When a function is named in a call, which function
declaration is being referenced and the validity of the call
are determined by comparing the types
of the arguments at the point of use with the types of the parameters
in the declarations in the overload set.

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*
that can
be called based on the context of the call.

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 implicit object parameter,
and certain other properties of the candidate function.

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

- invocation of a function named in the function call syntax;
- 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]);
- invocation of the operator referenced in an expression ([over.match.oper]);
- invocation of a constructor for default- or direct-initialization ([dcl.init]) of a class object ([over.match.ctor]);
- invocation of a user-defined conversion for copy-initialization of a class object ([over.match.copy]);
- invocation of a conversion function for initialization of an object of a non-class type from an expression of class type ([over.match.conv]); and
- invocation of a conversion function for conversion in which a reference ([dcl.init.ref]) will be directly bound.

Each of these contexts defines the set of candidate functions and
the list of arguments in its own unique way.

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

- 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 of viable functions ([over.match.viable]).
- Then the best viable function is selected based on the implicit conversion sequences needed to match each argument to the corresponding parameter of each viable function.

If a best viable function exists and is unique, overload
resolution succeeds and produces it as the result.

Otherwise
overload resolution fails and the invocation is ill-formed.

When overload resolution succeeds,
and the best viable function is not accessible in the context
in which it is used,
the program is ill-formed.

Overload resolution results in a *usable candidate*
if overload resolution succeeds and
the selected candidate
is either not a function ([over.built]), or
is a function that is not deleted and
is accessible from the context
in which overload resolution was performed.

The subclauses of [over.match.funcs] describe
the set of candidate functions and the argument list submitted to
overload resolution in each context in which
overload resolution is used.

The source transformations and constructions defined
in these subclauses are only for the purpose of describing the
overload resolution process.

An implementation is not required
to use such transformations and constructions.

The set of candidate functions can contain both member and non-member
functions to be resolved against the same argument list.

So that argument and parameter lists are comparable within this
heterogeneous set, a member function is considered to have an
extra first parameter, called the
*implicit object parameter*,
which represents the object for which the member function has been
called.

For the purposes of overload resolution, both static and
non-static member functions have an implicit object parameter,
but constructors do not.

Similarly, when appropriate, the context can construct an
argument list that contains an
*implied object argument*
as the first argument in the list to denote
the object to be operated on.

For non-static member functions, the type of the implicit object
parameter is

- “lvalue reference to cv X” for functions declared
without a
*ref-qualifier*or with the &*ref-qualifier* - “rvalue reference to cv X” for functions declared with the
&&
*ref-qualifier*

For conversion 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.

For non-conversion functions
nominated by a *using-declaration*
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.

For static member functions, the implicit object parameter is considered
to match any object (since if the function is selected, the object is
discarded).

[*Note 1*: *end note*]

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]).

— During overload resolution, the implied object argument is
indistinguishable from other arguments.

The implicit object
parameter, however, retains its identity since
no user-defined conversions can be applied to achieve a type
match with it.

For non-static member functions declared without a *ref-qualifier*,
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.

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],
[over.best.ics]).

[*Example 2*: class T {
public:
T();
};
class C : T {
public:
C(int);
};
T a = 1; // error: no viable conversion (T(C(1)) not considered)
— *end example*]

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* operator T
in S.

[*Note 3*: *end note*]

This search can find a specialization of
a conversion function template ([basic.lookup]).

—
Each such case also defines sets of *permissible types*
for explicit and non-explicit conversion functions;
each (non-template) conversion function
that

- is a non-hidden member of S,
- yields a permissible type, and,
- for the former set, is non-explicit

If initializing an object, for any permissible type cv U, any
*cv2* U, *cv2* U&, or *cv2* U&&
is also a permissible type.

If the set of permissible types for explicit conversion functions is empty,
any candidates that are explicit are discarded.

In each case where a candidate is a function template, candidate
function template specializations
are generated using template argument deduction ([temp.over],
[temp.deduct]).

If a constructor template or conversion function template
has an *explicit-specifier*
whose *constant-expression* is value-dependent ([temp.dep]),
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]),
it will be removed from the candidate set.

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.

In such a case, the
candidate functions generated from each function template are combined
with the set of non-template candidate functions.

A
defaulted move special member function ([class.copy.ctor], [class.copy.assign])
that is defined as deleted
is excluded from the set of candidate functions in all contexts.

A constructor inherited from class type C ([class.inhctor.init])
that has a first parameter of type “reference to *cv1* P”
(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 and
C is reference-related to P and
P is reference-related to D.

[*Example 3*: struct A {
A(); // #1
A(A &&); // #2
template<typename T> A(T &&); // #3
};
struct B : A {
using A::A;
B(const B &); // #4
B(B &&) = default; // #5, implicitly deleted
struct 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 candidates
struct C { operator B&&(); };
B b3 = C(); // calls #4
— *end example*]

In a function call
if the *postfix-expression* names at least one function or
function template,
overload resolution is applied as specified in [over.call.func].

If the *postfix-expression* denotes an object of class type, overload
resolution is applied as specified in [over.call.object].

If the *postfix-expression* is the address of an overload set,
overload resolution is applied using that set as described above.

If the function selected by overload resolution is a non-static member function,
the program is ill-formed.

[*Note 1*: *end note*]

The resolution of the address of an
overload set in other contexts is described in [over.over].

— Of interest in [over.call.func] are only those function calls in
which the *postfix-expression*
ultimately contains an *id-expression* that
denotes one or more functions.

Such a
*postfix-expression*,
perhaps nested arbitrarily deep in
parentheses, has one of the following forms:

These represent two syntactic subcategories of function calls:
qualified function calls and unqualified function calls.

In qualified function calls,
the function is named by an *id-expression*
preceded by an -> or . operator.

Since the
construct
A->B
is generally equivalent to
(*A).B,
the rest of
[over] assumes, without loss of generality, that all member
function calls have been normalized to the form that uses an
object and the
.
operator.

Furthermore, [over] assumes that
the
*postfix-expression*
that is the left operand of the
.
operator
has type “cv T”
where
T
denotes a class.112

The function declarations found by name lookup ([class.member.lookup])
constitute the set of candidate functions.

The argument list is the
*expression-list*
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]).

In unqualified function calls, the function is named by a
*primary-expression*.

The function declarations found by name lookup ([basic.lookup]) constitute the
set of candidate functions.

Because of the rules for name lookup, the set of candidate functions
consists (1) entirely of non-member functions or (2) entirely of
member functions of some class
T.

In case (1),
the argument list is
the same as the
*expression-list*
in the call.

