:](); // error: host scope is block scope}void h(int = 7);constexpr std::meta::info r = ^^h;void poison() {void h(int = 8);
 h(); // OK, calls h(8)[:^^h:](); // error: default argument provided by declarations from two scopes}void call_h() {[:^^h:](); // error: default argument provided by declarations from two scopes[:r:](); // error: default argument provided by declarations from two scopes}template<typename... Ts>int k(int = 3, Ts...);int i = k<int>(); // error: no default argument for the second parameterint j = k<>(); // OK â *end example*]

[3](#viable-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L1784)

Second, for a function to be viable, if it has associated constraints ([[temp.constr.decl]](temp.constr.decl "13.5.3 Constrained declarations")),
those constraints shall be satisfied ([[temp.constr.constr]](temp.constr.constr "13.5.2 Constraints"))[.](#viable-3.sentence-1)

[4](#viable-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L1788)

Third, forF to be a viable function, there shall exist for each
argument an[implicit conversion sequence](#def:conversion_sequence,implicit "12.2.4.2 Implicit conversion sequences [over.best.ics]") that
converts that argument to the corresponding parameter ofF[.](#viable-4.sentence-1)

If the parameter has reference type, the implicit conversion sequence
includes the operation of binding the reference, and the fact that
an lvalue reference to non-const cannot bind to an rvalue
and that an rvalue reference cannot bind to an lvalue
can affect
the viability of the function (see [[over.ics.ref]](#over.ics.ref "12.2.4.2.5 Reference binding"))[.](#viable-4.sentence-2)

### [12.2.4](#best) Best viable function [[over.match.best]](over.match.best)

#### [12.2.4.1](#best.general) General [[over.match.best.general]](over.match.best.general)

[1](#best.general-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L1808)

Define ICSi(F) as
the implicit conversion sequence that converts
the ith argument in the list to the type of
the ith parameter of viable function F[.](#best.general-1.sentence-1)

[[over.best.ics]](#over.best.ics "12.2.4.2 Implicit conversion sequences") defines the implicit conversion sequences and [[over.ics.rank]](#over.ics.rank "12.2.4.3 Ranking implicit conversion sequences") defines what it means for one implicit conversion sequence to be
a better conversion sequence or worse conversion sequence than
another[.](#best.general-1.sentence-2)

[2](#best.general-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L1819)

Given these definitions,
a viable function F1 is defined to be a[*better*](#def:overloading,resolution,better_viable_function "12.2.4.1 General [over.match.best.general]") function than another viable function F2 if for all arguments i,ICSi(F1) is not a worse conversion
sequence than ICSi(F2), and then

- [(2.1)](#best.general-2.1)

for some argument j,ICSj(F1) is a better conversion sequence thanICSj(F2), or, if not that,

- [(2.2)](#best.general-2.2)

the context is an initialization by user-defined conversion
(see [[dcl.init]](dcl.init "9.5 Initializers"),[[over.match.conv]](#conv "12.2.2.6 Initialization by conversion function"), and [[over.match.ref]](#ref "12.2.2.7 Initialization by conversion function for direct reference binding"))
and the standard conversion sequence
from the result of F1 to the destination type
(i.e., the type of the entity being initialized)
is a better conversion sequence than the standard conversion sequence
from the result of F2 to the destination type
  [*Example [1](#best.general-example-1)*: struct A { A(); operator int(); operator double();} a;int i = a; // a.operator int() followed by no conversion is better than// a.operator double() followed by a conversion to intfloat x = a; // ambiguous: both possibilities require conversions,// and neither is better than the other â *end example*]
 or, if not that,

- [(2.3)](#best.general-2.3)

the context is an initialization by conversion function for [direct
reference binding](#ref "12.2.2.7 Initialization by conversion function for direct reference binding [over.match.ref]") of a reference to function type, the
return type of F1 is the same kind of reference (lvalue or rvalue)
as the reference being initialized, and the return type of F2 is not
  [*Example [2](#best.general-example-2)*: template <class T> struct A {operator T&(); // #1operator T&&(); // #2};typedef int Fn();
A<Fn> a;
Fn& lf = a; // calls #1 Fn&& rf = a; // calls #2 â *end example*]
 or, if not that,

- [(2.4)](#best.general-2.4)

F1 is not a function template specialization andF2 is a
function template
specialization, or, if not that,

- [(2.5)](#best.general-2.5)

F1 andF2 are
function template specializations,
and the function template
forF1 is more specialized than the template forF2 according to the partial ordering rules described in [[temp.func.order]](temp.func.order "13.7.7.3 Partial ordering of function templates"),
or, if not that,

- [(2.6)](#best.general-2.6)

F1 and F2 are non-template functions andF1 is more partial-ordering-constrained thanF2 ([[temp.constr.order]](temp.constr.order "13.5.5 Partial ordering by constraints"))
  [*Example [3](#best.general-example-3)*: template <typename T = int>struct S {constexpr void f(); // #1constexpr void f(this S&) requires true; // #2};

    void test() { S<> s;
 s.f(); // calls #2} â *end example*]
 or, if not that,

- [(2.7)](#best.general-2.7)

F1 is a constructor for a class D,F2 is a constructor for a base class B of D, and
for all arguments
the corresponding parameters of F1 and F2 have the same type
  [*Example [4](#best.general-example-4)*: struct A { A(int = 0);};

    struct B: A {using A::A;
 B();};

    int main() { B b; // OK, B​::​B()} â *end example*]
 or, if not that,

- [(2.8)](#best.general-2.8)

F2 is a rewritten candidate ([[over.match.oper]](#oper "12.2.2.3 Operators in expressions")) andF1 is not
  [*Example [5](#best.general-example-5)*: struct S {friend auto operator<=>(const S&, const S&) = default; // #1friend bool operator<(const S&, const S&); // #2};bool b = S() < S(); // calls #2 â *end example*]
 or, if not that,

- [(2.9)](#best.general-2.9)

F1 and F2 are rewritten candidates, andF2 is a synthesized candidate
with reversed order of parameters
and F1 is not
  [*Example [6](#best.general-example-6)*: struct S {friend std::weak_ordering operator<=>(const S&, int); // #1friend std::weak_ordering operator<=>(int, const S&); // #2};bool b = 1 < S(); // calls #2 â *end example*]
 or, if not that,

- [(2.10)](#best.general-2.10)

F1 and F2 are generated
from class template argument deduction ([[over.match.class.deduct]](#class.deduct "12.2.2.9 Class template argument deduction"))
for a class D, andF2 is generated
from inheriting constructors from a base class of D while F1 is not, and
for each explicit function argument,
the corresponding parameters of F1 and F2 are either both ellipses or have the same type,
or, if not that,

- [(2.11)](#best.general-2.11)

