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

[Example 1: *end example*]

For a
const
member
function of class
X,
the extra parameter is assumed to have type
“reference to
const X”.

—
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
introduced by a
using-declaration
into 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 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 requires a candidate that
is 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 one or more function templates and also
to 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*]

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.

⮥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 a name that
denotes one or more functions that might be called.

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 name to be resolved is an
id-expression
and is 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.121

Under this
assumption, the
id-expression
in the call is looked up as a
member function of
T
following the rules for looking up names in
classes ([class.member.lookup]).

The function declarations found by that 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 name is not qualified by an
->
or
.
operator and has the more general form of a
primary-expression.

The name is looked up in the context of the function
call following the normal rules for name lookup
in expressions ([basic.lookup]).

The function declarations found by that 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 keyword
this is in scope and refers to
class
T,
or a derived class of
T,
then the implied object argument is
(*this).

If the keyword
this
is not in
scope or refers to another class, then
a contrived object of type
T
becomes the implied object
argument.122

If the argument list is augmented by a contrived object and overload
resolution selects one of the non-static member functions of
T,
the call is ill-formed.

Note that cv-qualifiers on the type of objects are
significant in overload
resolution for
both glvalue and class prvalue objects.

⮥An implied object argument must be 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 obtained by ordinary lookup of the name operator()
in the context of (E).operator().

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

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

where the optional
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.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*]

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 might 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 15
(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 15: 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 the qualified lookup of T1::operator@ ([over.call.func]); otherwise, the set of member candidates is empty.
- The set of non-member candidates is the result of the unqualified lookup of operator@ in the context of the expression according to the usual rules for name lookup in unqualified function calls ([basic.lookup.argdep]) except that all member functions are ignored.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].124

[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*]

The second operand of operator
->
is ignored in selecting an
operator->
function, and is not an argument when the
operator->
function is called.

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.

[Note 1: *end note*]

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.

—
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”, the non-explicit conversion functions of S and its base classes 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”, explicit conversion functions are also considered.Those that are not hidden within S and yield a type whose cv-unqualified version is the same type as T or is a derived class thereof are candidate functions.A call to a conversion function returning “reference to X” is a glvalue of type X, and such a conversion function is therefore considered to yield X for this process of selecting candidate functions.

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 “cv1 T” is the
type of the object being initialized, and “cv S” is the type
of the initializer expression, with
S
a class type,
the candidate functions are selected as follows:

- The conversion functions of
S
and its base classes are considered. Those non-explicit conversion functions that are not hidden within S and yield type T or a type that can be converted to type T via a standard conversion sequence are candidate functions.For direct-initialization, those explicit conversion functions that are not hidden within S and yield type T or a type that can be converted to type T with a qualification conversion are also candidate functions.Conversion functions that return a cv-qualified type are considered to yield the cv-unqualified version of that type for this process of selecting candidate functions.A call to a conversion function returning “reference to X” is a glvalue of type X, and such a conversion function is therefore considered to yield X for this process of selecting candidate functions.

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, and
“cv S” is the type
of the initializer expression, with
S
a class type,
the candidate functions are selected as follows:

- The conversion functions of
S
and its base classes are considered. Those non-explicit conversion functions that are not hidden within S and yield type “lvalue reference to cv2 T2” (when initializing an lvalue reference or an rvalue reference to function) or “cv2 T2” or “rvalue reference to cv2 T2” (when initializing an rvalue reference or an lvalue reference to function), where “cv1 T” is reference-compatible with “cv2 T2”, are candidate functions.For direct-initialization, those explicit conversion functions that are not hidden within S and yield type “lvalue reference to cv2 T2” (when initializing an lvalue reference or an rvalue reference to function) or “rvalue reference to cv2 T2” (when initializing an rvalue reference or an lvalue reference to function), where T2 is the same type as T or can be converted to type T with a qualification conversion, are also candidate functions.

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-guides for C,
the set contains an additional function template,
called the aggregate deduction candidate, defined as follows.

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

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

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

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

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

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

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

—