In case (2), the argument list is the
*expression-list*
in the call augmented by the addition of an implied object
argument as in a qualified function call.

If the current class is, or is derived from, T, and the keyword
this ([expr.prim.this]) refers to it,
then the implied object argument is (*this).

113)113)

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.

If the *postfix-expression* E
in the function call syntax evaluates
to a class object of type “cv T”,
then the set of candidate functions
includes at least the function call operators of T.

The function call operators of T
are the results of a search for the name operator()
in the scope of T.

In addition, for each non-explicit conversion function declared in T of the
form
*cv-qualifier-seq*
is the same cv-qualification as, or a greater cv-qualification than,
cv,
and where
*conversion-type-id*
denotes the type “pointer to function
of () returning R”,
or the type “reference to pointer to function
of () returning R”,
or the type
“reference to function of ()
returning R”, a *surrogate call function* with the unique name
*call-function*
and having the form
is also considered as a candidate function.

operator *conversion-type-id* ( ) *cv-qualifier-seq* *ref-qualifier* *noexcept-specifier* *attribute-specifier-seq* ;

where the optional
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).

[*Note 1*: *end note*]

When comparing the
call against the function call operators, the implied object
argument is compared against the implicit object parameter of the
function call operator.

When comparing the call against a
surrogate call function, the implied object argument is compared
against the first parameter of the surrogate call function.

The
conversion function from which the surrogate call function was
derived will be used in the conversion sequence for that
parameter since it converts the implied object argument to the
appropriate function pointer or reference required by that first
parameter.

— [*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
— *end example*]

114)114)

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.

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].

[*Note 1*: *end note*]

Because
.,
.*,
and
::
cannot be overloaded,
these operators are always built-in operators interpreted according to
[expr.compound].

?:
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]).

— [*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 type
int I = 1 + 1; // always evaluates to 2 even if class or enumeration types exist
// that would perform the operation.
}
— *end example*]

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.

In this case, overload resolution is used to determine
which operator function or built-in operator is to be invoked to implement the
operator.

Therefore, the operator notation is first transformed
to the equivalent function-call notation as summarized in
Table 17
(where @ denotes one of the operators covered in the specified subclause).

However, the operands are sequenced in the order prescribed
for the built-in operator ([expr.compound]).

Table 17: Relationship between operator and function call notation [tab:over.match.oper]

Subclause | Expression | As member function | As non-member function | |

@a | (a).operator@ ( ) | operator@(a) | ||

a@b | (a).operator@ (b) | operator@(a, b) | ||

a=b | (a).operator= (b) | |||

a[b] | (a).operator[](b) | |||

a-> | (a).operator->( ) | |||

a@ | (a).operator@ (0) | operator@(a, 0) |

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*,
*non-member candidates*,
*built-in candidates*,
and
*rewritten candidates*,
are constructed as follows:

- If T1 is a complete class type or a class currently being defined, the set of member candidates is the result of a search for operator@ in the scope of T1; otherwise, the set of member candidates is empty.
- For the operators =, [], or ->, the set of non-member candidates is empty; otherwise, it includes the result of unqualified lookup for operator@ in the rewritten function call ([basic.lookup.unqual], [basic.lookup.argdep]), ignoring all member functions.However, if no operand has a class type, only those non-member functions in the lookup set that have a first parameter of type T1 or “reference to cv T1”, when T1 is an enumeration type, or (if there is a right operand) a second parameter of type T2 or “reference to cv T2”, when T2 is an enumeration type, are candidate functions.
- For all other operators, the built-in candidates include all of the candidate operator functions defined in [over.built] that, compared to the given operator,
- have the same operator name, and
- accept the same number of operands, and
- accept operand types to which the given operand or operands can be converted according to [over.best.ics], and
- do not have the same parameter-type-list as any non-member candidate that is not a function template specialization.

- The rewritten candidate set is determined as follows:
- For the relational ([expr.rel]) operators, the rewritten candidates include all non-rewritten candidates for the expression x <=> y.
- For the relational ([expr.rel]) and three-way comparison ([expr.spaceship]) 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.
- 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.
- For all other operators, the rewritten candidate set is empty.

For the built-in assignment operators, conversions of the left
operand are restricted as follows:

- no temporaries are introduced to hold the left operand, and
- no user-defined conversions are applied to the left operand to achieve a type match with the left-most parameter of a built-in candidate.

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 @.

The argument list contains all of the
operands of the operator.

The best function from the set of candidate functions is selected
according to [over.match.viable]
and [over.match.best].115

[*Example 2*: 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)
}
— *end example*]

If a rewritten operator<=> candidate
is selected by overload resolution
for an operator @,
x @ y
is interpreted as
0 @ (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.

Rewritten candidates for the operator @
are not considered in the context of the resulting expression.

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

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

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 is not applied.

Then the operator is treated as the corresponding
built-in operator and interpreted according to [expr.compound].

[*Example 3*: struct X {
operator double();
};
struct Y {
operator int*();
};
int *a = Y() + 100.0; // error: pointer arithmetic requires integral operand
int *b = Y() + X(); // error: pointer arithmetic requires integral operand
— *end example*]

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].

[*Note 3*: *end note*]

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+
}

— When objects of class type are direct-initialized,
copy-initialized from an expression of the same or a
derived class type ([dcl.init]),
or default-initialized,
overload resolution selects the constructor.

For direct-initialization or default-initialization
that is not in the context of copy-initialization, the
candidate functions are
all the constructors of the class of the object being
initialized.

For copy-initialization (including default initialization
in the context of copy-initialization), the candidate functions are all
the converting constructors ([class.conv.ctor]) of that
class.

Under the conditions specified in [dcl.init], 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.

Overload resolution is used
to select the user-defined conversion to be invoked.

Assuming that
“*cv1* T” is the type of the object being initialized, with
T
a class type,
the candidate functions are selected as follows:

- When the type of the initializer expression is a class type “cv S”, conversion functions are considered.When initializing a temporary object ([class.mem]) to be bound to the first parameter of a constructor where the parameter is of type “reference to
*cv2*T” and the constructor is called with a single argument in the context of direct-initialization of an object of type “*cv3*T”, the permissible types for explicit conversion functions are the same; otherwise there are none.

Under the conditions specified in [dcl.init], 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.

Overload resolution is used to select the
conversion function to be invoked.

Assuming that “cv T” is the
type of the object being initialized,
the candidate functions are selected as follows:

- 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]). 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]); otherwise there are none.

Under the conditions specified in [dcl.init.ref], a reference can be bound directly
to the result of applying a conversion
function to an initializer expression.

Overload resolution is used to select the
conversion function to be invoked.

