Yet another approach for constrained declarations

Abstract

We propose a short syntax for the constrained declaration of function parameters, function return types and variables. The new syntax is a “constrained auto”, e.g. void sort(Sortable auto& c);.

Contents

1. Revision history
2. Proposal summary
3. Proposal details
   • Part 1: “Constrained auto”
   • Part 2: Relaxed “constrained auto” [not proposed]
   • Part 3: Meaning of “template <Concept T>”
   • Part 4: Meaning of “template <Concept... T>” and its friends
   • Part 5: Meaning of “-> Concept auto” and its friends [not proposed]
4. Proposed wording for Parts 1, 3, and 4
   • Changes in [expr]
   • Changes in [dcl]
   • Changes in [temp]

Revision history

• P1141R2 (this document): Added formal wording for parts 1, 3, and 4. Parts 2 and 5 are not being proposed.
• P1141R1: Added discussion in Parts 2 and 5 on return-type-requirements and in Part 4 on variadic concepts.
• P1141R0: Initial proposal.

Proposal summary

This paper proposes three things:

1. A syntax for constrained declarations that is practically a “constrained auto”; the principle being “wherever auto goes, a Constraint auto can also (non-recursively) go”. The semantics are to deduce like auto and additionally check a constraint. In a nutshell,

   void f(Sortable auto x);
   Sortable auto f();      // #1
   Sortable auto x = f();  // #2
   template <Sortable auto N> void f();

   and all combined:

   template <Sortable auto N> Sortable auto f(Sortable auto x)
   {
       Sortable auto y = init;
   }

   An unconstrained version of that is:

   template <auto N> auto f(auto x)
   {
       auto y = init;
   }

   So, this proposal includes auto-typed parameters for functions, which we already allow for lambdas.
2. Simplifying (and thus restricting) the rules in [temp.param]/10, so that template <Sortable S> always means that S is a type parameter, and template <Sortable auto S> always means that S is a non-type parameter. Template template-parameters are no longer supported in this short form. Moreover, Sortable is restricted to be a concept that takes a type parameter or type parameter pack; non-type and template concepts are no longer supported in this short form.
3. Changing the meaning of parameter packs, so that template <Sortable ...T> means requires Sortable<T> && ... && true, and not requires Sortable<T...>.

Sortable is a “type concept” in all the examples of this summary.

This paper specifically does not propose

• any new lead-in syntax for templates, or
• a new syntax for introducing names for placeholder types, or
• a shortcut syntax for applying multiple constraints to a placeholder type.

The idea of this approach is to provide a syntax that

• works for constrained function parameters, constrained return types, constrained variables, and type-constrained non-type template parameters;
• avoids inventing many adventurous new things;
• in particular, avoids inventing new type sigils;
• does not clash with explicit template instantiations; and
• is compatible with what we already have in polymorphic lambdas, and makes functions uniform with them.

The previous revision of this paper (P1141R1) also proposed (in Part 2) an optional relaxation where the auto would be optional for the cases #1 and #2 illustrated above, and (in Part 5) a change of the meaning of -> Concept auto. However, EWG decided to propose only parts 1, 3, and 4.

Proposal details

Part 1: “Constrained auto”

The approach proposed here borrows a subset of P0807R0 An Adjective Syntax for Concepts. The idea is that we don’t try to come up with a notation that does everything that P0807 does; in particular, there is no proposal for a new syntax to introduce a type name.

Function templates

The approach is simple: allow auto parameters to produce function templates (as they produce polymorphic lambdas), and allow the auto to be preceded by a concept name. In every case, such a parameter is a deduced parameter, and we can see which parameters are deduced and which ones are not:

[](auto a, auto& b, const auto& c, auto&& d) {...}; // unconstrained
[](Constraint auto a, Constraint auto& b, const Constraint auto& c, Constraint auto&& d) {...}; // constrained

void f1(auto a, auto& b, const auto& c, auto&& d) {...}; // unconstrained
void f2(Constraint auto a, Constraint auto& b, const Constraint auto& c, Constraint auto&& d) {...}; // constrained

[](Constraint auto&& a, SomethingElse&& b) {...}; // a constrained deduced forwarding reference and a concrete rvalue reference
void f3(Constraint auto&& a, SomethingElse&& b) {...}; // a constrained deduced forwarding reference and a concrete rvalue reference

The appearance of auto (including Constraint auto) in a parameter list tells us that we are dealing with a function template. For each parameter, we know whether it is deduced or not. We can tell apart concepts from types: concepts precede auto, types do not.

