[conventions] - C++17 → C++20

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@@ -1,31 +1,63 @@
 ### Other conventions <a id="conventions">[[conventions]]</a>
 This subclause describes several editorial conventions used to describe
 the contents of the C++ standard library. These conventions are for
-describing implementation-defined types ([[type.descriptions]]), and
-member functions ([[functions.within.classes]]).
 #### Type descriptions <a id="type.descriptions">[[type.descriptions]]</a>
 ##### General <a id="type.descriptions.general">[[type.descriptions.general]]</a>
 The Requirements subclauses may describe names that are used to specify
 constraints on template arguments.[^6] These names are used in library
-Clauses to describe the types that may be supplied as arguments by a
-C++program when instantiating template components from the library.
-Certain types defined in Clause  [[input.output]] are used to describe
 implementation-defined types. They are based on other types, but with
 added constraints.
 ##### Exposition-only types <a id="expos.only.types">[[expos.only.types]]</a>
-Several types defined in Clauses  [[language.support]] through
-[[thread]] and Annex  [[depr]] that are used as function parameter or
-return types are defined for the purpose of exposition only in order to
-capture their language linkage. The declarations of such types are
 followed by a comment ending in *exposition only*.
 [*Example 1*:
 ``` cpp
@@ -39,73 +71,73 @@ function that takes a callback parameter with C language linkage.
 — *end example*]
 ##### Enumerated types <a id="enumerated.types">[[enumerated.types]]</a>
-Several types defined in Clause  [[input.output]] are *enumerated
-types*. Each enumerated type may be implemented as an enumeration or as
-a synonym for an enumeration.[^7]
 The enumerated type `enumerated` can be written:
 ``` cpp
-enum enumerated { V₀, V₁, V₂, V₃, ..... };
 inline const enumerated C₀(V₀);
 inline const enumerated C₁(V₁);
 inline const enumerated C₂(V₂);
 inline const enumerated C₃(V₃);
- .....
 ```
 Here, the names `C₀`, `C₁`, etc. represent *enumerated elements* for
 this particular enumerated type. All such elements have distinct values.
 ##### Bitmask types <a id="bitmask.types">[[bitmask.types]]</a>
-Several types defined in Clauses  [[language.support]] through
-[[thread]] and Annex  [[depr]] are *bitmask types*. Each bitmask type
-can be implemented as an enumerated type that overloads certain
-operators, as an integer type, or as a `bitset` ([[template.bitset]]).
-The bitmask type *bitmask* can be written:
 ``` cpp
 // For exposition only.
 // int_type is an integral type capable of representing all values of the bitmask type.
 enum bitmask : int_type {
-  V₀ = 1 << 0, V₁ = 1 << 1, V₂ = 1 << 2, V₃ = 1 << 3, .....
 };
 inline constexpr bitmask C₀(V₀{});
 inline constexpr bitmask C₁(V₁{});
 inline constexpr bitmask C₂(V₂{});
 inline constexpr bitmask C₃(V₃{});
- .....
-constexpr bitmask{} operator&(bitmask{} X, bitmask{} Y) {
-  return static_cast<bitmask{}>(
     static_cast<int_type>(X) & static_cast<int_type>(Y));
 }
-constexpr bitmask{} operator|(bitmask{} X, bitmask{} Y) {
-  return static_cast<bitmask{}>(
     static_cast<int_type>(X) | static_cast<int_type>(Y));
 }
-constexpr bitmask{} operator^(bitmask{} X, bitmask{} Y){
-  return static_cast<bitmask{}>(
     static_cast<int_type>(X) ^ static_cast<int_type>(Y));
 }
-constexpr bitmask{} operator~(bitmask{} X){
-  return static_cast<bitmask{}>(~static_cast<int_type>(X));
 }
-bitmask{}& operator&=(bitmask{}& X, bitmask{} Y){
   X = X & Y; return X;
 }
-bitmask{}& operator|=(bitmask{}& X, bitmask{} Y) {
   X = X | Y; return X;
 }
-bitmask{}& operator^=(bitmask{}& X, bitmask{} Y) {
   X = X ^ Y; return X;
 }
 ```
 Here, the names `C₀`, `C₁`, etc. represent *bitmask elements* for this
@@ -132,73 +164,99 @@ sequences that follow a few uniform conventions:
   basic execution character set.