F1 is generated from a[*deduction-guide*](temp.deduct.guide#nt:deduction-guide "13.7.2.3 Deduction guides [temp.deduct.guide]") ([[over.match.class.deduct]](#class.deduct "12.2.2.9 Class template argument deduction"))
and F2 is not, or, if not that,

- [(2.12)](#best.general-2.12)

F1 is the [copy deduction candidate](#def:copy_deduction_candidate "12.2.2.9 Class template argument deduction [over.match.class.deduct]") and F2 is not, or, if not that,

- [(2.13)](#best.general-2.13)

F1 is generated from a non-template constructor
and F2 is generated from a constructor template[.](#best.general-2.sentence-1)
  [*Example [7](#best.general-example-7)*: template <class T> struct A {using value_type = T;
 A(value_type); // #1 A(const A&); // #2 A(T, T, int); // #3template<class U> A(int, T, U); // #4// #5 is the copy deduction candidate, A(A)};

    A x(1, 2, 3); // uses #3, generated from a non-template constructortemplate <class T> A(T) -> A<T>; // #6, less specialized than #5 A a(42); // uses #6 to deduce A<int> and #1 to initialize A b = a; // uses #5 to deduce A<int> and #2 to initializetemplate <class T> A(A<T>) -> A<A<T>>; // #7, as specialized as #5 A b2 = a; // uses #7 to deduce A<A<int>> and #1 to initialize â *end example*]

[3](#best.general-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2022)

If there is exactly one viable function that is a better function
than all other viable functions, then it is the one selected by
overload resolution; otherwise the call is ill-formed[.](#best.general-3.sentence-1)[105](#footnote-105 "The algorithm for selecting the best viable function is linear in the number of viable functions. Run a simple tournament to find a function W that is not worse than any opponent it faced. Although it is possible that another function F that W did not face is at least as good as W, F cannot be the best function because at some point in the tournament F encountered another function G such that F was not better than G. Hence, either W is the best function or there is no best function. So, make a second pass over the viable functions to verify that W is better than all other functions.")

[*Example [8](#best.general-example-8)*: void Fcn(const int*, short);void Fcn(int*, int);

int i;short s = 0;

void f() { Fcn(&i, s); // is ambiguous because &i â int* is better than &i â const int*// but s â short is also better than s â int Fcn(&i, 1L); // calls Fcn(int*, int), because &i â int* is better than &i â const int*// and 1L â short and 1L â int are indistinguishable Fcn(&i, 'c'); // calls Fcn(int*, int), because &i â int* is better than &i â const int*// and 'c' â int is better than 'c' â short} â *end example*]

[4](#best.general-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2083)

[*Note [1](#best.general-note-1)*:

If the best viable function was made viable by one or more default arguments,
additional requirements apply ([[over.match.viable]](#viable "12.2.3 Viable functions"))[.](#best.general-4.sentence-1)

â *end note*]

[105)](#footnote-105)[105)](#footnoteref-105)

The algorithm
for selecting the best viable function is linear in the number
of viable
functions[.](#footnote-105.sentence-1)

Run a simple tournament to find a functionW that is not
worse than any
opponent it faced[.](#footnote-105.sentence-2)

Although it is possible that another functionF thatW did not face
is at least as good asW,F cannot be the best function because at some point in the
tournamentF encountered another functionG such thatF was not better thanG[.](#footnote-105.sentence-3)

Hence,
either W is
the best function or there is no best function[.](#footnote-105.sentence-4)

So, make a second pass over
the viable
functions to verify thatW is better than all other functions[.](#footnote-105.sentence-5)

#### [12.2.4.2](#over.best.ics) Implicit conversion sequences [[over.best.ics]](over.best.ics)

#### [12.2.4.2.1](#over.best.ics.general) General [[over.best.ics.general]](over.best.ics.general)

[1](#over.best.ics.general-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2095)

An [*implicit conversion sequence*](#def:conversion_sequence,implicit "12.2.4.2.1 General [over.best.ics.general]") is a sequence of conversions used
to convert an argument in a function call to the type of the
corresponding parameter of the function being called[.](#over.best.ics.general-1.sentence-1)

The
sequence of conversions is an implicit conversion as defined in[[conv]](conv "7.3 Standard conversions"), which means it is governed by the rules for
initialization of an object or reference by a single
expression ([[dcl.init]](dcl.init "9.5 Initializers"), [[dcl.init.ref]](dcl.init.ref "9.5.4 References"))[.](#over.best.ics.general-1.sentence-2)

[2](#over.best.ics.general-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2106)

Implicit conversion sequences are concerned only with the type,
cv-qualification, and value category of the argument and how these
are converted to match the corresponding properties of the
parameter[.](#over.best.ics.general-2.sentence-1)

[*Note [1](#over.best.ics.general-note-1)*:

Other properties, such as the lifetime, storage duration, linkage,
alignment, accessibility of the argument, whether the argument is a bit-field,
and whether a function is [deleted](dcl.fct.def.delete "9.6.3 Deleted definitions [dcl.fct.def.delete]"), are ignored[.](#over.best.ics.general-2.sentence-2)

So, although an implicit
conversion sequence can be defined for a given argument-parameter
pair, the conversion from the argument to the parameter might still
be ill-formed in the final analysis[.](#over.best.ics.general-2.sentence-3)

â *end note*]

[3](#over.best.ics.general-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2121)

A
well-formed implicit conversion
sequence is one of the following forms:

- [(3.1)](#over.best.ics.general-3.1)

a [standard conversion sequence](#over.ics.scs "12.2.4.2.2 Standard conversion sequences [over.ics.scs]"),

- [(3.2)](#over.best.ics.general-3.2)

a [user-defined conversion sequence](#over.ics.user "12.2.4.2.3 User-defined conversion sequences [over.ics.user]"), or

- [(3.3)](#over.best.ics.general-3.3)

an [ellipsis conversion sequence](#over.ics.ellipsis "12.2.4.2.4 Ellipsis conversion sequences [over.ics.ellipsis]")[.](#over.best.ics.general-3.sentence-1)

[4](#over.best.ics.general-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2134)

However, if the target is

- [(4.1)](#over.best.ics.general-4.1)

the first parameter of a constructor or

- [(4.2)](#over.best.ics.general-4.2)

the object parameter of a user-defined conversion function

and the constructor or user-defined conversion function is a candidate by

- [(4.3)](#over.best.ics.general-4.3)

[[over.match.ctor]](#ctor "12.2.2.4 Initialization by constructor"), when the argument is the temporary in the second
step of a class copy-initialization,

- [(4.4)](#over.best.ics.general-4.4)

[[over.match.copy]](#copy "12.2.2.5 Copy-initialization of class by user-defined conversion"), [[over.match.conv]](#conv "12.2.2.6 Initialization by conversion function"), or [[over.match.ref]](#ref "12.2.2.7 Initialization by conversion function for direct reference binding") (in all cases), or