Return types and variable declarations

Constrained return types work the same way:

auto f4();                  // unconstrained, deduced.

Constraint auto f5();       // constrained, deduced.

Whatever f6();              // See part 2. If Whatever is a type, not deduced.
                            // If Whatever is a concept, constrained and deduced.

Note that f4, f5 and f6 are not templates (whereas the previous f1, f2 and f3 are templates). Here, there is no mention of auto in the parameter list. Users have the choice of adopting a style where it is explicit as to whether the return type is deduced.

Constrained types for variables work the same way:

auto x1 = f1();             // unconstrained, deduced.

Constraint auto x2 = f2();  // constrained, deduced.

Whatever x3 = f3();         // See part 2. If Whatever is a type, not deduced.
                            // If Whatever is a concept, constrained and deduced.

Again, users can make it so that it is easy to see when deduction occurs.

Since non-type template parameters can be deduced via auto (as in template <auto N> void f();), we also allow a constraint there:

template <Constraint auto N> void f7();

Note, however, that this can only be a type constraint; non-type concepts (including auto concepts) are not allowed in this form.

Other uses of auto

In concert with the general approach that “Constraint auto goes wherever auto goes”, new-expressions and conversion operators work:

auto alloc_next() { return new Sortable auto(this->next_val()); }

operator Sortable auto() { }

A “Constraint auto” cannot be used to indicate that a function declarator has a trailing return type:

Constraint auto f() -> auto; // ill-formed; shall be the single type-specifier auto

decltype(auto) can also be constrained:

auto f() -> Constraint decltype(auto);
Constraint decltype(auto) x = f();

Structured bindings do deduce auto in some cases; however, the auto is deduced from the whole (and not from the individual components). It is somewhat doubtful that applying the constraint to the whole, as opposed to (for example) applying separately to each component, is the correct semantic. Therefore, we propose to defer enabling the application of constraints to structured bindings to separate papers.

General rules

The constraint applies directly to the deduced type. It does not apply to the possibly cv-qualified type described by the type specifiers, nor does it apply to the type declared for the variable:

const Assignable<int> auto&& c = *static_cast<int *>(p); // Assignable<int &, int>

Naturally, if the deduced type is cv-qualified (or a reference), the constraint applies to that type.

To keep things simple, an auto (or decltype(auto)) being constrained is always immediately preceded by the constraint. So, cv-qualifiers and concept-identifiers cannot be freely mixed:

const Contraint auto x = foo(); // ok
Constraint const auto x = foo(); // ill-formed
Constraint auto const y = foo(); // ok

We propose only the ability to apply one single constraint for a parameter, return type, or non-type template parameter. Any proposal to consider multiple constraints should happen separately after C++20.

Partial concept identifiers also work. Given a concept template <typename T, typename... Args> concept Constructible = /* ... */;, we can say:

void f(Constructible<int> auto x);   // Constructible<decltype(x), int> is satisfied

Constructible<int> auto f();

Constructible<int> auto x = f();

template <Constructible<int> auto N> void f();

Part 2: Relaxed “constrained auto” [not proposed]

Part 3: Meaning of “template <Concept T>”

In [temp.param]/10 we have:

A constrained-parameter declares a template parameter whose kind (type, non-type, template) and type match that of the prototype parameter (17.6.8) of the concept designated by the type-constraint in the constrained-parameter. Let X be the prototype parameter of the designated concept. The declared template parameter is determined by the kind of X (type, non-type, template) and the optional ellipsis in the constrained-parameter as follows.

• If X is a type template-parameter, the declared parameter is a type template-parameter.
• If X is a non-type template-parameter, the declared parameter is a non-type template-parameter having the same type as X.
• If X is a template template-parameter, the declared parameter is a template template-parameter having the same template-parameter-list as X, excluding default template arguments.
• If the type-constraint is followed by an ellipsis, then the declared parameter is a template parameter pack (17.6.3).