 - The *decimal-point character* is the (single-byte) character used by
   functions that convert between a (single-byte) character sequence and
   a value of one of the floating-point types. It is used in the
   character sequence to denote the beginning of a fractional part. It is
-  represented in Clauses  [[language.support]] through  [[thread]] and
- Annex  [[depr]] by a period, `'.'`, which is also its value in the
- `"C"` locale, but may change during program execution by a call to
   `setlocale(int, const char*)`,[^8] or by a change to a `locale`
-  object, as described in Clauses  [[locales]] and  [[input.output]].
-- A *character sequence* is an array object ([[dcl.array]]) `A` that
- can be declared as `T A[N]`, where `T` is any of the types `char`,
-  `unsigned char`, or `signed char` ([[basic.fundamental]]), optionally
   qualified by any combination of `const` or `volatile`. The initial
   elements of the array have defined contents up to and including an
   element determined by some predicate. A character sequence can be
   designated by a pointer value `S` that points to its first element.
 ###### Byte strings <a id="byte.strings">[[byte.strings]]</a>
 A *null-terminated byte string*, or NTBS, is a character sequence whose
 highest-addressed element with defined content has the value zero (the
-*terminating null* character); no other element in the sequence has the
 value zero. [^9]
-The *length* of an NTBS is the number of elements that precede the
-terminating null character. An *empty* NTBS has a length of zero.
-The *value* of an NTBS is the sequence of values of the elements up to
 and including the terminating null character.
-A *static* NTBS is an NTBSwith static storage duration.[^10]
 ###### Multibyte strings <a id="multibyte.strings">[[multibyte.strings]]</a>
 A *null-terminated multibyte string*, or NTMBS, is an NTBS that
 constitutes a sequence of valid multibyte characters, beginning and
 ending in the initial shift state.[^11]
-A *static* NTMBS is an NTMBSwith static storage duration.
 #### Functions within classes <a id="functions.within.classes">[[functions.within.classes]]</a>
-For the sake of exposition, Clauses  [[language.support]] through
-[[thread]] and Annex  [[depr]] do not describe copy/move constructors,
-assignment operators, or (non-virtual) destructors with the same
-apparent semantics as those that can be generated by default (
-[[class.ctor]], [[class.dtor]], [[class.copy]]). It is unspecified
-whether the implementation provides explicit definitions for such member
-function signatures, or for virtual destructors that can be generated by
-default.
-For the sake of exposition, the library clauses sometimes annotate
-constructors with \EXPLICIT. Such a constructor is conditionally
-declared as either explicit or non-explicit ([[class.conv.ctor]]).
-[*Note 1*: This is typically implemented by declaring two such
-constructors, of which at most one participates in overload
-resolution. — *end note*]
 #### Private members <a id="objects.within.classes">[[objects.within.classes]]</a>
-Clauses  [[language.support]] through  [[thread]] and Annex  [[depr]] do
-not specify the representation of classes, and intentionally omit
-specification of class members ([[class.mem]]). An implementation may
-define static or non-static class members, or both, as needed to
-implement the semantics of the member functions specified in Clauses
-[[language.support]] through  [[thread]] and Annex  [[depr]].
 For the sake of exposition, some subclauses provide representative
 declarations, and semantic requirements, for private members of classes
 that meet the external specifications of the classes. The declarations
 for such members are followed by a comment that ends with *exposition

 ### Other conventions <a id="conventions">[[conventions]]</a>
 This subclause describes several editorial conventions used to describe
 the contents of the C++ standard library. These conventions are for
+describing implementation-defined types [[type.descriptions]], and
+member functions [[functions.within.classes]].
+#### Exposition-only functions <a id="expos.only.func">[[expos.only.func]]</a>
+Several function templates defined in [[support]] through [[thread]] and
+[[depr]] are only defined for the purpose of exposition. The declaration
+of such a function is followed by a comment ending in *exposition only*.