- [(4.5)](#over.best.ics.general-4.5)

the second phase of [[over.match.list]](#list "12.2.2.8 Initialization by list-initialization") when the initializer list has exactly one element that
is itself an initializer list, and
the target is the first parameter of a constructor of class X, and
the conversion is to X or reference to cv X,

user-defined conversion sequences are not considered[.](#over.best.ics.general-4.sentence-1)

[*Note [2](#over.best.ics.general-note-2)*:

These rules prevent more than one user-defined conversion from being
applied during overload resolution, thereby avoiding infinite recursion[.](#over.best.ics.general-4.sentence-2)

â *end note*]

[*Example [1](#over.best.ics.general-example-1)*: struct Y { Y(int); };struct A { operator int(); };
Y y1 = A(); // error: A​::​operator int() is not a candidatestruct X { X(); };struct B { operator X(); };
B b;
X x{{b}}; // error: B​::​operator X() is not a candidate â *end example*]

[5](#over.best.ics.general-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2170)

For the case where the parameter type is a reference, see [[over.ics.ref]](#over.ics.ref "12.2.4.2.5 Reference binding")[.](#over.best.ics.general-5.sentence-1)

[6](#over.best.ics.general-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2173)

When the parameter type is not a reference, the implicit conversion
sequence models a copy-initialization of the parameter from the argument
expression[.](#over.best.ics.general-6.sentence-1)

The implicit conversion sequence is the one required to convert the
argument expression to a prvalue of the type of
the parameter[.](#over.best.ics.general-6.sentence-2)

[*Note [3](#over.best.ics.general-note-3)*:

When the parameter has a class type, this is a conceptual conversion
defined for the purposes of [[over]](over "12 Overloading"); the actual initialization is
defined in terms of constructors and is not a conversion[.](#over.best.ics.general-6.sentence-3)

â *end note*]

[7](#over.best.ics.general-7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2186)

When the cv-unqualified version of the type of the argument expression
is the same as the parameter type,
the implicit conversion sequence is an identity conversion[.](#over.best.ics.general-7.sentence-1)

When the parameter has a class type and the argument expression has a
(possibly cv-qualified)
derived class type, the implicit conversion sequence is a
derived-to-baseconversion from the derived class to the base class[.](#over.best.ics.general-7.sentence-2)

A derived-to-base conversion has Conversion rank ([[over.ics.scs]](#over.ics.scs "12.2.4.2.2 Standard conversion sequences"))[.](#over.best.ics.general-7.sentence-3)

[*Note [4](#over.best.ics.general-note-4)*:

There is no such standard conversion; this derived-to-base conversion exists
only in the description of implicit conversion sequences[.](#over.best.ics.general-7.sentence-4)

â *end note*]

[*Example [2](#over.best.ics.general-example-2)*:

An implicit conversion sequence from an argument of type const A to a parameter of type A can be formed,
even if overload resolution for copy-initialization of A from the argument would not find a viable function ([[over.match.ctor]](#ctor "12.2.2.4 Initialization by constructor"), [[over.match.viable]](#viable "12.2.3 Viable functions"))[.](#over.best.ics.general-7.sentence-5)

The implicit conversion sequence for that case is the identity sequence; it
contains no âconversionâ from const A to A[.](#over.best.ics.general-7.sentence-6)

â *end example*]

[8](#over.best.ics.general-8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2210)

When the parameter is the implicit object parameter of a static member function,
the implicit conversion sequence is a standard conversion sequence
that is neither better nor worse than any other standard conversion sequence[.](#over.best.ics.general-8.sentence-1)

[9](#over.best.ics.general-9)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2215)

In all contexts, when converting to the implicit object parameter
or when converting to the left operand of an assignment operation
only standard conversion sequences are allowed[.](#over.best.ics.general-9.sentence-1)

[*Note [5](#over.best.ics.general-note-5)*:

When a conversion to the explicit object parameter occurs,
it can include user-defined conversion sequences[.](#over.best.ics.general-9.sentence-2)

â *end note*]

[10](#over.best.ics.general-10)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2224)

If no conversions are required to match an argument to a
parameter type, the implicit conversion sequence is the standard
conversion sequence consisting of the identity conversion ([[over.ics.scs]](#over.ics.scs "12.2.4.2.2 Standard conversion sequences"))[.](#over.best.ics.general-10.sentence-1)

[11](#over.best.ics.general-11)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2229)

If no sequence of conversions can be found to convert an argument
to a parameter type, an implicit conversion sequence cannot be formed[.](#over.best.ics.general-11.sentence-1)

[12](#over.best.ics.general-12)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2233)

If there are multiple well-formed implicit conversion sequences
converting the argument to the parameter type, the implicit
conversion sequence associated with the parameter is defined to be
the unique conversion sequence designated the[*ambiguous conversion sequence*](#def:conversion_sequence,ambiguous "12.2.4.2.1 General [over.best.ics.general]")[.](#over.best.ics.general-12.sentence-1)

For the purpose of ranking implicit conversion sequences as described
in [[over.ics.rank]](#over.ics.rank "12.2.4.3 Ranking implicit conversion sequences"), the ambiguous conversion sequence is treated
as a user-defined conversion sequence that is indistinguishable from any
other user-defined conversion sequence[.](#over.best.ics.general-12.sentence-2)

[*Note [6](#over.best.ics.general-note-6)*:

This rule prevents a function from becoming non-viable because of an ambiguous
conversion sequence for one of its parameters[.](#over.best.ics.general-12.sentence-3)

[*Example [3](#over.best.ics.general-example-3)*: class B;class A { A (B&);};class B { operator A (); };class C { C (B&); };void f(A) { }void f(C) { } B b;
f(b); // error: ambiguous because there is a conversion b â C (via constructor)// and an (ambiguous) conversion b â A (via constructor or conversion function)void f(B) { } f(b); // OK, unambiguous â *end example*]

â *end note*]

If a function that uses the ambiguous conversion sequence is selected
as the best viable function, the call will be ill-formed because the conversion
of one of the arguments in the call is ambiguous[.](#over.best.ics.general-12.sentence-4)

[13](#over.best.ics.general-13)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2266)

The three forms of implicit conversion sequences mentioned above
are defined in the following subclauses[.](#over.best.ics.general-13.sentence-1)

#### [12.2.4.2.2](#over.ics.scs) Standard conversion sequences [[over.ics.scs]](over.ics.scs)

[1](#over.ics.scs-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2272)

Table [19](#tab:over.ics.scs "Table 19: Conversions") summarizes the conversions defined in [[conv]](conv "7.3 Standard conversions") and
partitions them into four disjoint categories: Lvalue Transformation,
Qualification Adjustment, Promotion, and Conversion[.](#over.ics.scs-1.sentence-1)