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

- Let R be a set of types including
- “lvalue reference to
*cv2*T2” (when initializing an lvalue reference or an rvalue reference to function) and - “
*cv2*T2” and “rvalue reference to*cv2*T2” (when initializing an rvalue reference or an lvalue reference to function)

The permissible types for non-explicit conversion functions are the members of R where “*cv1*T” is reference-compatible ([dcl.init.ref]) with “*cv2*T2”.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]); otherwise there are none. - “lvalue reference to

When objects of non-aggregate class type T are
list-initialized such that [dcl.init.list] 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],
overload resolution selects the constructor in two phases:

- 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]) of the class T and the argument list consists of the initializer list as a single argument.
- 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.

In copy-list-initialization, if an explicit constructor is
chosen, the initialization is ill-formed.

[*Note 1*: *end note*]

This differs from other situations ([over.match.ctor], [over.match.copy]),
where only converting constructors are considered for copy-initialization.

This restriction only
applies if this initialization is part of the final result of overload
resolution.

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

- If C is defined, for each constructor of C, a function template with the following properties:
- The template parameters are the template parameters of C followed by the template parameters (including default template arguments) of the constructor, if any.
- The types of the function parameters are those of the constructor.
- The return type is the class template specialization designated by C and template arguments corresponding to the template parameters of C.

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

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

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

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

- brace elision is not considered for any aggregate element that has a dependent non-array type or an array type with a value-dependent bound, and
- each non-trailing aggregate element that is a pack expansion is assumed to correspond to no elements of the initializer list, and
- a trailing aggregate element that is a pack expansion is assumed to correspond to all remaining elements of the initializer list (if any).

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

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

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

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

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

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

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

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

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

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

The following exceptions apply:

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

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

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

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

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

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

— From the set of candidate functions constructed for a given
context ([over.match.funcs]), 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])
for the best fit ([over.match.best]).

The selection of viable functions considers
associated constraints, if any, and
relationships between arguments and function parameters other
than the ranking of conversion sequences.

First, to be a viable function, a candidate function shall have
enough parameters to agree in number with the arguments in the
list.

- If there are m arguments in the list, all candidate functions having exactly m parameters are viable.
- A candidate function having fewer than m parameters is viable only if it has an ellipsis in its parameter list ([dcl.fct]).For the purposes of overload resolution, any argument for which there is no corresponding parameter is considered to “match the ellipsis” ([over.ics.ellipsis]).
- A candidate function having more than m parameters is viable only if all parameters following the have default arguments ([dcl.fct.default]).For the purposes of overload resolution, the parameter list is truncated on the right, so that there are exactly m parameters.

Second, for a function to be viable, if it has associated constraints ([temp.constr.decl]),
those constraints shall be satisfied ([temp.constr.constr]).

Third, for
F
to be a viable function, there shall exist for each
argument an
implicit conversion sequence that
converts that argument to the corresponding parameter of
F.

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 be bound to an rvalue
and that an rvalue reference cannot be bound to an lvalue
can affect
the viability of the function (see [over.ics.ref]).

Define as follows:

- If F is a static member function, is defined such that is neither better nor worse than for any function G, and, symmetrically, is neither better nor worse than ;117 otherwise,
- let denote the implicit conversion sequence that converts
the argument in the list to the type of
the parameter of viable function F. [over.best.ics] defines the implicit conversion sequences and [over.ics.rank] defines what it means for one implicit conversion sequence to be a better conversion sequence or worse conversion sequence than another.

Given these definitions,
a viable function is defined to be a
*better*
function than another viable function
if for all arguments i,
is not a worse conversion
sequence than , and then

- for some argument j, is a better conversion sequence than , or, if not that,
- the context is an initialization by user-defined conversion
(see [dcl.init],
[over.match.conv], and [over.match.ref])
and the standard conversion sequence
from the return type of 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 return type of to the destination type
or, if not that,[
*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 int float x = a; // ambiguous: both possibilities require conversions, // and neither is better than the other —*end example*] - the context is an initialization by conversion function for direct
reference binding 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
or, if not that,[
*Example 2*: template <class T> struct A { operator T&(); // #1 operator T&&(); // #2 }; typedef int Fn(); A<Fn> a; Fn& lf = a; // calls #1 Fn&& rf = a; // calls #2 —*end example*] - F1 is not a function template specialization and F2 is a function template specialization, or, if not that,
- F1 and F2 are function template specializations, and the function template for F1 is more specialized than the template for F2 according to the partial ordering rules described in [temp.func.order], or, if not that,
- F1 and F2 are non-template functions with the same parameter-type-lists, and F1 is more constrained than F2 according to the partial ordering of constraints described in [temp.constr.order], or if not that,
- 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
or, if not that,[
*Example 3*: struct A { A(int = 0); }; struct B: A { using A::A; B(); }; int main() { B b; // OK, B::B() } —*end example*] - F2 is a rewritten candidate ([over.match.oper]) and
F1 is not
or, if not that,[
*Example 4*: struct S { friend auto operator<=>(const S&, const S&) = default; // #1 friend bool operator<(const S&, const S&); // #2 }; bool b = S() < S(); // calls #2 —*end example*] - F1 and F2 are rewritten candidates, and
F2 is a synthesized candidate
with reversed order of parameters
and F1 is not
or, if not that[
*Example 5*: struct S { friend std::weak_ordering operator<=>(const S&, int); // #1 friend std::weak_ordering operator<=>(int, const S&); // #2 }; bool b = 1 < S(); // calls #2 —*end example*] - F1 is generated from a
*deduction-guide*([over.match.class.deduct]) and F2 is not, or, if not that, - F1 is the copy deduction candidate and F2 is not, or, if not that,
- F1 is generated from a non-template constructor
and F2 is generated from a constructor template. [
*Example 6*: template <class T> struct A { using value_type = T; A(value_type); // #1 A(const A&); // #2 A(T, T, int); // #3 template<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 constructor template <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 initialize template <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 —*end example*]

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.118

[*Example 7*: void Fcn(const int*, short);
void Fcn(int*, int);
int i;
short s = 0;
void f() {
Fcn(&i, s); // is ambiguous because &i → int* is better than &i → const int*
// but s → short is also better than s → int
Fcn(&i, 1L); // calls Fcn(int*, int), because &i → int* is better than &i → const int*
// and 1L → short and 1L → int are indistinguishable
Fcn(&i, 'c'); // calls Fcn(int*, int), because &i → int* is better than &i → const int*
// and c → int is better than c → short
}
— *end example*]

If the best viable function resolves to a function
for which multiple declarations were found, and
if any two of these declarations inhabit different scopes and
specify a default argument that made the function viable,
the program is ill-formed.