+The following are defined for exposition only to aid in the
+specification of the library:
+``` cpp
+template<class T> constexpr decay_t<T> decay-copy(T&& v)
+    noexcept(is_nothrow_convertible_v<T, decay_t<T>>)           // exposition only
+  { return std::forward<T>(v); }
+constexpr auto synth-three-way =
+  []<class T, class U>(const T& t, const U& u)
+    requires requires {
+      { t < u } -> boolean-testable;
+      { u < t } -> boolean-testable;
+    }
+  {
+    if constexpr (three_way_comparable_with<T, U>) {
+      return t <=> u;
+    } else {
+      if (t < u) return weak_ordering::less;
+      if (u < t) return weak_ordering::greater;
+      return weak_ordering::equivalent;
+    }
+  };
+template<class T, class U=T>
+using synth-three-way-result = decltype(synth-three-way(declval<T&>(), declval<U&>()));
+```
 #### Type descriptions <a id="type.descriptions">[[type.descriptions]]</a>
 ##### General <a id="type.descriptions.general">[[type.descriptions.general]]</a>
 The Requirements subclauses may describe names that are used to specify
 constraints on template arguments.[^6] These names are used in library
+Clauses to describe the types that may be supplied as arguments by a C++
+program when instantiating template components from the library.
+Certain types defined in [[input.output]] are used to describe
 implementation-defined types. They are based on other types, but with
 added constraints.
 ##### Exposition-only types <a id="expos.only.types">[[expos.only.types]]</a>
+Several types defined in [[support]] through [[thread]] and [[depr]] are
+defined for the purpose of exposition. The declaration of such a type is
 followed by a comment ending in *exposition only*.
 [*Example 1*:
 ``` cpp
 — *end example*]
 ##### Enumerated types <a id="enumerated.types">[[enumerated.types]]</a>
+Several types defined in [[input.output]] are *enumerated types*. Each
+enumerated type may be implemented as an enumeration or as a synonym for
+an enumeration.[^7]
 The enumerated type `enumerated` can be written:
 ``` cpp
+enum enumerated { V₀, V₁, V₂, V₃, … };
 inline const enumerated C₀(V₀);
 inline const enumerated C₁(V₁);
 inline const enumerated C₂(V₂);
 inline const enumerated C₃(V₃);
+ ⋮
 ```
 Here, the names `C₀`, `C₁`, etc. represent *enumerated elements* for
 this particular enumerated type. All such elements have distinct values.
 ##### Bitmask types <a id="bitmask.types">[[bitmask.types]]</a>
+Several types defined in [[support]] through [[thread]] and [[depr]] are
+*bitmask types*. Each bitmask type can be implemented as an enumerated
+type that overloads certain operators, as an integer type, or as a
+`bitset` [[template.bitset]].
+The bitmask type `bitmask` can be written:
 ``` cpp
 // For exposition only.
 // int_type is an integral type capable of representing all values of the bitmask type.
 enum bitmask : int_type {
+  V₀ = 1 << 0, V₁ = 1 << 1, V₂ = 1 << 2, V₃ = 1 << 3, …
 };
 inline constexpr bitmask C₀(V₀{});
 inline constexpr bitmask C₁(V₁{});
 inline constexpr bitmask C₂(V₂{});
 inline constexpr bitmask C₃(V₃{});
+ ⋮
+constexpr bitmask operator&(bitmask X, bitmask Y) {
+  return static_cast<bitmask>(
     static_cast<int_type>(X) & static_cast<int_type>(Y));
 }
+constexpr bitmask operator|(bitmask X, bitmask Y) {
+  return static_cast<bitmask>(
     static_cast<int_type>(X) | static_cast<int_type>(Y));
 }
+constexpr bitmask operator^(bitmask X, bitmask Y){
+  return static_cast<bitmask>(
     static_cast<int_type>(X) ^ static_cast<int_type>(Y));
 }
+constexpr bitmask operator~(bitmask X){
+  return static_cast<bitmask>(~static_cast<int_type>(X));
 }
+bitmask& operator&=(bitmask& X, bitmask Y){
   X = X & Y; return X;
 }
+bitmask& operator|=(bitmask& X, bitmask Y) {
   X = X | Y; return X;
 }
+bitmask& operator^=(bitmask& X, bitmask Y) {
   X = X ^ Y; return X;
 }
 ```
 Here, the names `C₀`, `C₁`, etc. represent *bitmask elements* for this
   basic execution character set.