[*Note [1](#over.ics.scs-note-1)*:

These categories are orthogonal with respect to value category,
cv-qualification, and data representation: the Lvalue Transformations
do not change the cv-qualification or data
representation of the type; the Qualification Adjustments do not
change the value category or data representation of the type; and
the Promotions and Conversions do not change the
value category or cv-qualification of the type[.](#over.ics.scs-1.sentence-2)

â *end note*]

[2](#over.ics.scs-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2287)

[*Note [2](#over.ics.scs-note-2)*:

As described in [[conv]](conv "7.3 Standard conversions"),
a standard conversion sequence either is the Identity conversion
by itself (that is, no conversion) or consists of one to three
conversions from the other
four categories[.](#over.ics.scs-2.sentence-1)

If there are two or more conversions in the sequence, the
conversions are applied in the canonical order:**Lvalue Transformation**,**Promotion** or**Conversion**,**Qualification Adjustment**[.](#over.ics.scs-2.sentence-2)

â *end note*]

[3](#over.ics.scs-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2303)

Each conversion in Table [19](#tab:over.ics.scs "Table 19: Conversions") also has an associated rank (Exact
Match, Promotion, or Conversion)[.](#over.ics.scs-3.sentence-1)

These are used
to [rank standard conversion sequences](#over.ics.rank "12.2.4.3 Ranking implicit conversion sequences [over.ics.rank]")[.](#over.ics.scs-3.sentence-2)

The rank of a conversion sequence is determined by considering the
rank of each conversion in the sequence and the rank of any [reference
binding](#over.ics.ref "12.2.4.2.5 Reference binding [over.ics.ref]")[.](#over.ics.scs-3.sentence-3)

If any of those has Conversion rank, the
sequence has Conversion rank; otherwise, if any of those has Promotion rank,
the sequence has Promotion rank; otherwise, the sequence has Exact
Match rank[.](#over.ics.scs-3.sentence-4)

Table [19](#tab:over.ics.scs) — Conversions [[tab:over.ics.scs]](./tab:over.ics.scs)  

| [ð](#tab:over.ics.scs-row-1)<br>**Conversion** | **Category** | **Rank** | **Subclause** |
| --- | --- | --- | --- |
| [ð](#tab:over.ics.scs-row-2)<br>No conversions required | Identity |  |  |
| [ð](#tab:over.ics.scs-row-3)<br> Lvalue-to-rvalue conversion |  |  | [[conv.lval]](conv.lval "7.3.2 Lvalue-to-rvalue conversion") |
| [ð](#tab:over.ics.scs-row-4)<br> Array-to-pointer conversion | Lvalue Transformation |  | [[conv.array]](conv.array "7.3.3 Array-to-pointer conversion") |
| [ð](#tab:over.ics.scs-row-5)<br> Function-to-pointer conversion |  | Exact Match | [[conv.func]](conv.func "7.3.4 Function-to-pointer conversion") |
| [ð](#tab:over.ics.scs-row-6)<br> Qualification conversions |  |  | [[conv.qual]](conv.qual "7.3.6 Qualification conversions") |
| [ð](#tab:over.ics.scs-row-7)<br> Function pointer conversion | Qualification Adjustment |  | [[conv.fctptr]](conv.fctptr "7.3.14 Function pointer conversions") |
| [ð](#tab:over.ics.scs-row-8)<br>Integral promotions |  |  | [[conv.prom]](conv.prom "7.3.7 Integral promotions") |
| [ð](#tab:over.ics.scs-row-9)<br> Floating-point promotion | Promotion | Promotion | [[conv.fpprom]](conv.fpprom "7.3.8 Floating-point promotion") |
| [ð](#tab:over.ics.scs-row-10)<br>Integral conversions |  |  | [[conv.integral]](conv.integral "7.3.9 Integral conversions") |
| [ð](#tab:over.ics.scs-row-11)<br> Floating-point conversions |  |  | [[conv.double]](conv.double "7.3.10 Floating-point conversions") |
| [ð](#tab:over.ics.scs-row-12)<br> Floating-integral conversions |  |  | [[conv.fpint]](conv.fpint "7.3.11 Floating-integral conversions") |
| [ð](#tab:over.ics.scs-row-13)<br> Pointer conversions | Conversion | Conversion | [[conv.ptr]](conv.ptr "7.3.12 Pointer conversions") |
| [ð](#tab:over.ics.scs-row-14)<br> Pointer-to-member conversions |  |  | [[conv.mem]](conv.mem "7.3.13 Pointer-to-member conversions") |
| [ð](#tab:over.ics.scs-row-15)<br> Boolean conversions |  |  | [[conv.bool]](conv.bool "7.3.15 Boolean conversions") |

#### [12.2.4.2.3](#over.ics.user) User-defined conversion sequences [[over.ics.user]](over.ics.user)

[1](#over.ics.user-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2339)

A [*user-defined conversion sequence*](#def:conversion_sequence,user-defined "12.2.4.2.3 User-defined conversion sequences [over.ics.user]") consists of an initial
standard conversion sequence followed by a user-defined
conversion ([[class.conv]](class.conv "11.4.8 Conversions")) followed by a second standard
conversion sequence[.](#over.ics.user-1.sentence-1)

If the user-defined conversion is specified
by a constructor ([[class.conv.ctor]](class.conv.ctor "11.4.8.2 Conversion by constructor")), the initial standard
conversion sequence converts the source type to the type of the
first parameter of that constructor[.](#over.ics.user-1.sentence-2)

If the user-defined
conversion is specified by a [conversion function](class.conv.fct "11.4.8.3 Conversion functions [class.conv.fct]"), the
initial standard conversion sequence
converts the source type to the type of the
object parameter of that conversion function[.](#over.ics.user-1.sentence-3)

[2](#over.ics.user-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2354)

The second standard conversion sequence converts the result of
the user-defined conversion to the target type for the sequence;
any reference binding is included in the second standard
conversion sequence[.](#over.ics.user-2.sentence-1)

Since an implicit conversion sequence is an initialization, the
special rules for initialization by user-defined conversion apply
when selecting the best user-defined conversion for a
user-defined conversion sequence (see [[over.match.best]](#best "12.2.4 Best viable function") and [[over.best.ics]](#over.best.ics "12.2.4.2 Implicit conversion sequences"))[.](#over.ics.user-2.sentence-2)

[3](#over.ics.user-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2364)

If the user-defined conversion is specified by a
specialization of a conversion function template,
the second standard conversion sequence shall have Exact Match rank[.](#over.ics.user-3.sentence-1)

[4](#over.ics.user-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2369)