[*Example 8*: namespace A {
extern "C" void f(int = 5);
}
namespace B {
extern "C" void f(int = 5);
}
using A::f;
using B::f;
void use() {
f(3); // OK, default argument was not used for viability
f(); // error: found default argument twice
}
— *end example*]

118)118)

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.

An *implicit conversion sequence*
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.

The
sequence of conversions is an implicit conversion as defined in
[conv], which means it is governed by the rules for
initialization of an object or reference by a single
expression ([dcl.init], [dcl.init.ref]).

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.

[*Note 1*: *end note*]

Other properties, such as the lifetime, storage class,
alignment, accessibility of the argument, whether the argument is a bit-field,
and whether a function is deleted, are ignored.

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.

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

However, if the target is

- the first parameter of a constructor or
- the implicit object parameter of a user-defined conversion function

- [over.match.ctor], when the argument is the temporary in the second step of a class copy-initialization,
- [over.match.copy], [over.match.conv], or [over.match.ref] (in all cases), or
- the second phase of [over.match.list] 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,

[*Example 1*: struct Y { Y(int); };
struct A { operator int(); };
Y y1 = A(); // error: A::operator int() is not a candidate
struct X { X(); };
struct B { operator X(); };
B b;
X x{{b}}; // error: B::operator X() is not a candidate
— *end example*]

When the parameter type is not a reference, the implicit conversion
sequence models a copy-initialization of the parameter from the argument
expression.

The implicit conversion sequence is the one required to convert the
argument expression to a prvalue of the type of
the parameter.

Any difference in top-level cv-qualification is
subsumed by the initialization itself and does not constitute a conversion.

When the parameter has a class type and the argument expression has the
same type, the implicit conversion sequence is an identity conversion.

When the parameter has a class type and the argument expression has a
derived class type, the implicit conversion sequence is a
derived-to-base
conversion from the derived class to the base class.

A derived-to-base conversion has Conversion rank ([over.ics.scs]).

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.

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]).

If no sequence of conversions can be found to convert an argument
to a parameter type, an implicit conversion sequence cannot be formed.

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*.

For the purpose of ranking implicit conversion sequences as described
in [over.ics.rank], the ambiguous conversion sequence is treated
as a user-defined conversion sequence that is indistinguishable from any
other user-defined conversion sequence.

[*Note 5*: — *end note*]

This rule prevents a function from becoming non-viable because of an ambiguous
conversion sequence for one of its parameters.

[*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 → C (via constructor)
// and an (ambiguous) conversion b → A (via constructor or conversion function)
void f(B) { }
f(b); // OK, unambiguous
— *end example*]

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.

Table 18
summarizes the conversions defined in [conv] and
partitions them into four disjoint categories: Lvalue Transformation,
Qualification Adjustment, Promotion, and Conversion.

[*Note 1*: *end note*]

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.

— [*Note 2*: *end note*]

As described in [conv],
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.

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**.

— These are used
to rank standard conversion sequences.

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.

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.

Table 18: Conversions [tab:over.ics.scs]

Conversion | Category | Rank | Subclause | |

No conversions required | Identity | |||

Lvalue-to-rvalue conversion | ||||

Array-to-pointer conversion | Lvalue Transformation | |||

Function-to-pointer conversion | Exact Match | |||

Qualification conversions | ||||

Function pointer conversion | Qualification Adjustment | |||

Integral promotions | ||||

Floating-point promotion | Promotion | Promotion | ||

Integral conversions | ||||

Floating-point conversions | ||||

Floating-integral conversions | ||||

Pointer conversions | Conversion | Conversion | ||

Pointer-to-member conversions | ||||

Boolean conversions |

A *user-defined conversion sequence* consists of an initial
standard conversion sequence followed by a user-defined
conversion ([class.conv]) followed by a second standard
conversion sequence.

If the user-defined conversion is specified
by a constructor ([class.conv.ctor]), the initial standard
conversion sequence converts the source type to the type of the
first parameter of that constructor.

If the user-defined
conversion is specified by a conversion function, the
initial standard conversion sequence
converts the source type to the type of the
implicit object parameter of that conversion function.

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.

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] and [over.best.ics]).

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.

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.

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]).

When a parameter of reference type binds directly to an
argument expression, the implicit conversion sequence is the identity conversion,
unless the argument expression has a type that is a derived class of the parameter
type, in which case the implicit conversion sequence is a derived-to-base
conversion ([over.best.ics]).

[*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 conversion
— *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,
with the second standard conversion sequence either an identity conversion or,
if the conversion function returns an entity of a type that is a derived class
of the parameter type, a derived-to-base conversion.

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].

Conceptually, this conversion sequence corresponds to copy-initializing a
temporary of the referenced type with the argument expression.

Any difference
in top-level cv-qualification is subsumed by the initialization itself and
does not constitute a conversion.

Except for an implicit object parameter, for which see [over.match.funcs],
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 other than a function lvalue.

[*Note 1*: *end note*]

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]).

— 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.

[*Example 2*: *end example*]

A function with an “lvalue reference to int” parameter can
be a viable candidate even if the corresponding argument is an
int
bit-field.

The formation of implicit conversion sequences
treats the
int
bit-field as an
int
lvalue and finds an exact
match with the parameter.

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]).

— When an argument is an initializer list ([dcl.init.list]), it is not an expression and special rules apply for converting it to a parameter type.

If the initializer list is a *designated-initializer-list*,
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]),
in which case the implicit conversion sequence is
a user-defined conversion sequence
whose second standard conversion sequence
is an identity conversion.

[*Note 1*: — *end note*]

Aggregate initialization does not require that
the members are declared in designation order.

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]).

[*Example 1*: struct A { int x, y; };
struct B { int y, x; };
void f(A a, int); // #1
void f(B b, ...); // #2
void g(A a); // #3
void g(B b); // #4
void 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])
g({.x = 1, .y = 2}); // error: ambiguous between #3 and #4
}
— *end example*]

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.

Otherwise, if the parameter type is a character array119
and the initializer list has a single element that is an appropriately-typed
*string-literal* ([dcl.init.string]), the implicit conversion
sequence is the identity conversion.

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.

This conversion can be a user-defined conversion even in
the context of a call to an initializer-list constructor.

[*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: narrowing
struct A {
A(std::initializer_list<double>); // #1
A(std::initializer_list<complex<double>>); // #2
A(std::initializer_list<std::string>); // #3
};
A a{ 1.0,2.0 }; // OK, uses #1
void g(A);
g({ "foo", "bar" }); // OK, uses #3
typedef int IA[3];
void h(const IA&);
h({ 1, 2, 3 }); // OK: identity conversion
— *end example*]

Otherwise, if the parameter type is “array of N X”
or “array of unknown bound of X”,
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.

