[over.match.best] - C++17 → C++20

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@@ -3,11 +3,11 @@
 Define ICS*i*(`F`) as follows:
 - If `F` is a static member function, ICS*1*(`F`) is defined such that
   ICS*1*(`F`) is neither better nor worse than ICS*1*(`G`) for any
   function `G`, and, symmetrically, ICS*1*(`G`) is neither better nor
-  worse than ICS*1*(`F`);[^9] otherwise,
 - let ICS*i*(`F`) denote the implicit conversion sequence that converts
   the *i*-th argument in the list to the type of the *i*-th 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
@@ -40,14 +40,14 @@ and then
   ```
   — *end example*]
   or, if not that,
 - the context is an initialization by conversion function for direct
-  reference binding ([[over.match.ref]]) of a reference to function
- type, the return type of `F1` is the same kind of reference (i.e.
- lvalue or rvalue) as the reference being initialized, and the return
- type of `F2` is not
   \[*Example 2*:
   ``` cpp
   template <class T> struct A {
     operator T&();    // #1
     operator T&&();   // #2
@@ -64,17 +64,67 @@ and then
   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` is generated from a *deduction-guide* (
- [[over.match.class.deduct]]) and `F2` is not, or, if not that,
-- `F1` is the copy deduction candidate ([[over.match.class.deduct]])
   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 3*:
   ``` cpp
   template <class T> struct A {
     using value_type = T;
     A(value_type);    // #1
     A(const A&);      // #2
@@ -100,13 +150,13 @@ and then
   — *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.[^10]
-[*Example 4*:
 ``` cpp
 void Fcn(const int*,  short);
 void Fcn(int*, int);
@@ -131,11 +181,11 @@ If the best viable function resolves to a function for which multiple
 declarations were found, and if at least two of these declarations — or
 the declarations they refer to in the case of *using-declaration*s —
 specify a default argument that made the function viable, the program is
 ill-formed.
-[*Example 5*:
 ``` cpp
 namespace A {
   extern "C" void f(int = 5);
 }
@@ -146,41 +196,43 @@ namespace B {
 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*]
 #### Implicit conversion sequences <a id="over.best.ics">[[over.best.ics]]</a>
 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 Clause  [[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. 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 ([[dcl.fct.def.delete]]), 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:
-- a *standard conversion sequence* ([[over.ics.scs]]),
-- a *user-defined conversion sequence* ([[over.ics.user]]), or
-- an *ellipsis conversion sequence* ([[over.ics.ellipsis]]).
 However, if the target is
 - the first parameter of a constructor or
 - the implicit object parameter of a user-defined conversion function
@@ -197,25 +249,25 @@ by
   is the first parameter of a constructor of class `X`, and the
   conversion is to `X` or reference to cv `X`,
 user-defined conversion sequences are not considered.
-[*Note 1*: These rules prevent more than one user-defined conversion
 from being applied during overload resolution, thereby avoiding infinite
 recursion. — *end note*]
 [*Example 1*:
 ``` cpp
 struct Y { Y(int); };
 struct A { operator int(); };
 Y y1 = A();         // error: A::operator int() is not a candidate
- struct X { };
 struct B { operator X(); };
 B b;
- X x({b}); // error: B::operator X() is not a candidate
 ```
 — *end example*]
 For the case where the parameter type is a reference, see
@@ -225,12 +277,12 @@ 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.
-[*Note 2*: When the parameter has a class type, this is a conceptual
-conversion defined for the purposes of Clause  [[over]]; the actual
 initialization is defined in terms of constructors and is not a
 conversion. — *end note*]
 Any difference in top-level cv-qualification is subsumed by the
 initialization itself and does not constitute a conversion.
@@ -242,39 +294,39 @@ case is the identity sequence; it contains no “conversion” from
 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.