[Example:

template<typename T> concept C1 = true;
template<template<typename> class X> concept C2 = true;
template<int N> concept C3 = true;
template<typename... Ts> concept C4 = true;
template<char... Cs> concept C5 = true;

template<C1 T> void f1();       // OK, T is a type template-parameter
template<C2 X> void f2();       // OK, X is a template with one type-parameter
template<C3 N> void f3();       // OK, N has type int
template<C4... Ts> void f4();   // OK, Ts is a template parameter pack of types
template<C4 T> void f5();       // OK, T is a type template-parameter
template<C5... Cs> void f6();   // OK, Cs is a template parameter pack of chars

—end example]

Does that seem like a mouthful?

That’s because it is. In template <Constraint T>, the kind of T depends on the kind of the prototype parameter of Constraint.

We instead propose that, for such a constrained-parameter syntax:

• T should always be a type, and
• Constraint would always need to be a concept that has a corresponding type parameter or type parameter pack.

To be clear, we are not proposing that concepts in general should not have non-type or template template parameters. We are merely proposing for it to be the case that the constrained parameter shortcut is not provided for concepts with such prototype parameters; such concepts would need to be used with a requires-clause. The constrained parameter syntax should mean just one thing. Note that the same syntax template <A T> is still a non-type parameter when A is a type name rather than a concept. We are willing to tolerate this small potential for ambiguity.

The rationale for this part is as follows:

1. It seems desirable to have the constrained template parameter syntax.
2. It would be nice if that syntax covered the most common case.
3. It would further be nice if that syntax covered only the most common case.
4. The other cases are expected to be so rare that there’s no need to provide a shortcut for them, and they are certainly rare enough that they shouldn’t use the same syntax.

So, to clarify:

• template <MyIntTypeDef N> means a non-type parameter, like it always did.
• template <ConceptName T> means a type parameter constrained by ConceptName, and the prototype parameter of ConceptName needs to be a type parameter or a type parameter pack.
• template <auto N> means a non-type parameter with a deduced type.
• template <ConceptName auto N> means a non-type parameter with a deduced type constrained by ConceptName, and the prototype parameter of ConceptName needs to be a type parameter or a type parameter pack.

Other use cases can be done with requires-clauses.

Part 4: Meaning of “template <Concept... T>” and its friends

In [temp.param]/11 we have:

template<C2... T> struct s3; // associates C2<T...>

This seems to be doing an unexpected thing, which is having the constraint apply to more than one type in a pack at a time. We propose that, regardless of whether the prototype parameter of the named concept is a pack:

• For a simple pack of constrained types, the concept mentioned is applied, as a unary concept, to each type in the pack in turn.
• For a pack of constrained types that use partial-concept-ids, the concept mentioned is applied, as an n-ary concept whose arity is unaffected by the size of the pack, individually to each type in the pack in turn.

In other words,

• template <ConceptName... T> void f(T...); means a variadic function template where each type in the pack T needs to satisfy ConceptName as a unary concept, applied as ConceptName<Tn>.
• Similarly, void f(ConceptName auto... T); means exactly the same thing.
• template <ConceptName<int>... U> void f(U...); means a variadic function template where each type in the pack U needs to satisfy ConceptName as a binary concept, applied as ConceptName<Un, int>.
• Similarly, void f(ConceptName<int> auto... U); means exactly the same thing.
• template <ConceptName<0u, void, wchar_t>... U> void f(U...); means a variadic function template where each type in the pack U needs to satisfy ConceptName as a n-ary concept, applied as ConceptName<Un, 0u, void, wchar_t>.

Part 5: Meaning of “-> Concept auto” and its friends [not proposed]

Proposed wording for Parts 1, 3, and 4

Changes in [expr]

Update [expr.prim.lambda, 7.5.5], paragraph 5, to allow placeholder type specifiers as lambda parameters.

A lambda is a generic lambda if the auto type-specifier appears as one of the decl-specifiersthere is a decl-specifier that is a placeholder-type-specifier in the decl-specifier-seq of a parameter-declaration of the lambda-expression, or if the lambda has a template-parameter-list. [Example: […] —end example]

In [expr.prim.lambda.closure, 7.5.5.1], modify paragraph 3.