A conversion of an expression of class type
to the same class type is given Exact Match rank, and
a conversion of an expression of class type
to a base class of that type is given Conversion rank,
in spite of the
fact that a constructor (i.e., a user-defined conversion
function) is called for those cases[.](#over.ics.user-4.sentence-1)

#### [12.2.4.2.4](#over.ics.ellipsis) Ellipsis conversion sequences [[over.ics.ellipsis]](over.ics.ellipsis)

[1](#over.ics.ellipsis-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2380)

An ellipsis conversion sequence occurs when an argument in a
function call is matched with the ellipsis parameter
specification of the function called (see [[expr.call]](expr.call "7.6.1.3 Function call"))[.](#over.ics.ellipsis-1.sentence-1)

#### [12.2.4.2.5](#over.ics.ref) Reference binding [[over.ics.ref]](over.ics.ref)

[1](#over.ics.ref-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2388)

When a parameter of type âreference to cv Tâ
binds directly ([[dcl.init.ref]](dcl.init.ref "9.5.4 References")) to an argument expression:

- [(1.1)](#over.ics.ref-1.1)

  If the argument expression has a type that
is a derived class of the parameter type,
the implicit conversion sequence is a derived-to-base
conversion ([[over.best.ics]](#over.best.ics "12.2.4.2 Implicit conversion sequences"))[.](#over.ics.ref-1.1.sentence-1)

- [(1.2)](#over.ics.ref-1.2)

  Otherwise,
if the type of the argument is possibly cv-qualified T, or
if T is an array type of unknown bound with element type U and
the argument has an array type of known bound whose
element type is possibly cv-qualified U,
the implicit conversion sequence is the identity conversion[.](#over.ics.ref-1.2.sentence-1)

- [(1.3)](#over.ics.ref-1.3)

  Otherwise,
if T is a function type,
the implicit conversion sequence is a function pointer conversion[.](#over.ics.ref-1.3.sentence-1)

- [(1.4)](#over.ics.ref-1.4)

  Otherwise, the implicit conversion sequence is a qualification conversion[.](#over.ics.ref-1.4.sentence-1)

[*Example [1](#over.ics.ref-example-1)*: struct A {};struct B : public A {} b;int f(A&);int f(B&);int i = f(b); // calls f(B&), an exact match, rather than f(A&), a conversionvoid g() noexcept;int h(void (&)() noexcept); // #1int h(void (&)()); // #2int j = h(g); // calls #1, an exact match, rather than #2, a function pointer conversion â *end example*]

If the parameter binds directly to the result of
applying a conversion function to the argument expression, the implicit
conversion sequence is a user-defined conversion sequence ([[over.ics.user]](#over.ics.user "12.2.4.2.3 User-defined conversion sequences"))
whose second standard conversion sequence is
determined by the above rules[.](#over.ics.ref-1.sentence-2)

[2](#over.ics.ref-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2435)

When a parameter of reference type is not bound directly to an argument
expression, the conversion sequence is the one required to convert the argument
expression to the referenced type according to [[over.best.ics]](#over.best.ics "12.2.4.2 Implicit conversion sequences")[.](#over.ics.ref-2.sentence-1)

Conceptually, this conversion sequence corresponds to copy-initializing a
temporary of the referenced type with the argument expression[.](#over.ics.ref-2.sentence-2)

Any difference
in top-level cv-qualification is subsumed by the initialization itself and
does not constitute a conversion[.](#over.ics.ref-2.sentence-3)

[3](#over.ics.ref-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2445)

Except for an implicit object parameter, for which see [[over.match.funcs]](#funcs "12.2.2 Candidate functions and argument lists"),
an implicit conversion sequence cannot be formed if it requires
binding an lvalue reference
other than a reference to a non-volatile const type
to an rvalue
or binding an rvalue reference to an lvalue of object type[.](#over.ics.ref-3.sentence-1)

[*Note [1](#over.ics.ref-note-1)*:

This means, for example, that a candidate function cannot be a viable
function if it has a non-const lvalue reference parameter (other than
the implicit object parameter) and the corresponding argument
would require a temporary to be created to initialize the lvalue
reference (see [[dcl.init.ref]](dcl.init.ref "9.5.4 References"))[.](#over.ics.ref-3.sentence-2)

â *end note*]

[4](#over.ics.ref-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2460)

Other restrictions on binding a reference to a particular argument
that are not based on the types of the reference and the argument
do not affect the formation of an implicit conversion
sequence, however[.](#over.ics.ref-4.sentence-1)

[*Example [2](#over.ics.ref-example-2)*:

A function with an âlvalue reference to intâ parameter can
be a viable candidate even if the corresponding argument is anint bit-field[.](#over.ics.ref-4.sentence-2)

The formation of implicit conversion sequences
treats theint bit-field as anint lvalue and finds an exact
match with the parameter[.](#over.ics.ref-4.sentence-3)

If the function is selected by overload
resolution, the call will nonetheless be ill-formed because of
the prohibition on binding a non-const lvalue reference to a bit-field ([[dcl.init.ref]](dcl.init.ref "9.5.4 References"))[.](#over.ics.ref-4.sentence-4)

â *end example*]

#### [12.2.4.2.6](#over.ics.list) List-initialization sequence [[over.ics.list]](over.ics.list)

[1](#over.ics.list-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2484)

When an argument is an initializer list ([[dcl.init.list]](dcl.init.list "9.5.5 List-initialization")), it is not an expression and special rules apply for converting it to a parameter type[.](#over.ics.list-1.sentence-1)

[2](#over.ics.list-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2488)

If the initializer list is a [*designated-initializer-list*](dcl.init.general#nt:designated-initializer-list "9.5.1 General [dcl.init.general]") and the parameter is not a reference,
a conversion is only possible if
the parameter has an aggregate type
that can be initialized from the initializer list
according to the rules for aggregate initialization ([[dcl.init.aggr]](dcl.init.aggr "9.5.2 Aggregates")),
in which case the implicit conversion sequence is
a user-defined conversion sequence
whose second standard conversion sequence
is an identity conversion[.](#over.ics.list-2.sentence-1)

[*Note [1](#over.ics.list-note-1)*:

Aggregate initialization does not require that
the members are declared in designation order[.](#over.ics.list-2.sentence-2)

If, after overload resolution, the order does not match
for the selected overload,
the initialization of the parameter will be ill-formed ([[dcl.init.list]](dcl.init.list "9.5.5 List-initialization"))[.](#over.ics.list-2.sentence-3)

[*Example [1](#over.ics.list-example-1)*: struct A { int x, y; };struct B { int y, x; };void f(A a, int); // #1void f(B b, ...); // #2void g(A a); // #3void g(B b); // #4void h() { f({.x = 1, .y = 2}, 0); // OK; calls #1 f({.y = 2, .x = 1}, 0); // error: selects #1, initialization of a fails// due to non-matching member order ([[dcl.init.list]](dcl.init.list "9.5.5 List-initialization")) g({.x = 1, .y = 2}); // error: ambiguous between #3 and #4} â *end example*]

