[over.best.ics] - C++14 → C++17

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@@ -8,16 +8,16 @@ 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, or
-accessibility of the argument and whether or not the argument is a
-bit-field 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]]),
@@ -35,17 +35,21 @@ by
 -  [[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, and the target is the first parameter of a
- constructor of class `X`, and the conversion is to `X` or reference to
- (possibly cv-qualified) `X`,
-user-defined conversion sequences are not considered. These rules
-prevent more than one user-defined conversion from being applied during
-overload resolution, thereby avoiding infinite recursion.
 ``` cpp
   struct Y { Y(int); };
   struct A { operator int(); };
   Y y1 = A();       // error: A::operator int() is not a candidate
@@ -54,83 +58,120 @@ struct Y { Y(int); };
   struct B { operator X(); };
   B b;
   X x({b});         // error: B::operator X() is not a candidate
 ```
 For the case where the parameter type is a reference, see
 [[over.ics.ref]].
 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. 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. Any difference in top-level cv-qualification is subsumed by
-the initialization itself and does not constitute a conversion. a
-parameter of type `A` can be initialized from an argument of type
-`const A`. The implicit conversion sequence for that case is the
-identity sequence; it contains no “conversion” from `const A` to `A`.
 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.
-There is no such standard conversion; this derived-to-base Conversion
-exists only in the description of implicit conversion sequences. 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 that create no temporary object for the
-result 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 or the conversion is otherwise ill-formed, 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 sequence that is indistinguishable from any other
-user-defined conversion sequence[^11]. 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.
 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. 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.
-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. At
-most one conversion from each category is allowed in a single standard
-conversion sequence. 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**.
 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
@@ -191,70 +232,88 @@ 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]]).
 ``` cpp
 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
 ```
 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 underlying type of the reference according
-to  [[over.best.ics]]. Conceptually, this conversion sequence
-corresponds to copy-initializing a temporary of the underlying 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. 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 is a temporary or would require one 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 a standard conversion sequence, however. 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]]).
 ##### 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 `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.
 ``` 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
 f( {'a','b'} );         // OK: f(initializer_list<int>) integral promotion
@@ -273,27 +332,38 @@ g({ "foo", "bar" });        // OK, uses #3
 typedef int IA[3];
 void h(const IA&);
 h({ 1, 2, 3 });         // OK: identity conversion
 ```
-Otherwise, if the parameter type is “array of `N` `X`”[^12], if the
-initializer list has exactly `N` elements or if it has fewer than `N`
-elements and `X` is default-constructible, and if 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`.
 Otherwise, if the parameter is a non-aggregate class `X` and overload
-resolution per  [[over.match.list]] chooses a single best constructor of
-`X` to perform the initialization of an object of type `X` from the
-argument initializer list, 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]].
 ``` cpp
 struct A {
   A(std::initializer_list<int>);
 };
@@ -321,16 +391,20 @@ struct D {
 };
 void i(D);
 i({ {1,2}, {"bar"} });  // OK: i(D(A(std::initializer_list<int>{1,2\), C(std::string("bar"))))}
 ```
 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.
 ``` cpp
 struct A {
   int m1;
   double m2;
 };
@@ -338,13 +412,18 @@ struct A {
 void f(A);
 f( {'a', 'b'} );        // OK: f(A(int,double)) user-defined conversion
 f( {1.0} );             // error: narrowing
 ```
-Otherwise, if the parameter is a reference, see  [[over.ics.ref]]. The
-rules in this section will apply for initializing the underlying
-temporary for the reference.
 ``` cpp
 struct A {
   int m1;
   double m2;
@@ -356,25 +435,33 @@ f( {1.0} );                 // error: narrowing
 void g(const double &);
 g({1});                 // same conversion as int to double
 ```
 Otherwise, if the parameter type is not a class:
-- if the initializer list has one element, the implicit conversion
- sequence is the one required to convert the element to the parameter
-  type;
   ``` cpp
   void f(int);
   f( {'a'} );             // OK: same conversion as char to int
   f( {1.0} );             // error: narrowing
   ```
 - if the initializer list has no elements, the implicit conversion
   sequence is the identity conversion.
   ``` cpp
   void f(int);
   f( { } );               // OK: identity conversion
   ```
 In all cases other than those enumerated above, no conversion is
 possible.

 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]]),
 -  [[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`,
+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 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
 [[over.ics.ref]].
 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.
+[*Example 2*: A parameter of type `A` can be initialized from an
+argument of type `const A`. The implicit conversion sequence for that
+case is the identity sequence; it contains no “conversion” from
+`const A` to `A`. — *end example*]
 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*:
+``` cpp
+class B;
+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
+```
+— *end example*]
+— *end note*]
+If a function that uses the ambiguous conversion sequence is selected as
+the best viable function, the call will be ill-formed because the
+conversion of one of the arguments in the call is ambiguous.
 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
 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 {};
 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 ([[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
 f( {'a','b'} );         // OK: f(initializer_list<int>) integral promotion
 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`”, 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:
+- 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 7*:
 ``` cpp
 struct A {
   A(std::initializer_list<int>);
 };
 };
 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 ([[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;
 };
 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]].
+[*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;
 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 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
   ```
+  — *end example*]
 In all cases other than those enumerated above, no conversion is
 possible.

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