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

- 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.
- Otherwise, the implicit conversion sequence is a user-defined conversion sequence with the second standard conversion sequence an identity conversion.

If multiple constructors are viable but none is better than
the others, the implicit conversion sequence is the ambiguous conversion
sequence.

User-defined conversions are allowed for conversion of the initializer
list elements to the constructor parameter types except as noted
in [over.best.ics].

[*Example 3*: struct A {
A(std::initializer_list<int>);
};
void f(A);
f( {'a', 'b'} ); // OK: f(A(std::initializer_list<int>)) user-defined conversion
struct B {
B(int, double);
};
void g(B);
g( {'a', 'b'} ); // OK: g(B(int, double)) user-defined conversion
g( {1.0, 1.0} ); // error: narrowing
void 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"))))
— *end example*]

Otherwise, if the parameter has an aggregate type which can be initialized from
the initializer list according to the rules for aggregate
initialization, the implicit conversion sequence is a
user-defined conversion sequence with the second standard conversion
sequence an identity conversion.

[*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
— *end example*]

Otherwise, if the parameter is a reference, see [over.ics.ref].

[*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: narrowing
void g(const double &);
g({1}); // same conversion as int to double
— *end example*]

Otherwise, if the parameter type is not a class:

- 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*: void f(int); f( {'a'} ); // OK: same conversion as char to int f( {1.0} ); // error: narrowing —*end example*] - if the initializer list has no elements, the implicit conversion sequence
is the identity conversion.

This subclause defines a partial ordering of implicit conversion
sequences based on the relationships
*better conversion sequence*
and
*better conversion*.

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*
than S1.

If conversion sequence S1 is neither better
than nor worse than conversion sequence S2, S1 and S2 are said to
be
*indistinguishable conversion sequences*.

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

- a standard conversion sequence is a better conversion sequence than a user-defined conversion sequence or an ellipsis conversion sequence, and
- a user-defined conversion sequence is a better conversion sequence than an ellipsis conversion sequence.

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

- List-initialization sequence L1 is a better conversion sequence than list-initialization sequence L2 if
- L1 converts to std::initializer_list<X> for some X and L2 does not, or, if not that,
- L1 and L2 convert to arrays of the same element type, and either the number of elements initialized by L1 is less than the number of elements initialized by L2, or and L2 converts to an array of unknown bound and L1 does not,

[*Example 1*: void f1(int); // #1 void f1(std::initializer_list<long>); // #2 void g1() { f1({42}); } // chooses #2 void f2(std::pair<const char*, const char*>); // #3 void f2(std::initializer_list<std::string>); // #4 void g2() { f2({"foo","bar"}); } // chooses #4 —*end example*][*Example 2*: void f(int (&&)[] ); // #1 void f(double (&&)[] ); // #2 void 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 —*end example*] - Standard conversion sequence S1 is a better conversion sequence than standard conversion sequence S2 if
- S1 is a proper subsequence of S2 (comparing the conversion sequences in the canonical form defined by [over.ics.scs], excluding any Lvalue Transformation; the identity conversion sequence is considered to be a subsequence of any non-identity conversion sequence) or, if not that,
- the rank of S1 is better than the rank of S2, or S1 and S2 have the same rank and are distinguishable by the rules in the paragraph below, or, if not that,
- S1 and S2 include reference bindings ([dcl.init.ref]) and neither refers to an implicit object parameter of a non-static member function declared without a
*ref-qualifier*, and S1 binds an rvalue reference to an rvalue and S2 binds an lvalue referenceor, if not that,[*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()& —*end example*] - S1 and S2 include reference bindings ([dcl.init.ref]) and S1 binds an lvalue reference to a function lvalue and S2 binds an rvalue reference to a function lvalueor, if not that,[
*Example 4*: int f(void(&)()); // #1 int f(void(&&)()); // #2 void g(); int i1 = f(g); // calls #1 —*end example*] - S1 and S2 differ only in their qualification conversion ([conv.qual]) and yield similar types T1 and T2, respectively, where T1 can be converted to T2 by a qualification conversion.[
*Example 5*: int f(const volatile int *); int f(const int *); int i; int j = f(&i); // calls f(const int*) —*end example*]or, if not that, - S1 and S2 include reference bindings ([dcl.init.ref]), and the types to which the references refer are the same type except for top-level cv-qualifiers, and the type to which the reference initialized by S2 refers is more cv-qualified than the type to which the reference initialized by S1 refers.[
*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); // ambiguous struct X { void f() const; void f(); }; void g(const X& a, X b) { a.f(); // calls X::f() const b.f(); // calls X::f() } —*end example*]

- User-defined conversion sequence U1 is a better conversion sequence than another user-defined conversion sequence U2 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 of U1 is better than the second standard conversion sequence of U2.[
*Example 7*: struct A { operator short(); } a; int f(int); int f(float); int i = f(a); // calls f(int), because short → int is // better than short → float. —*end example*]

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.

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

- A conversion that does not convert a pointer or a pointer to member to bool is better than one that does.
- 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.
- If class B is derived directly or indirectly from class A, conversion of B* to A* is better than conversion of B* to void*, and conversion of A* to void* is better than conversion of B* to void*.
- If class B is derived directly or indirectly from class A and class C is derived directly or indirectly from B,
- conversion of
C*
to
B*
is better than conversion of
C*
to
A*,
[
*Example 8*: 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*) —*end example*] - binding of an expression of type C to a reference to type B is better than binding an expression of type C to a reference to type A,
- conversion of A::* to B::* is better than conversion of A::* to C::*,
- conversion of C to B is better than conversion of C to A,
- conversion of B* to A* is better than conversion of C* to A*,
- binding of an expression of type B to a reference to type A is better than binding an expression of type C to a reference to type A,
- conversion of B::* to C::* is better than conversion of A::* to C::*, and
- conversion of B to A is better than conversion of C to A.

[*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]); in all other contexts, the source types will be the same and the target types will be different.—*end note*] - conversion of
C*
to
B*
is better than conversion of
C*
to
A*,

An *id-expression*
whose terminal name refers to an overload set S and
that appears without arguments
is resolved to
a function,
a pointer to function, or
a pointer to member function
for a specific function
that is chosen from a set of functions selected from S
determined based on the target type required in the context (if any),
as described below.

The target can be

- an object or reference being initialized ([dcl.init], [dcl.init.ref], [dcl.init.list]),
- the left side of an assignment,
- a parameter of a function ([expr.call]),
- a parameter of a user-defined operator,
- the return value of a function, operator function, or conversion ([stmt.return]),
- an explicit type conversion ([expr.type.conv], [expr.static.cast], [expr.cast]), or
- a non-type
*template-parameter*([temp.arg.nontype]).