â *end note*]

[3](#over.ics.list-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2523)

Otherwise,
if the parameter type is an aggregate class X and the initializer list has a
single element of type cv U, where U is X or a class derived from X, the implicit conversion sequence is the one
required to convert the element to the parameter type[.](#over.ics.list-3.sentence-1)

[4](#over.ics.list-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2530)

Otherwise, if the parameter type is a character array[106](#footnote-106 "Since there are no parameters of array type, this will only occur as the referenced type of a reference parameter.") and the initializer list has a single element that is an appropriately-typed[*string-literal*](lex.string#nt:string-literal "5.13.5 String literals [lex.string]") ([[dcl.init.string]](dcl.init.string "9.5.3 Character arrays")), the implicit conversion
sequence is the identity conversion[.](#over.ics.list-4.sentence-1)

[5](#over.ics.list-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2540)

Otherwise, if the parameter type is std​::​initializer_list<X> and all the elements
of the initializer list can be implicitly converted to X, the implicit
conversion sequence is the worst conversion necessary to convert an element of
the list to X, or if the initializer list has no elements, the identity
conversion[.](#over.ics.list-5.sentence-1)

This conversion can be a user-defined conversion even in
the context of a call to an initializer-list constructor[.](#over.ics.list-5.sentence-2)

[*Example [2](#over.ics.list-example-2)*: void f(std::initializer_list<int>);
f( {} ); // OK, f(initializer_list<int>) identity conversion f( {1,2,3} ); // OK, f(initializer_list<int>) identity conversion f( {'a','b'} ); // OK, f(initializer_list<int>) integral promotion f( {1.0} ); // error: narrowingstruct A { A(std::initializer_list<double>); // #1 A(std::initializer_list<std::complex<double>>); // #2 A(std::initializer_list<std::string>); // #3};
A a{ 1.0,2.0 }; // OK, uses #1void g(A);
g({ "foo", "bar" }); // OK, uses #3typedef int IA[3];void h(const IA&);
h({ 1, 2, 3 }); // OK, identity conversion â *end example*]

[6](#over.ics.list-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2572)

Otherwise, if the parameter type is âarray of N Xâ
or âarray of unknown bound of Xâ,
if there exists an implicit conversion sequence
from each element of the initializer list
(and from {} in the former case
if N exceeds the number of elements in the initializer list)
to X, the implicit conversion sequence is
the worst such implicit conversion sequence[.](#over.ics.list-6.sentence-1)

[7](#over.ics.list-7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2582)

Otherwise, if the parameter is a non-aggregate class X and overload
resolution per [[over.match.list]](#list "12.2.2.8 Initialization by list-initialization") chooses a single best constructor C ofX to perform the initialization of an object of type X from the
argument initializer list:

- [(7.1)](#over.ics.list-7.1)

  If C is not an initializer-list constructor
and the initializer list has a single element of type cv U,
where U is X or a class derived from X,
the implicit conversion sequence has Exact Match rank if U is X,
or Conversion rank if U is derived from X[.](#over.ics.list-7.1.sentence-1)

- [(7.2)](#over.ics.list-7.2)

  Otherwise, the implicit conversion sequence is a user-defined
conversion sequence whose second standard conversion sequence is an
identity conversion[.](#over.ics.list-7.2.sentence-1)

If multiple constructors are viable but none is better than
the others, the implicit conversion sequence is the ambiguous conversion
sequence[.](#over.ics.list-7.sentence-2)

User-defined conversions are allowed for conversion of the initializer
list elements to the constructor parameter types except as noted
in [[over.best.ics]](#over.best.ics "12.2.4.2 Implicit conversion sequences")[.](#over.ics.list-7.sentence-3)

[*Example [3](#over.ics.list-example-3)*: struct A { A(std::initializer_list<int>);};void f(A);
f( {'a', 'b'} ); // OK, f(A(std​::​initializer_list<int>)) user-defined conversionstruct B { B(int, double);};void g(B);
g( {'a', 'b'} ); // OK, g(B(int, double)) user-defined conversion g( {1.0, 1.0} ); // error: narrowingvoid f(B);
f( {'a', 'b'} ); // error: ambiguous f(A) or f(B)struct C { C(std::string);};void h(C);
h({"foo"}); // OK, h(C(std​::​string("foo")))struct D { D(A, C);};void i(D);
i({ {1,2}, {"bar"} }); // OK, i(D(A(std​::​initializer_list<int>{1,2}), C(std​::​string("bar")))) â *end example*]

[8](#over.ics.list-8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2636)

Otherwise, if the parameter has an aggregate type which can be initialized from
the initializer list according to the rules for aggregate
initialization ([[dcl.init.aggr]](dcl.init.aggr "9.5.2 Aggregates")), the implicit conversion sequence is a
user-defined conversion sequence whose second standard conversion
sequence is an identity conversion[.](#over.ics.list-8.sentence-1)

[*Example [4](#over.ics.list-example-4)*: struct A {int m1; double m2;};

void f(A);
f( {'a', 'b'} ); // OK, f(A(int,double)) user-defined conversion f( {1.0} ); // error: narrowing â *end example*]

[9](#over.ics.list-9)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2655)

Otherwise, if the parameter is a reference, see [[over.ics.ref]](#over.ics.ref "12.2.4.2.5 Reference binding")[.](#over.ics.list-9.sentence-1)

[*Note [2](#over.ics.list-note-2)*:

The rules in this subclause will apply for initializing the underlying temporary
for the reference[.](#over.ics.list-9.sentence-2)

â *end note*]

[*Example [5](#over.ics.list-example-5)*: struct A {int m1; double m2;};

void f(const A&);
f( {'a', 'b'} ); // OK, f(A(int,double)) user-defined conversion f( {1.0} ); // error: narrowingvoid g(const double &);
g({1}); // same conversion as int to double â *end example*]

[10](#over.ics.list-10)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2677)

Otherwise, if the parameter type is not a class:

- [(10.1)](#over.ics.list-10.1)

if the initializer list has one element that is not itself an initializer list,
the implicit conversion sequence is the one required to convert the element to
the parameter type;
  [*Example [6](#over.ics.list-example-6)*: void f(int);
f( {'a'} ); // OK, same conversion as char to int f( {1.0} ); // error: narrowing â *end example*]

- [(10.2)](#over.ics.list-10.2)

if the initializer list has no elements, the implicit conversion sequence
is the identity conversion[.](#over.ics.list-10.sentence-1)
  [*Example [7](#over.ics.list-example-7)*: void f(int);
f( { } ); // OK, identity conversion â *end example*]