The closure type for a non-generic lambda-expression has a public inline function call operator (for a non-generic lambda) or function call operator template (for a generic lambda) (11.5.4) whose parameters and return type are described by the lambda-expression’s parameter-declaration-clause and trailing-return-type respectively. For a generic lambda, the closure type has a public inline function call operator member template (12.6.2) , and whose template-parameter-list consists of the specified template-parameter-list, if any, to which is appended one invented type template-parameter for each occurrence of auto in the lambda’s parameter-declaration-clause, in order of appearance. The invented type template-parameter is a template parameter pack if the corresponding parameter-declaration declares a function parameter pack (9.2.3.5). The return type and function parameters of the function call operator template are derived from the lambda-expression’s trailing-return-type and parameter-declaration-clause by replacing each occurrence of auto in the decl-specifiers of the parameter-declaration-clause with the name of the corresponding invented template-parameter. The requires-clause of the function call operator template is the requires-clause immediately following < template-parameter-list >, if any. The trailing requires-clause of the function call operator or operator template is the requires-clause following the lambda-declarator, if any. [Note: The function call operator for a generic lambda might be an abbreviated function template (9.2.3.5). —end note] [Example: […] —end example]

Modify paragraph 6 as follows.

[Note: The function call operator or operator template may be constrained (12.4.2) by a constrained-parametertype-constraint (12.1), a requires-clause (Clause 12), or a trailing requires-clause (9.2). [Example:

template <typename T> concept C1 = /* ... */;
template <std::size_t N> concept C2 = /* ... */;
template <typename A, typename B> concept C3 = /* ... */;

auto f = []<typename T1, C1 T2> requires C2<sizeof(T1) + sizeof(T2)>
         (T1 a1, T1 b1, T2 a2, auto a3, auto a4) requires C3<decltype(a4), T2> {
  // T2 is a constrained parameterconstrained by a type-constraint,
  // T1 and T2 are constrained by a requires-clause, and
  // T2 and the type of a4 are constrained by a trailing requires-clause.
};

—end example] —end note]

Changes in [dcl]

Change [dcl.type.simple, 9.1.7.2] paragraph 1 to add placeholder-type-specifiers.

simple-type-specifier:
        nested-name-specifieropt type-name
        nested-name-specifier template simple-template-id
        nested-name-specifieropt template-name
        char
        char16_t
        char32_t
        wchar_t
        bool
        short
        int
        long
        signed
        unsigned
        float
        double
        void
        auto
        decltype-specifier
        placeholder-type-specifier

type-name:
        class-name
        enum-name
        typedef-name
        simple-template-id

decltype-specifier:
        decltype ( expression )
        decltype ( auto )

placeholder-type-specifier:
        type-constraintopt auto
        type-constraintopt decltype ( auto )

Modify paragraph 2 as follows.

The simple-type-specifier autoA placeholder-type-specifier is a placeholder for a type to be deduced (9.1.7.4).

Add placeholder-type-specifiers to the table of simple-type-specifiers and their meaning.

Specifier(s)	Type
type-name	the type named
simple-template-id	the as defined in 12.2
...	...
void	“void”
auto	placeholder for a type to be deduced
decltype(auto)	placeholder for a type to be deduced
decltype(expression)	the type as described below
placeholder-type-specifier	placeholder for a type to be deduced

In [dcl.spec.auto, 9.1.7.4], modify and split paragraph 1 as follows.

The auto and decltype(auto) type-specifiers are used toA placeholder-type-specifier designates a placeholder type that will be replaced later by deduction from an initializer.

A placeholder-type-specifier of the form type-constraint_opt auto can be used in the decl-specifier-seq of a parameter-declaration of a function declaration or lambda-expression and signifies that the function is an abbreviated function template (9.2.3.5) or theThe auto type-specifier is also used to introduce a function type having a trailing-return-type or to signify that a lambda is a generic lambda (7.5.5). The auto type-specifier is also used to introduce a structured binding declaration (9.5).

Modify (old) paragraph 3 as follows.

The type of a variable declared using auto or decltype(auto)a placeholder type is deduced from its initializer. This use is allowed in an initializing declaration (9.3) of a variable. auto or decltype(auto)The placeholder type shall appear as one of the decl-specifiers in the decl-specifier-seq and the decl-specifier-seq shall be followed by one or more declarators, each of which shall be followed by a non-empty initializer. […]—end example] The auto type-specifier can also be used to introduce a structured binding declaration (9.5).

Modify (old) paragraph 5 as follows.