-[*Note 3*: There is no such standard conversion; this derived-to-base
-Conversion exists only in the description of implicit conversion
 sequences. — *end note*]
-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 several different sequences of conversions exist that each convert
-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 4*:
 This rule prevents a function from becoming non-viable because of an
 ambiguous conversion sequence for one of its parameters.
 [*Example 3*:
@@ -285,11 +337,11 @@ class A { A (B&);};
 class B { operator A (); };
 class C { C (B&); };
 void f(A) { }
 void f(C) { }
 B b;
-f(b);               // ill-formed: 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
 ```
@@ -304,69 +356,66 @@ conversion of one of the arguments in the call is ambiguous.
 The three forms of implicit conversion sequences mentioned above are
 defined in the following subclauses.
 ##### Standard conversion sequences <a id="over.ics.scs">[[over.ics.scs]]</a>
-Table  [[tab:over.conversions]] summarizes the conversions defined in
-Clause  [[conv]] and partitions them into four disjoint categories:
-Lvalue Transformation, Qualification Adjustment, Promotion, and
-Conversion.
-[*Note 5*: 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. — *end note*]
-[*Note 6*: As described in Clause  [[conv]], a standard conversion
-sequence is either 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**. — *end note*]
-Each conversion in Table  [[tab:over.conversions]] also has an
-associated rank (Exact Match, Promotion, or Conversion). These are used
-to rank standard conversion sequences ([[over.ics.rank]]). The rank of
-a conversion sequence is determined by considering the rank of each
-conversion in the sequence and the rank of any reference binding (
-[[over.ics.ref]]). 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: Conversions** <a id="tab:over.conversions">[tab:over.conversions]</a>
 | Conversion              | Category | Rank | Subclause         |
 | ----------------------- | -------- | ---- | ----------------- |
 | No conversions required | Identity |      |                   |
 | Integral promotions     |          |      | [[conv.prom]]     |
 | Integral conversions    |          |      | [[conv.integral]] |
 ##### User-defined conversion sequences <a id="over.ics.user">[[over.ics.user]]</a>
-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 required by the argument of the constructor.
-If the user-defined conversion is specified by a conversion function (
-[[class.conv.fct]]), the initial standard conversion sequence converts
-the source type to the implicit object parameter of the conversion
-function.
 The second standard conversion sequence converts the result of the
-user-defined conversion to the target type for the 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.
@@ -382,15 +431,15 @@ 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]]).
 ##### Reference binding <a id="over.ics.ref">[[over.ics.ref]]</a>
-When a parameter of reference type binds directly ([[dcl.init.ref]]) 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 4*:
 ``` cpp
 struct A {};
@@ -402,73 +451,108 @@ int i = f(b);       // calls f(B&), an exact match, rather than f(A&), a convers
 — *end example*]
 If the parameter binds directly to the result of applying a conversion
 function to the argument expression, the implicit conversion sequence is
-a user-defined conversion sequence ([[over.ics.user]]), 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]], a standard 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 7*: 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]]). — *end note*]
 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 a standard conversion sequence, however.
 [*Example 5*: 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]]). — *end example*]
 ##### List-initialization sequence <a id="over.ics.list">[[over.ics.list]]</a>
-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 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 array [^11] 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 6*:
 ``` cpp
 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
@@ -490,15 +574,16 @@ void h(const IA&);
 h({ 1, 2, 3 });                 // OK: identity conversion
 ```
 — *end example*]
-Otherwise, if the parameter type is “array of `N` `X`”, if there exists
-an implicit conversion sequence for each element of the array from the
-corresponding element of the initializer list (or from `{}` if there is
-no such element), 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:
@@ -515,11 +600,11 @@ 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 7*:
 ``` cpp
 struct A {
   A(std::initializer_list<int>);
 };
@@ -551,15 +636,15 @@ i({ {1,2}, {"bar"} });  // OK: i(D(A(std::initializer_list<int>{1,2\), C(std::st
 — *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 ([[dcl.init.aggr]]), the implicit conversion
 sequence is a user-defined conversion sequence with the second standard
 conversion sequence an identity conversion.