[11](#over.ics.list-11)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2701)

In all cases other than those enumerated above, no conversion is possible[.](#over.ics.list-11.sentence-1)

[106)](#footnote-106)[106)](#footnoteref-106)

Since there are no parameters of array type,
this will only occur as the referenced type of a reference parameter[.](#footnote-106.sentence-1)

#### [12.2.4.3](#over.ics.rank) Ranking implicit conversion sequences [[over.ics.rank]](over.ics.rank)

[1](#over.ics.rank-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2706)

This subclause defines a partial ordering of implicit conversion
sequences based on the relationships[*better conversion sequence*](#def:conversion_sequence,better "12.2.4.3 Ranking implicit conversion sequences [over.ics.rank]") and[*better conversion*](#def:conversion,better "12.2.4.3 Ranking implicit conversion sequences [over.ics.rank]")[.](#over.ics.rank-1.sentence-1)

If an implicit conversion sequence S1 is
defined by these rules to be a better conversion sequence than
S2, then it is also the case that S2 is a[*worse conversion sequence*](#def:conversion_sequence,worse "12.2.4.3 Ranking implicit conversion sequences [over.ics.rank]") than S1[.](#over.ics.rank-1.sentence-2)

If conversion sequence S1 is neither better
than nor worse than conversion sequence S2, S1 and S2 are said to
be[*indistinguishable conversion sequences*](#def:conversion_sequence,indistinguishable "12.2.4.3 Ranking implicit conversion sequences [over.ics.rank]")[.](#over.ics.rank-1.sentence-3)

[2](#over.ics.rank-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2722)

When comparing the basic forms of implicit conversion sequences
(as defined in [[over.best.ics]](#over.best.ics "12.2.4.2 Implicit conversion sequences"))

- [(2.1)](#over.ics.rank-2.1)

a [standard conversion sequence](#over.ics.scs "12.2.4.2.2 Standard conversion sequences [over.ics.scs]") is a better
conversion sequence than a user-defined conversion sequence
or an ellipsis conversion sequence, and

- [(2.2)](#over.ics.rank-2.2)

a [user-defined conversion sequence](#over.ics.user "12.2.4.2.3 User-defined conversion sequences [over.ics.user]") is a
better conversion sequence than an [ellipsis conversion
sequence](#over.ics.ellipsis "12.2.4.2.4 Ellipsis conversion sequences [over.ics.ellipsis]")[.](#over.ics.rank-2.sentence-1)

[3](#over.ics.rank-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2736)

Two implicit conversion sequences of the same form are
indistinguishable conversion sequences unless one of the
following rules applies:

- [(3.1)](#over.ics.rank-3.1)

  List-initialization sequence L1 is a better conversion sequence than
list-initialization sequence L2 if
  * [(3.1.1)](#over.ics.rank-3.1.1)

L1 converts to std​::​initializer_list<X> for some X andL2 does not, or, if not that,

  * [(3.1.2)](#over.ics.rank-3.1.2)

L1 and L2 convert to arrays of the same element type, and
either the number of elements n1 initialized by L1 is less than the number of elements n2 initialized by L2, orn1=n2 andL2 converts to an array of unknown bound and L1 does not,

even if one of the other rules in this paragraph would otherwise apply[.](#over.ics.rank-3.1.sentence-1)
  [*Example [1](#over.ics.rank-example-1)*: void f1(int); // #1void f1(std::initializer_list<long>); // #2void g1() { f1({42}); } // chooses #2void f2(std::pair<const char*, const char*>); // #3void f2(std::initializer_list<std::string>); // #4void g2() { f2({"foo","bar"}); } // chooses #4 â *end example*]
  [*Example [2](#over.ics.rank-example-2)*: void f(int (&&)[] ); // #1void f(double (&&)[] ); // #2void f(int (&&)[2]); // #3 f( {1} ); // Calls #1: Better than #2 due to conversion, better than #3 due to bounds f( {1.0} ); // Calls #2: Identity conversion is better than floating-integral conversion f( {1.0, 2.0} ); // Calls #2: Identity conversion is better than floating-integral conversion f( {1, 2} ); // Calls #3: Converting to array of known bound is better than to unknown bound,// and an identity conversion is better than floating-integral conversion â *end example*]

- [(3.2)](#over.ics.rank-3.2)

  Standard conversion sequenceS1 is a better conversion
sequence than standard conversion sequenceS2 if
  * [(3.2.1)](#over.ics.rank-3.2.1)

S1 is a proper subsequence ofS2 (comparing the conversion sequences in the canonical form defined
by [[over.ics.scs]](#over.ics.scs "12.2.4.2.2 Standard conversion sequences"), excluding any Lvalue Transformation;
the identity conversion sequence is considered to be a
subsequence of any non-identity conversion sequence)
or, if not that,

  * [(3.2.2)](#over.ics.rank-3.2.2)

the rank ofS1 is better than the rank ofS2,
orS1 andS2 have the same rank and are distinguishable by the rules
in the paragraph below,
or, if not that,

  * [(3.2.3)](#over.ics.rank-3.2.3)

S1 and S2 include reference bindings ([[dcl.init.ref]](dcl.init.ref "9.5.4 References")) and
neither refers to an implicit object parameter of a non-static member function
declared without a [*ref-qualifier*](dcl.decl.general#nt:ref-qualifier "9.3.1 General [dcl.decl.general]"),
and S1 binds an rvalue reference to an
rvalue and S2 binds an lvalue reference
      [*Example [3](#over.ics.rank-example-3)*: int i;int f1();int&& f2();int g(const int&);int g(const int&&);int j = g(i); // calls g(const int&)int k = g(f1()); // calls g(const int&&)int l = g(f2()); // calls g(const int&&)struct A { A& operator<<(int); void p() &; void p() &&;};
A& operator<<(A&&, char);
A() << 1; // calls A​::​operator<<(int) A() << 'c'; // calls operator<<(A&&, char) A a;
a << 1; // calls A​::​operator<<(int) a << 'c'; // calls A​::​operator<<(int) A().p(); // calls A​::​p()&& a.p(); // calls A​::​p()& â *end example*]
 or, if not that,

  * [(3.2.4)](#over.ics.rank-3.2.4)

S1 and S2 include reference bindings ([[dcl.init.ref]](dcl.init.ref "9.5.4 References")) andS1 binds an lvalue reference to an lvalue of function type andS2 binds an rvalue reference to an lvalue of function type
      [*Example [4](#over.ics.rank-example-4)*: int f(void(&)()); // #1int f(void(&&)()); // #2void g();int i1 = f(g); // calls #1 â *end example*]
 or, if not that,

  * [(3.2.5)](#over.ics.rank-3.2.5)