A program that uses auto or decltype(auto)a placeholder type in a context not explicitly allowed in this subclause is ill-formed.

In [dcl.type.auto.deduct, 9.1.7.4.1], modify the last sentence of paragraph 2 as follows.

[…] In the case of a return statement with no operand or with an operand of type void, T shall be either type-constraint_opt decltype(auto)or cv type-constraint_opt auto.

Modify paragraph 4 as follows.

If the placeholder is the auto type-specifierplaceholder-type-specifier is of the form type-constraint_opt auto, the deduced type T′ replacing T is determined using the rules for template argument deduction. Obtain P from T by replacing the occurrences of type-constraint_opt auto with either a new invented type template parameter U or, if the initialization is copy-list-initialization, with std::initiali­zer_list<U>. […]

Modify paragraph 5 as follows.

If the placeholder is the decltype(auto) type-specifierplaceholder-type-specifier is of the form type-constraint_opt decltype(auto), T shall be the placeholder alone. The type deduced for T is deter­mined […]

Append a new paragraph as follows.

?. For a placeholder-type-specifier with a type-constraint, if the type deduced for the placeholder does not satisfy its immediately-declared constraint ([temp, 12]), the program is ill-formed.

Add the following paragraphs to [dcl.fct, 9.2.3.5], after paragraph 16.

?. An abbreviated function template is a function declaration whose parameter-type-list includes one or more placeholders (9.1.7.4). An abbreviated function template is equivalent to a function template (17.6.5) whose template-parameter-list includes one invented type template-parameter for each occurrence of a placeholder type in the decl-specifier-seq of a parameter-declaration in the function’s parameter-type-list, in order of appearance. For a placeholder-type-specifier of the form auto, the invented parameter is an unconstrained type-parameter. For a placeholder-type-specifier of the form type-constraint auto, the invented parameter is a type-parameter with that type-constraint. The invented type template-parameter is a template parameter pack if the corresponding parameter-declaration declares a function parameter pack (9.2.3.5). If the placeholder contains decltype(auto), the program is ill-formed. The adjusted function parameters of an abbreviated function template are derived from the parameter-declaration-clause by replacing each occurrence of a placeholder with the name of the corresponding invented template-parameter.

[Example:

template<typename T>     concept C1 = /* ... */;
template<typename T>     concept C2 = /* ... */;
template<typename... Ts> concept C4 = /* ... */;

void g1(const C1 auto*, C2 auto&);
void g3(C1 auto&...);
void g5(C4 auto...);
void g7(C4 auto);

These declarations are functionally equivalent (but not equivalent) to the following declarations.

template<C1 T, C2 U> void g1(const T*, U&);
template<C1... Ts>   void g3(Ts&...);
template<C4... Ts>   void g5(Ts...);
template<C4 T>       void g7(T);

Abbreviated function templates can be specialized like all function templates.

template<> void g1<int>(const int*, const double&);   // OK, specialization of g1<int, const double>

—end example]

?. An abbreviated function template can have a template-head. The invented template-parameters are appended to the template-parameter-list after the explicitly declared template-parameters.

[Example:

template<typename> concept C = /* ... */;

template <typename T, C U>
  void g(T x, U y, C auto z);

This is functionally equivalent to each of the following two declarations.

template<typename T, C U, C W>
  void g(T x, U y, W z);

template<typename T, typename U, typename W>
  requires C<U> && C<W>
  void g(T x, U y, W z);

—end example]

?. A function declaration at block scope shall not declare an abbreviated function template.

Changes in [temp]

Add to the grammar in [temp, 12] the following.

concept-name:
      identifier

type-constraint:
      nested-name-specifieropt concept-name
      nested-name-specifieropt concept-name < template-argument-listopt >

Append a new paragraph as follows.

?. A type-constraint Q that designates a concept C can be used to constrain a contextually-determined type or template type parameter pack T with a constraint-expression E defined as follows. If Q is of the form C<A1, ..., An>, then let E′ be C<T, A1, ..., An>. Otherwise, let E′ be C<T>. If T is not a pack, then E is E′, otherwise E is (E′ && ...). This is called the immediately-declared constraint of T. The concept designated by a type-constraint shall be a type concept (12.6.8).

Change [temp.param, 12.1] paragraph 1 to remove the grammar for constrained-parameter and to enhance the grammar of type-parameter.