-[*Example 8*:
 ``` cpp
 struct A {
   int m1;
   double m2;
@@ -572,14 +657,14 @@ f( {1.0} );             // error: narrowing
 — *end example*]
 Otherwise, if the parameter is a reference, see  [[over.ics.ref]].
-[*Note 8*: The rules in this section will apply for initializing the
 underlying temporary for the reference. — *end note*]
-[*Example 9*:
 ``` cpp
 struct A {
   int m1;
   double m2;
@@ -598,21 +683,21 @@ g({1});                 // same conversion as int to double
 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 10*:
   ``` cpp
   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.
-  \[*Example 11*:
   ``` cpp
   void f(int);
   f( { } );               // OK: identity conversion
   ```
@@ -632,26 +717,28 @@ 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 ([[over.ics.scs]]) is a better
- conversion sequence than a user-defined conversion sequence or an
- ellipsis conversion sequence, and
-- a user-defined conversion sequence ([[over.ics.user]]) is a better
-  conversion sequence than an ellipsis conversion sequence (
-  [[over.ics.ellipsis]]).
 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` converts to type “array of `N1` `T`”, `L2` converts to type
- “array of `N2` `T`”, and `N1` is smaller than `N2`,
   even if one of the other rules in this paragraph would otherwise
   apply.
   \[*Example 1*:
   ``` cpp
@@ -662,10 +749,24 @@ conversion sequences unless one of the following rules applies:
   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*]
 - 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]],
@@ -673,15 +774,15 @@ conversion sequences unless one of the following rules applies:
     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` are 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 reference
-    \[*Example 2*:
     ``` cpp
     int i;
     int f1();
     int&& f2();
     int g(const int&);
@@ -705,45 +806,43 @@ conversion sequences unless one of the following rules applies:
     a.p();                          // calls A::p()&
     ```
     — *end example*]
     or, if not that,
-  - `S1` and `S2` are reference bindings ([[dcl.init.ref]]) and `S1`
     binds an lvalue reference to a function lvalue and `S2` binds an
     rvalue reference to a function lvalue
-    \[*Example 3*:
     ``` cpp
     int f(void(&)());               // #1
     int f(void(&&)());              // #2
     void g();
     int i1 = f(g);                  // calls #1
     ```
     — *end example*]
     or, if not that,
-  - `S1`
-    and `S2` differ only in their qualification conversion and yield
- similar types `T1` and `T2` ([[conv.qual]]), respectively, and the
- cv-qualification signature of type `T1` is a proper subset of the
-    cv-qualification signature of type `T2`
-    \[*Example 4*:
     ``` cpp
     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` are 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 5*:
     ``` cpp
     int f(const int &);
     int f(int &);
     int g(const int &);
     int g(int);
@@ -767,11 +866,11 @@ conversion sequences unless one of the following rules applies:
   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 6*:
   ``` cpp
   struct A {
     operator short();
   } a;
   int f(int);
@@ -785,12 +884,12 @@ conversion sequences unless one of the following rules applies:
 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, a pointer to member, or
-  `std::nullptr_t` 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
@@ -798,11 +897,11 @@ indistinguishable unless one of the following rules applies:
   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 7*:
     ``` cpp
     struct A {};
     struct B : public A {};
     struct C : public B {};
     C* pc;

 Define ICS*i*(`F`) as follows:
 - If `F` is a static member function, ICS*1*(`F`) is defined such that
   ICS*1*(`F`) is neither better nor worse than ICS*1*(`G`) for any
   function `G`, and, symmetrically, ICS*1*(`G`) is neither better nor
+  worse than ICS*1*(`F`);[^8] otherwise,
 - let ICS*i*(`F`) denote the implicit conversion sequence that converts
   the *i*-th argument in the list to the type of the *i*-th 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
   ```
   — *end example*]
   or, if not that,
 - the context is an initialization by conversion function for direct
+  reference binding [[over.match.ref]] of a reference to function type,
+  the return type of `F1` is the same kind of reference (lvalue or
+  rvalue) as the reference being initialized, and the return type of
+  `F2` is not
   \[*Example 2*:
   ``` cpp
   template <class T> struct A {
     operator T&();    // #1
     operator T&&();   // #2
   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.