[*Note 1*: *end note*]

Any redundant set of parentheses surrounding the function name is
ignored ([expr.prim.paren]).

— If there is no target, all non-template functions named are selected.

Otherwise, a non-template function with type F
is selected for the function type FT of the target type
if F
(after possibly applying the function pointer conversion ([conv.fctptr]))
is identical to FT.

The specialization, if any, generated by template argument deduction
([temp.over], [temp.deduct.funcaddr], [temp.arg.explicit])
for each function template named
is added to the set of selected functions considered.

Non-member functions and static member functions
match targets of function pointer type or
reference to function type.

Non-static member functions match targets of
pointer-to-member-function type.

[*Note 3*: *end note*]

If a non-static member function is chosen,
the result can be used only to form a pointer to member ([expr.unary.op]).

— All functions with
associated constraints
that are not satisfied ([temp.constr.decl])
are eliminated from the set of selected functions.

If more than one function in the set remains,
all function template specializations
in the set
are eliminated if the set also contains a function that is not a
function template specialization.

Any given non-template function
F0
is eliminated if the set contains a second
non-template function that
is more constrained than
F0
according to
the partial ordering rules of [temp.constr.order].

Any given
function template specialization
F1
is eliminated if the set contains a second
function template specialization whose function template
is more specialized than the
function template of
F1
according to
the partial ordering rules of [temp.func.order].

After such eliminations,
if any, there shall remain exactly one selected function.

[*Example 1*: int f(double);
int f(int);
int (*pfd)(double) = &f; // selects f(double)
int (*pfi)(int) = &f; // selects f(int)
int (*pfe)(...) = &f; // error: type mismatch
int (&rfi)(int) = f; // selects f(int)
int (&rfd)(double) = f; // selects f(double)
void g() {
(int (*)(int))&f; // cast expression as selector
}

The initialization of
pfe
is ill-formed because no
f()
with type
int(...)
has been declared, and not because of any ambiguity.

For another example,

struct X {
int f(int);
static int f(long);
};
int (X::*p1)(int) = &X::f; // OK
int (*p2)(int) = &X::f; // error: mismatch
int (*p3)(long) = &X::f; // OK
int (X::*p4)(long) = &X::f; // error: mismatch
int (X::*p5)(int) = &(X::f); // error: wrong syntax for
// pointer to member
int (*p6)(long) = &(X::f); // OK
— *end example*]

A declaration
whose *declarator-id* is an *operator-function-id*
shall declare a function or function template or
an explicit instantiation or specialization of a function template.

A function so declared is an *operator function*.

A function template so declared is
an *operator function template*.

A specialization of an operator function template is also an operator function.

[*Note 2*: *end note*]

The following operators cannot be overloaded:

— . .* :: ?:

nor can the preprocessing symbols
# ([cpp.stringize])
and
## ([cpp.concat]).Operator functions are usually not called directly; instead they are invoked
to evaluate the operators they implement ([over.unary] – [over.inc]).

They can be explicitly called, however, using the
*operator-function-id*
as the name of the function in the function call syntax ([expr.call]).

[*Example 1*: complex z = a.operator+(b); // complex z = a+b;
void* p = operator new(sizeof(int)*n);
— *end example*]

The allocation and deallocation functions,
operator new,
operator new[],
operator delete, and
operator delete[],
are described completely in [basic.stc.dynamic].

The attributes and restrictions
found in the rest of [over.oper] do not apply to them unless explicitly
stated in [basic.stc.dynamic].

The attributes and restrictions
found in the rest of [over.oper] do not apply to it unless explicitly
stated in [expr.await].

An operator function
shall either be a non-static member function or be a non-member function that
has at least one parameter whose type is a class, a reference to a class, an
enumeration, or a reference to an enumeration.

It is not possible to change the precedence, grouping, or number of operands
of operators.

The meaning of
the operators =, (unary) &, and , (comma),
predefined for each type, can be changed for specific class types by
defining operator functions that implement these operators.

Likewise, the meaning of the operators (unary) & and , (comma)
can be changed for specific enumeration types.

Operator functions are inherited in the same manner as other base class
functions.

An operator function shall be a
prefix unary, binary, function call, subscripting, class member access, increment, or decrement
operator function.

[*Note 3*: *end note*]

The identities among certain predefined operators applied to basic types
(for example,
++a ≡
a+=1)
need not hold for operator functions.

Some predefined operators, such as
+=,
require an operand to be an lvalue when applied to basic types;
this is not required by operator functions.

— Operator
functions cannot have more or fewer parameters than the
number required for the corresponding operator, as
described in the rest of [over.oper].

Operators not mentioned explicitly in subclauses [over.ass] through [over.inc]
act as ordinary unary and binary
operators obeying the rules of [over.unary] or [over.binary].

A *prefix unary operator function*
is a function named operator@
for a prefix *unary-operator* @ ([expr.unary.op])
that is either
a non-static member function ([class.mfct]) with no parameters or
a non-member function with one parameter.

For a *unary-expression*
of the form @ *cast-expression*,
the operator function is selected by overload resolution ([over.match.oper]).

If a member function is selected,
the expression is interpreted as
Otherwise, if a non-member function is selected,
the expression is interpreted as

A *binary operator function*
is a function named operator@
for a binary operator @ that is either
a non-static member function ([class.mfct]) with one parameter or
a non-member function with two parameters.

For an expression x @ y with subexpressions x and y,
the operator function is selected by overload resolution ([over.match.oper]).

If a member function is selected,
the expression is interpreted as

x . operator @ ( y )

Otherwise, if a non-member function is selected,
the expression is interpreted as
operator @ ( x , y )

A *three-way comparison operator function* is an operator function
for the three-way comparison operator ([expr.spaceship]).

A *comparison operator function* is
an equality operator function,
a relational operator function, or
a three-way comparison operator function.

A simple assignment operator function shall be a non-static member function.

[*Note 1*: *end note*]

Because only standard conversion sequences are considered when converting
to the left operand of an assignment operation ([over.best.ics]),
an expression x = y with a subexpression x of class type
is always interpreted as x.operator=(y).

— [*Note 2*: *end note*]

Since a copy assignment operator is implicitly declared for a class
if not declared by the user ([class.copy.assign]),
a base class assignment operator function is always hidden by
the copy assignment operator function of the derived class.

— [*Note 3*: — *end note*]

Any assignment operator function, even the copy and move assignment operators,
can be virtual.