Editorial note: No further appearances of “qualified-concept-name” should remain in the working draft after application of P1084R2 and P1141R2 (this paper).

template-parameter:
      type-parameter
      parameter-declaration
      constrained-parameter

type-parameter:
      type-parameter-key ...opt identifieropt
      type-parameter-key identifieropt = type-id
      type-constraint ...opt identifieropt
      type-constraint identifieropt = type-id
      template-head type-parameter-key ...opt identifieropt
      template-head type-parameter-key identifieropt = id-expression

type-parameter-key:
      class
      typename

constrained-parameter:
      qualified-concept-name ... identifieropt
      qualified-concept-name identifieropt default-template-argumentopt

qualified-concept-name:
      nested-name-specifieropt concept-name nested-name-specifieroptpartial-concept-id

partial-concept-id:
      concept-name < template-argument-listopt >

Change [12.1, temp.param] paragraph 9 as follows.

A partial-concept-id is a concept-name followed by a sequence of template-arguments. These template arguments are used to form a constraint-expression as described below.A type-parameter that starts with a type-constraint introduces the immediately-declared constraint of the parameter.

Delete [12.1, temp.param] paragraph 10.

A constrained-parameter declares a template parameter whose kind (type, non-type, template) and type match that of the prototype parameter (12.6.8) of the concept designated by the qualified-concept-name in the constrained-parameter. Let X be the prototype parameter of the designated concept. The declared template parameter is determined by the kind of X (type, non-type, template) and the optional ellipsis in the constrained-parameter as follows.

• If X is a type template-parameter, the declared parameter is a type template-parameter.
• If X is a non-type template-parameter, the declared parameter is a non-type template-parameter having the same type as X.
• If X is a template template-parameter, the declared parameter is a template template-parameter having the same template-parameter-list as X, excluding default template arguments.
• If the qualified-concept-name is followed by an ellipsis, then the declared parameter is a template parameter pack (temp.variadic, 12.6.3).

[Example:

template<typename T> concept C1 = true;
template<template<typename> class X> concept C2 = true;
template<int N> concept C3 = true;
template<typename... Ts> concept C4 = true;
template<char... Cs> concept C5 = true;

template<C1 T> void f1();           // OK, T is a type template-parameter
template<C2 X> void f2();           // OK, X is a template with one type-parameter
template<C3 N> void f3();           // OK, N has type int
template<C4... Ts> void f4();       // OK, Ts is a template parameter pack of types
template<C4 T> void f5();           // OK, T is a type template-parameter
template<C5... Cs> void f6();       // OK, Cs is a template parameter pack of chars

—end example]

In [12.1, temp.param], delete the normative wording of (old) paragraph 11 and merge the (modified) example into paragraph 9 as follows.

Editorial note: This change effects the design change of Part 4 (changing the meaning of ...). The new pack expansion behaviour is subsumed by the “immediately-declared constraint” facility.

A constrained-parameter constraint-expression. The expression is derived from the qualified-concept-name Q in the constrained-parameter, its designated concept C, and the declared template parameter P.

• First, a template argument A is invented from P. If P declares a template parameter pack ([temp.variadic]) and C is a variadic concept ([temp.concept]), then A is the pack expansion P.... Otherwise, A is the id-expression P.
• Then, an id-expression E is formed as follows. If Q is a concept-name, then E is C<A>. Otherwise, Q is a partial-concept-id of the form C<A1, A2, ..., An>, and E is C<A, A1, A2, ..., An>.
• Finally, if P declares a template parameter pack and C is not a variadic concept, E is adjusted to be the fold-expression (E && ...) (7.5.6).

E is the introduced constraint-expression.

[Example:

template<typename T> concept C1 = true;
template<typename... Ts> concept C2 = true;
template<typename T, typename U> concept C3 = true;

template<C1 T> struct s1;          // associates C1<T>
template<C1... T> struct s2;       // associates (C1<T> && ...)
template<C2... T> struct s3;       // associates C2<T...>(C2<T> && ...)
template<C3<int> T> struct s4;     // associates C3<T, int>
template<C3<int>... T> struct s5;  // associates (C3<T, int> && ...)

—end example]

Insert a new paragraph after (old) paragraph 11.