+  \[*Example 3*:
+  ``` cpp
+  struct A {
+    A(int = 0);
+  };
+  struct B: A {
+    using A::A;
+    B();
+  };
+  int main() {
+    B b;              // OK, B::B()
+  }
+  ```
+  — *end example*]
+  or, if not that,
+- `F2` is a rewritten candidate [[over.match.oper]] and `F1` is not
+  \[*Example 4*:
+  ``` cpp
+  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*]
+  or, if not that,
+- `F1` and `F2` are rewritten candidates, and `F2` is a synthesized
+  candidate with reversed order of parameters and `F1` is not
+  \[*Example 5*:
+  ``` cpp
+  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*]
+  or, if not that
+- `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 [[over.match.class.deduct]] 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*:
   ``` cpp
   template <class T> struct A {
     using value_type = T;
     A(value_type);    // #1
     A(const A&);      // #2
   — *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.[^9]
+[*Example 7*:
 ``` cpp
 void Fcn(const int*,  short);
 void Fcn(int*, int);
 declarations were found, and if at least two of these declarations — or
 the declarations they refer to in the case of *using-declaration*s —
 specify a default argument that made the function viable, the program is
 ill-formed.
+[*Example 8*:
 ``` cpp
 namespace A {
   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*]
 #### Implicit conversion sequences <a id="over.best.ics">[[over.best.ics]]</a>
 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*: 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 [[dcl.fct.def.delete]], 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. — *end note*]
 A well-formed implicit conversion sequence is one of the following
 forms:
+- a standard conversion sequence [[over.ics.scs]],
+- a user-defined conversion sequence [[over.ics.user]], or
+- an ellipsis conversion sequence [[over.ics.ellipsis]].
 However, if the target is
 - the first parameter of a constructor or
 - the implicit object parameter of a user-defined conversion function
   is the first parameter of a constructor of class `X`, and the
   conversion is to `X` or reference to cv `X`,
 user-defined conversion sequences are not considered.
+[*Note 2*: These rules prevent more than one user-defined conversion
 from being applied during overload resolution, thereby avoiding infinite
 recursion. — *end note*]
 [*Example 1*:
 ``` cpp
 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*]
 For the case where the parameter type is a reference, see
 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.
+[*Note 3*: When the parameter has a class type, this is a conceptual
+conversion defined for the purposes of [[over]]; the actual
 initialization is defined in terms of constructors and is not a
 conversion. — *end note*]
 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.
+[*Note 4*: There is no such standard conversion; this derived-to-base
+conversion exists only in the description of implicit conversion
 sequences. — *end note*]
+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*:
 This rule prevents a function from becoming non-viable because of an
 ambiguous conversion sequence for one of its parameters.
 [*Example 3*:
 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
 ```
 The three forms of implicit conversion sequences mentioned above are
 defined in the following subclauses.
 ##### Standard conversion sequences <a id="over.ics.scs">[[over.ics.scs]]</a>
+summarizes the conversions defined in [[conv]] and partitions them into
+four disjoint categories: Lvalue Transformation, Qualification
+Adjustment, Promotion, and Conversion.