For a derived class D with a base class B
for which a virtual copy/move assignment has been declared,
the copy/move assignment operator in D does not override
B's virtual copy/move assignment operator.

[*Example 1*: struct B {
virtual int operator= (int);
virtual B& operator= (const B&);
};
struct D : B {
virtual int operator= (int);
virtual D& operator= (const B&);
};
D dobj1;
D dobj2;
B* bptr = &dobj1;
void f() {
bptr->operator=(99); // calls D::operator=(int)
*bptr = 99; // ditto
bptr->operator=(dobj2); // calls D::operator=(const B&)
*bptr = dobj2; // ditto
dobj1 = dobj2; // calls implicitly-declared D::operator=(const D&)
}
— *end example*]

A *function call operator function*
is a function named operator()
that is a non-static member function with an arbitrary number of parameters.

It may have default arguments.

For an expression of the form
where the *postfix-expression* is of class type,
the operator function
is selected by overload resolution ([over.call.object]).

If a surrogate call function for a conversion function
named operator *conversion-type-id* is selected,
the expression is interpreted as
Otherwise, the expression is interpreted as

A *subscripting operator function*
is a function named operator[]
that is a non-static member function with exactly one parameter.

For an expression of the form
the operator function
is selected by overload resolution ([over.match.oper]).

If a member function is selected,
the expression is interpreted as

A *class member access operator function*
is a function named operator->
that is a non-static member function taking no parameters.

For an expression of the form
the operator function
is selected by overload resolution ([over.match.oper]),
and the expression is interpreted as

If this function is a non-static member function with no parameters, or a non-member
function with one parameter,
it defines the prefix increment operator
++
for objects of that type.

If the function is a non-static member function with one parameter (which shall be of type
int)
or a non-member function with two parameters (the second of which shall be of type
int),
it defines the postfix increment operator
++
for objects of that type.

When the postfix increment is called as a result of using the
++
operator, the
int
argument will have value zero.120

[*Example 1*: struct X {
X& operator++(); // prefix ++a
X operator++(int); // postfix a++
};
struct Y { };
Y& operator++(Y&); // prefix ++b
Y operator++(Y&, int); // postfix b++
void f(X a, Y b) {
++a; // a.operator++();
a++; // a.operator++(0);
++b; // operator++(b);
b++; // operator++(b, 0);
a.operator++(); // explicit call: like ++a;
a.operator++(0); // explicit call: like a++;
operator++(b); // explicit call: like ++b;
operator++(b, 0); // explicit call: like b++;
}
— *end example*]

A *decrement operator function*
is a function named operator--
and is handled analogously to an increment operator function.

The candidate operator functions that represent the built-in operators
defined in [expr.compound] are specified in this subclause.

These candidate
functions participate in the operator overload resolution process as
described in [over.match.oper] and are used for no other purpose.

[*Note 1*: *end note*]

Because built-in operators take only operands with non-class type,
and operator overload resolution occurs only when an operand expression
originally has class or enumeration type,
operator overload resolution can resolve to a built-in operator only
when an operand has a class type that has a user-defined conversion to
a non-class type appropriate for the operator, or when an operand has
an enumeration type that can be converted to a type appropriate
for the operator.

Also note that some of the candidate operator functions given in this subclause are
more permissive than the built-in operators themselves.

As
described in [over.match.oper], after a built-in operator is selected
by overload resolution the expression is subject to the requirements for
the built-in operator given in [expr.compound], and therefore to any
additional semantic constraints given there.

In some cases, user-written candidates
with the same name and parameter types as a built-in
candidate operator function cause the built-in operator function
to not be included in the set of candidate functions.

— In this subclause, the term
*promoted integral type*
is used to refer to those integral types which are preserved by
integral promotion (including e.g.
int
and
long
but excluding e.g.
char).

For every pair
(*T*,
*vq*),
where
*T*
is an arithmetic type other than bool,
there exist candidate operator functions of the form
*vq* *T*& operator++(*vq* *T*&);
*T* operator++(*vq* *T*&, int);

For every pair
(*T*,
*vq*),
where
*T*
is an arithmetic type other than
bool,
there exist candidate operator functions of the form
*vq* *T*& operator--(*vq* *T*&);
*T* operator--(*vq* *T*&, int);

For every pair
(*T*,
*vq*),
where
*T*
is a cv-qualified or cv-unqualified object type,
there exist candidate operator functions of the form
*T***vq*& operator++(*T***vq*&);
*T***vq*& operator--(*T***vq*&);
*T** operator++(*T***vq*&, int);
*T** operator--(*T***vq*&, int);

For every cv-qualified or cv-unqualified object type
*T*,
there exist candidate operator functions of the form
*T*& operator*(*T**);

For every function type
*T* that does not have cv-qualifiers or a *ref-qualifier*,
there exist candidate operator functions of the form
*T*& operator*(*T**);

For every floating-point or promoted integral type *T*,
there exist candidate operator functions of the form
*T* operator+(*T*);
*T* operator-(*T*);

For every promoted integral type
*T*,
there exist candidate operator functions of the form
*T* operator~(*T*);

For every quintuple
(*C1*,
*C2*,
*T*,
*cv1*,
*cv2*),
where
*C2*
is a class type,
*C1*
is the same type as *C2* or is a derived class of *C2*, and
*T*
is an object type or a function type,
there exist candidate operator functions of the form
*cv12* *T*& operator->*(*cv1* *C1**, *cv2* *T C2*::*);
where *cv12* is the union of *cv1* and *cv2*.

The return type is shown for exposition only; see [expr.mptr.oper] for the
determination of the operator's result type.

For every pair of types *L* and *R*,
where each of *L* and *R* is a
floating-point or promoted integral type,
there exist candidate operator functions of the form
*LR* operator*(*L*, *R*);
*LR* operator/(*L*, *R*);
*LR* operator+(*L*, *R*);
*LR* operator-(*L*, *R*);
bool operator==(*L*, *R*);
bool operator!=(*L*, *R*);
bool operator<(*L*, *R*);
bool operator>(*L*, *R*);
bool operator<=(*L*, *R*);
bool operator>=(*L*, *R*);
where
*LR*
is the result of the usual arithmetic conversions ([expr.arith.conv]) between types
*L*
and
*R*.