?. A non-type template parameter declared with a type that contains a placeholder type with a type-constraint introduces the immediately-declared constraint of the invented type corresponding to the placeholder.

Delete (old) paragraph 13.

The default template-argument of a constrained-parameter shall match the kind (type, non-type, template) of the declared template parameter. [Example: […] —end example]

Modify (old) paragraph 19.

If a template-parameter is a type-parameter with an ellipsis prior to its optional identifier or is a parameter-declaration that declares a pack (9.2.3.5), then the template-parameter is a template parameter pack (12.6.3). A template parameter pack that is a parameter-declaration whose type contains one or more unexpanded packs is a pack expansion. Similarly, a template parameter pack that is a type-parameter with a template-parameter-list containing one or more unexpanded packs is a pack expansion. A type parameter pack with a type-constraint that contains an unexpanded parameter pack is a pack expansion. A template parameter pack that is a pack expansion shall not expand a template parameter pack declared in the same template-parameter-list.

Modify [temp.constr.decl, 12.4.2] paragraph 2 as follows.

Constraints can also be associated with a declaration through the use of constrained-parameterstype-constraints in a template-parameter-list. Each of these forms introduces additional constraint-expressions that are used to constrain the declaration.

Modify paragraph 3 as follows.

A template’s associated constraints are defined as follows:

• […]
• Otherwise, the associated constraints are the normal form of a logical AND expression (7.6.14) whose operands are in the following order:
  • the constraint-expression introduced by each constrained-parametertype-constraint (12.1) in the declaration’s template-parameter-list, in order of appearance, and
  • the constraint-expression introduced by a requires-clause following a template-parameter-list (Clause12), and
  • the constraint-expression introduced by type-constraint in the parameter-type-list of a function declaration
  • the constraint-expression introduced by a trailing requires-clause (9.2) of a function declaration (9.2.3.5).

Modify [temp.decls, 12.6] paragraph 2 as follows.

For purposes of name lookup and instantiation, default arguments, partial-concept-idstype-constraints, requires-clauses (Clause 12), and noexcept-specifiers of function templates and of member functions of class templates are considered definitions; each default argument, partial-concept-idstype-constraint, requires-clause, or noexcept-specifier is a separate definition which is unrelated to the templated function definition or to any other default arguments partial-concept-ids, type-constraints, requires-clauses, or noexcept-specifiers. For the purpose of instantiation, the substatements of a constexpr if statement (8.4.1) are considered definitions.

Modify [temp.variadic, 12.6.3] paragraph 5 bullet (5.3.2) as follows.

A pack expansion consists of […]

• […]
• In a template parameter pack that is a pack expansion (12.1):
  • if the template parameter pack is a parameter-declaration; the pattern is the parameter-declaration without the ellipsis;
  • if the template parameter pack is a type-parameter with a template-parameter-list; the pattern is the corresponding type-parameter without the ellipsis.
• In an initializer-list (9.3); the pattern is an initializer-clause.
• […]

Modify the example in [temp.concept, 12.6.8] paragraph 2 as follows.

...
template<C T>  // C, as a type-constraint, constrains f2(T) as a constrained-parameter
...

Modify paragraph 6 as follows.

The first declared template parameter of a concept definition is its prototype parameter. A type concept is a concept whose prototype parameter is a type template-parameter.A variadic concept is a concept whose prototype parameter is a template parameter pack.

Modify [temp.res, 12.7] paragraph 8 item (8.2) as follows.

• no valid specialization can be generated for a template or a substatement of a constexpr if statement (8.4.1) within a template and the template is not instantiated, or
• no substitution of template arguments into a partial-concept-idtype-constraint or requires-clause would result in a valid expression, or
• every valid specialization of a variadic template requires an empty template parameter pack, or
• […]

Modify the note in [temp.inst, 12.8.1] paragraph 1 as follows.

[…] [Note: Within a template declaration, a local class (10.5) or enumeration and the members of a local class are never considered to be entities that can be separately instantiated (this includes their default arguments, noexcept-specifiers, and non-static data member initializers, if any, but not their partial-concept-idstype-constraints or requires-clauses). […]

Modify paragarph 17 as follows.

The partial-concept-idstype-constraints and requires-clause of a template specialization or member function are not instantiated along with the specialization or function itself, even for a member function of a local class; substitution into the atomic constraints […]