+[*Note 6*: 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. — *end note*]
+[*Note 7*: 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**. — *end note*]
+Each conversion in [[over.ics.scs]] also has an associated rank (Exact
+Match, Promotion, or Conversion). These are used to rank standard
+conversion sequences [[over.ics.rank]]. The rank of a conversion
+sequence is determined by considering the rank of each conversion in the
+sequence and the rank of any reference binding [[over.ics.ref]]. 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: Conversions** <a id="over.ics.scs">[over.ics.scs]</a>
 | Conversion              | Category | Rank | Subclause         |
 | ----------------------- | -------- | ---- | ----------------- |
 | No conversions required | Identity |      |                   |
 | Integral promotions     |          |      | [[conv.prom]]     |
 | Integral conversions    |          |      | [[conv.integral]] |
 ##### User-defined conversion sequences <a id="over.ics.user">[[over.ics.user]]</a>
+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 required by the argument of the constructor. If the user-defined
+conversion is specified by a conversion function [[class.conv.fct]], the
+initial standard conversion sequence converts the source type to the
+implicit object parameter of the 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.
 call is matched with the ellipsis parameter specification of the
 function called (see  [[expr.call]]).
 ##### Reference binding <a id="over.ics.ref">[[over.ics.ref]]</a>
+When a parameter of reference type binds directly [[dcl.init.ref]] 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 4*:
 ``` cpp
 struct A {};
 — *end example*]
 If the parameter binds directly to the result of applying a conversion
 function to the argument expression, the implicit conversion sequence is
+a user-defined conversion sequence [[over.ics.user]], 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 8*: 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]]). — *end note*]
 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 5*: 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]]. — *end example*]
 ##### List-initialization sequence <a id="over.ics.list">[[over.ics.list]]</a>
+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 9*:
+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 6*:
+``` cpp
+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*]
+— *end note*]
+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 array [^10] 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 7*:
 ``` cpp
 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
 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:
 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 8*:
 ``` cpp
 struct A {
   A(std::initializer_list<int>);
 };
 — *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 [[dcl.init.aggr]], the implicit conversion
 sequence is a user-defined conversion sequence with the second standard
 conversion sequence an identity conversion.
+[*Example 9*:
 ``` cpp
 struct A {
   int m1;
   double m2;
 — *end example*]
 Otherwise, if the parameter is a reference, see  [[over.ics.ref]].
+[*Note 10*: The rules in this subclause will apply for initializing the
 underlying temporary for the reference. — *end note*]
+[*Example 10*:
 ``` cpp
 struct A {
   int m1;
   double m2;
 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 11*:
   ``` cpp
   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.
+  \[*Example 12*:
   ``` cpp
   void f(int);
   f( { } );               // OK: identity conversion
   ```
 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 [[over.ics.scs]] is a better conversion
+  sequence than a user-defined conversion sequence or an ellipsis
+  conversion sequence, and
+- a user-defined conversion sequence [[over.ics.user]] is a better
+  conversion sequence than an ellipsis conversion sequence
+  [[over.ics.ellipsis]].
 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 n₁ initialized by `L1` is less than the
+    number of elements n₂ initialized by `L2`, or n₁ = n₂ and `L2`
+    converts to an array of unknown bound and `L1` does not,
   even if one of the other rules in this paragraph would otherwise
   apply.
   \[*Example 1*:
   ``` cpp
   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*:
+  ``` cpp
+  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]],
     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 reference
+    \[*Example 3*:
     ``` cpp
     int i;
     int f1();
     int&& f2();
     int g(const int&);
     a.p();                          // calls A::p()&
     ```
     — *end example*]
     or, if not that,
+  - `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 lvalue
+    \[*Example 4*:
     ``` cpp
     int f(void(&)());               // #1
     int f(void(&&)());              // #2
     void g();
     int i1 = f(g);                  // calls #1
     ```
     — *end example*]
     or, if not that,
+  - `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*:
     ``` cpp
     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*:
     ``` cpp
     int f(const int &);
     int f(int &);
     int g(const int &);
     int g(int);
   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*:
   ``` cpp
   struct A {
     operator short();
   } a;
   int f(int);
 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
   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*:
     ``` cpp
     struct A {};
     struct B : public A {};
     struct C : public B {};
     C* pc;

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