S1 and S2 differ only
in their qualification conversion ([[conv.qual]](conv.qual "7.3.6 Qualification conversions")) and
yield similar types T1 and T2, respectively
(where a standard conversion sequence that is a reference binding
is considered to yield the cv-unqualified referenced type),
where T1 and T2 are not the same type, andconst T2 is reference-compatible with T1 ([[dcl.init.ref]](dcl.init.ref "9.5.4 References"))
      [*Example [5](#over.ics.rank-example-5)*: int f(const volatile int *);int f(const int *);int i;int j = f(&i); // calls f(const int*)int g(const int*);int g(const volatile int* const&);int* p;int k = g(p); // calls g(const int*) â *end example*]
 or, if not that,

  * [(3.2.6)](#over.ics.rank-3.2.6)

S1 andS2 bind âreference to T1â and âreference to T2â,
respectively ([[dcl.init.ref]](dcl.init.ref "9.5.4 References")),
where T1 and T2 are not the same type, andT2 is reference-compatible with T1
      [*Example [6](#over.ics.rank-example-6)*: int f(const int &);int f(int &);int g(const int &);int g(int);

        int i;int j = f(i); // calls f(int &)int k = g(i); // ambiguousstruct X {void f() const; void f();};void g(const X& a, X b) { a.f(); // calls X​::​f() const b.f(); // calls X​::​f()}int h(int (&)[]);int h(int (&)[1]);void g2() {int a[1];
 h(a); // calls h(int (&)[1])} â *end example*]
 or, if not that,

  * [(3.2.7)](#over.ics.rank-3.2.7)

S1 and S2 bind the same reference type âreference to Tâ and
have source types V1 and V2, respectively,
where the standard conversion sequence from V1* to T* is better than the standard conversion sequence from V2* to T*[.](#over.ics.rank-3.2.sentence-1)
      [*Example [7](#over.ics.rank-example-7)*: struct Z {};

        struct A {operator Z&(); operator const Z&(); // #1};

        struct B {operator Z(); operator const Z&&(); // #2};

        const Z& r1 = A(); // OK, uses #1const Z&& r2 = B(); // OK, uses #2 â *end example*]

- [(3.3)](#over.ics.rank-3.3)

  User-defined conversion sequenceU1 is a better conversion sequence than another user-defined conversion
sequenceU2 if they contain the same user-defined conversion function or
constructor or they initialize the same class in an aggregate
initialization and in either case the second standard conversion
sequence ofU1 is better than
the second standard conversion sequence ofU2[.](#over.ics.rank-3.3.sentence-1)
  [*Example [8](#over.ics.rank-example-8)*: struct A {operator short();} a;int f(int);int f(float);int i = f(a); // calls f(int), because short â int is// better than short â float. â *end example*]

[4](#over.ics.rank-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2973)

Standard conversion sequences are ordered by their ranks: an Exact Match is a
better conversion than a Promotion, which is a better conversion than
a Conversion[.](#over.ics.rank-4.sentence-1)

Two conversion sequences with the same rank are indistinguishable unless
one of the following rules applies:

- [(4.1)](#over.ics.rank-4.1)

  A conversion that does not convert a pointer or a pointer to member
tobool is better than one that does[.](#over.ics.rank-4.1.sentence-1)

- [(4.2)](#over.ics.rank-4.2)

  A conversion that promotes an enumeration whose underlying type is fixed to its underlying
type is better than one that promotes to the promoted underlying type, if the two are
different[.](#over.ics.rank-4.2.sentence-1)

- [(4.3)](#over.ics.rank-4.3)

  A conversion in either direction
between floating-point type FP1 and floating-point type FP2 is better than a conversion in the same direction
between FP1 and arithmetic type T3 if
  * [(4.3.1)](#over.ics.rank-4.3.1)

the floating-point conversion rank ([[conv.rank]](conv.rank "6.9.6 Conversion ranks")) of FP1 is equal to the rank of FP2, and

  * [(4.3.2)](#over.ics.rank-4.3.2)

T3 is not a floating-point type, orT3 is a floating-point type
whose rank is not equal to the rank of FP1, or
the floating-point conversion subrank ([[conv.rank]](conv.rank "6.9.6 Conversion ranks")) of FP2 is greater than the subrank of T3[.](#over.ics.rank-4.3.sentence-1)
      [*Example [9](#over.ics.rank-example-9)*: int f(std::float32_t);int f(std::float64_t);int f(long long);float x;
std::float16_t y;int i = f(x); // calls f(std​::​float32_t) on implementations where// float and std​::​float32_t have equal conversion ranksint j = f(y); // error: ambiguous, no equal conversion rank â *end example*]

- [(4.4)](#over.ics.rank-4.4)

  If classB is derived directly or indirectly from classA,
conversion ofB* toA* is better than conversion ofB* tovoid*,
and conversion ofA* tovoid* is better than conversion
ofB* tovoid*[.](#over.ics.rank-4.4.sentence-1)

- [(4.5)](#over.ics.rank-4.5)

  If classB is derived directly or indirectly from classA and classC is derived directly or indirectly fromB,
  * [(4.5.1)](#over.ics.rank-4.5.1)

conversion ofC* toB* is better than conversion ofC* toA*,
      [*Example [10](#over.ics.rank-example-10)*: struct A {};struct B : public A {};struct C : public B {};
C* pc;int f(A*);int f(B*);int i = f(pc); // calls f(B*) â *end example*]

  * [(4.5.2)](#over.ics.rank-4.5.2)

binding of an expression of typeC to a reference to typeB is better than binding an expression of typeC to a reference to typeA,

  * [(4.5.3)](#over.ics.rank-4.5.3)

conversion ofA​::​* toB​::​* is better than conversion ofA​::​* toC​::​*,

  * [(4.5.4)](#over.ics.rank-4.5.4)

conversion ofC toB is better than conversion ofC toA,

  * [(4.5.5)](#over.ics.rank-4.5.5)

conversion ofB* toA* is better than conversion ofC* toA*,

  * [(4.5.6)](#over.ics.rank-4.5.6)

binding of an expression of typeB to a reference to typeA is better than binding an expression of typeC to a
reference to typeA,

  * [(4.5.7)](#over.ics.rank-4.5.7)

conversion ofB​::​* toC​::​* is better than conversion
ofA​::​* toC​::​*,
and

  * [(4.5.8)](#over.ics.rank-4.5.8)

conversion ofB toA is better than conversion ofC toA[.](#over.ics.rank-4.5.sentence-1)

  [*Note [1](#over.ics.rank-note-1)*:
  Compared conversion sequences will have different source types only in the
context of comparing the second standard conversion sequence of an
initialization by user-defined conversion (see [[over.match.best]](#best "12.2.4 Best viable function")); in
all other contexts, the source types will be the same and the target
types will be different[.](#over.ics.rank-4.5.sentence-2)
 â *end note*]