For every integral type *T*
there exists a candidate operator function of the form
std::strong_ordering operator<=>(*T*, *T*);

For every pair of floating-point types
*L* and *R*,
there exists a candidate operator function of the form
std::partial_ordering operator<=>(*L*, *R*);

For every cv-qualified or cv-unqualified object type
*T*
there exist candidate operator functions of the form
*T** operator+(*T**, std::ptrdiff_t);
*T*& operator[](*T**, std::ptrdiff_t);
*T** operator-(*T**, std::ptrdiff_t);
*T** operator+(std::ptrdiff_t, *T**);
*T*& operator[](std::ptrdiff_t, *T**);

For every
*T*,
where
*T*
is a pointer to object type,
there exist candidate operator functions of the form
std::ptrdiff_t operator-(*T*, *T*);

For every *T*, where *T* is an enumeration type or a pointer type,
there exist candidate operator functions of the form
bool operator==(*T*, *T*);
bool operator!=(*T*, *T*);
bool operator<(*T*, *T*);
bool operator>(*T*, *T*);
bool operator<=(*T*, *T*);
bool operator>=(*T*, *T*);
*R* operator<=>(*T*, *T*);
where *R* is the result type specified in [expr.spaceship].

For every *T*, where *T*
is a pointer-to-member type or std::nullptr_t,
there exist candidate operator functions of the form
bool operator==(*T*, *T*);
bool operator!=(*T*, *T*);

For every pair of promoted integral types
*L*
and
*R*,
there exist candidate operator functions of the form
*LR* operator%(*L*, *R*);
*LR* operator&(*L*, *R*);
*LR* operator^(*L*, *R*);
*LR* operator|(*L*, *R*);
*L* operator<<(*L*, *R*);
*L* operator>>(*L*, *R*);
where
*LR*
is the result of the usual arithmetic conversions ([expr.arith.conv]) between types
*L*
and
*R*.

For every triple
(*L*, *vq*, *R*),
where *L* is an arithmetic type,
and *R* is a floating-point or promoted integral type,
there exist candidate operator functions of the form
*vq* *L*& operator=(*vq* *L*&, *R*);
*vq* *L*& operator*=(*vq* *L*&, *R*);
*vq* *L*& operator/=(*vq* *L*&, *R*);
*vq* *L*& operator+=(*vq* *L*&, *R*);
*vq* *L*& operator-=(*vq* *L*&, *R*);

For every pair (*T*, *vq*),
where *T* is any type,
there exist candidate operator functions of the form
*T***vq*& operator=(*T***vq*&, *T**);

For every pair
(*T*,
*vq*),
where
*T*
is an enumeration or pointer-to-member type,
there exist candidate operator functions of the form
*vq* *T*& operator=(*vq* *T*&, *T*);

For every pair
(*T*,
*vq*),
where
*T*
is a cv-qualified or cv-unqualified object type,
there exist candidate operator functions of the form
*T***vq*& operator+=(*T***vq*&, std::ptrdiff_t);
*T***vq*& operator-=(*T***vq*&, std::ptrdiff_t);

For every triple
(*L*,
*vq*,
*R*),
where
*L*
is an integral type, and
*R*
is a promoted integral type,
there exist candidate operator functions of the form
*vq* *L*& operator%=(*vq* *L*&, *R*);
*vq* *L*& operator<<=(*vq* *L*&, *R*);
*vq* *L*& operator>>=(*vq* *L*&, *R*);
*vq* *L*& operator&=(*vq* *L*&, *R*);
*vq* *L*& operator^=(*vq* *L*&, *R*);
*vq* *L*& operator|=(*vq* *L*&, *R*);

There also exist candidate operator functions of the form
bool operator!(bool);
bool operator&&(bool, bool);
bool operator||(bool, bool);

For every pair of types *L* and *R*,
where each of *L* and *R* is a
floating-point or promoted integral type,
there exist candidate operator functions of the form
*LR* operator?:(bool, *L*, *R*);
where
*LR*
is the result of the usual arithmetic conversions ([expr.arith.conv]) between types
*L*
and
*R*.

The *string-literal* or *user-defined-string-literal*
in a *literal-operator-id* shall have no
*encoding-prefix* and shall contain no characters other than the
implicit terminating '\0'.

The *ud-suffix* of the *user-defined-string-literal* or
the *identifier* in a *literal-operator-id* is called a
*literal suffix identifier*.

Some literal suffix identifiers are reserved for future standardization;
see [usrlit.suffix].

A declaration whose *literal-operator-id* uses
such a literal suffix identifier is ill-formed, no diagnostic required.

A declaration whose *declarator-id* is a
*literal-operator-id* shall declare a function or function template
that belongs to a namespace (it could be a friend function ([class.friend])) or
an explicit instantiation or specialization of a function template.

The declaration of a literal operator shall have a
*parameter-declaration-clause* equivalent to one of the following:
const char*
unsigned long long int
long double
char
wchar_t
char8_t
char16_t
char32_t
const char*, std::size_t
const wchar_t*, std::size_t
const char8_t*, std::size_t
const char16_t*, std::size_t
const char32_t*, std::size_t

If a parameter has a default argument ([dcl.fct.default]), the program is
ill-formed.

A *numeric literal operator template*
is a literal operator template whose *template-parameter-list*
has a single *template-parameter*
that is a non-type template parameter pack ([temp.variadic])
with element type char.

A *string literal operator template*
is a literal operator template whose *template-parameter-list*
comprises
a single non-type *template-parameter* of class type.

The declaration of a literal operator template
shall have an empty *parameter-declaration-clause*
and shall declare either a numeric literal operator template
or a string literal operator template.

[*Note 1*: *end note*]

Literal operators and literal operator templates are usually invoked
implicitly through user-defined literals ([lex.ext]).

However, except for
the constraints described above, they are ordinary namespace-scope functions and
function templates.

In particular, they are looked up like ordinary functions
and function templates and they follow the same overload resolution rules.

Also,
they can be declared inline or constexpr,
they can have internal, module, or external linkage,
they can be called explicitly, their addresses can be
taken, etc.

— [*Example 1*: void operator "" _km(long double); // OK
string operator "" _i18n(const char*, std::size_t); // OK
template <char...> double operator "" _\u03C0(); // OK: UCN for lowercase pi
float operator ""_e(const char*); // OK
float operator ""E(const char*); // ill-formed, no diagnostic required:
// reserved literal suffix ([usrlit.suffix], [lex.ext])
double operator""_Bq(long double); // OK: does not use the reserved *identifier* _Bq ([lex.name])
double operator"" _Bq(long double); // ill-formed, no diagnostic required:
// uses the reserved *identifier* _Bq ([lex.name])
float operator " " B(const char*); // error: non-empty *string-literal*
string operator "" 5X(const char*, std::size_t); // error: invalid literal suffix identifier
double operator "" _miles(double); // error: invalid *parameter-declaration-clause*
template <char...> int operator "" _j(const char*); // error: invalid *parameter-declaration-clause*
extern "C" void operator "" _m(long double); // error: C language linkage
— *end example*]