 - The *decimal-point character* is the (single-byte) character used by
   functions that convert between a (single-byte) character sequence and
   a value of one of the floating-point types. It is used in the
   character sequence to denote the beginning of a fractional part. It is
+  represented in [[support]] through [[thread]] and [[depr]] by a
+  period, `'.'`, which is also its value in the `"C"` locale, but may
+  change during program execution by a call to
   `setlocale(int, const char*)`,[^8] or by a change to a `locale`
+  object, as described in [[locales]] and [[input.output]].
+- A *character sequence* is an array object [[dcl.array]] `A` that can
+  be declared as `T A[N]`, where `T` is any of the types `char`,
+  `unsigned char`, or `signed char` [[basic.fundamental]], optionally
   qualified by any combination of `const` or `volatile`. The initial
   elements of the array have defined contents up to and including an
   element determined by some predicate. A character sequence can be
   designated by a pointer value `S` that points to its first element.
 ###### Byte strings <a id="byte.strings">[[byte.strings]]</a>
 A *null-terminated byte string*, or NTBS, is a character sequence whose
 highest-addressed element with defined content has the value zero (the
+*terminating null character*); no other element in the sequence has the
 value zero. [^9]
+The *length of an NTBS* is the number of elements that precede the
+terminating null character. An *empty NTBS* has a length of zero.
+The *value of an NTBS* is the sequence of values of the elements up to
 and including the terminating null character.
+A *static NTBS* is an NTBS with static storage duration.[^10]
 ###### Multibyte strings <a id="multibyte.strings">[[multibyte.strings]]</a>
 A *null-terminated multibyte string*, or NTMBS, is an NTBS that
 constitutes a sequence of valid multibyte characters, beginning and
 ending in the initial shift state.[^11]
+A *static NTMBS* is an NTMBS with static storage duration.
+##### Customization Point Object types <a id="customization.point.object">[[customization.point.object]]</a>
+A *customization point object* is a function object [[function.objects]]
+with a literal class type that interacts with program-defined types
+while enforcing semantic requirements on that interaction.
+The type of a customization point object, ignoring cv-qualifiers, shall
+model `semiregular` [[concepts.object]].
+All instances of a specific customization point object type shall be
+equal [[concepts.equality]].
+The type `T` of a customization point object shall model
+`invocable<const T&, Args...>` [[concept.invocable]] when the types in
+`Args...` meet the requirements specified in that customization point
+object’s definition. When the types of `Args...` do not meet the
+customization point object’s requirements, `T` shall not have a function
+call operator that participates in overload resolution.
+Each customization point object type constrains its return type to model
+a particular concept.
+[*Note 1*: Many of the customization point objects in the library
+evaluate function call expressions with an unqualified name which
+results in a call to a program-defined function found by argument
+dependent name lookup [[basic.lookup.argdep]]. To preclude such an
+expression resulting in a call to unconstrained functions with the same
+name in namespace `std`, customization point objects specify that lookup
+for these expressions is performed in a context that includes deleted
+overloads matching the signatures of overloads defined in namespace
+`std`. When the deleted overloads are viable, program-defined overloads
+need be more specialized [[temp.func.order]] or more constrained
+[[temp.constr.order]] to be used by a customization point
+object. — *end note*]
 #### Functions within classes <a id="functions.within.classes">[[functions.within.classes]]</a>
+For the sake of exposition, [[support]] through [[thread]] and [[depr]]
+do not describe copy/move constructors, assignment operators, or
+(non-virtual) destructors with the same apparent semantics as those that
+can be generated by default ([[class.copy.ctor]],
+[[class.copy.assign]], [[class.dtor]]). It is unspecified whether the
+implementation provides explicit definitions for such member function
+signatures, or for virtual destructors that can be generated by default.
 #### Private members <a id="objects.within.classes">[[objects.within.classes]]</a>
+[[support]] through [[thread]] and [[depr]] do not specify the
+representation of classes, and intentionally omit specification of class
+members [[class.mem]]. An implementation may define static or non-static
+class members, or both, as needed to implement the semantics of the
+member functions specified in [[support]] through [[thread]] and
+[[depr]].
 For the sake of exposition, some subclauses provide representative
 declarations, and semantic requirements, for private members of classes
 that meet the external specifications of the classes. The declarations
 for such members are followed by a comment that ends with *exposition

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