Yet another approach for constrained declarations (original) (raw)

Document number: P1141R2

Ville Voutilainen
Thomas Köppe
Andrew Sutton
Herb Sutter
Gabriel Dos Reis
Bjarne Stroustrup
Jason Merrill
Hubert Tong
Eric Niebler
Casey Carter
Tom Honermann
Erich Keane
Walter E. Brown
Michael Spertus
Richard Smith
2018-11-09

Abstract

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

Contents

  1. Revision history
  2. Proposal summary
  3. Proposal details
  4. Proposed wording for Parts 1, 3, and 4

Revision history

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, andtemplate <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 notrequires Sortable<T...>.

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

This paper specifically does not propose

The idea of this approach is to provide a syntax that

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 ofP0807R0 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 whereverauto 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 concepttemplate <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.

[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 ofT depends on the kind of the prototype parameter of Constraint.

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

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:

Other use cases can be done with _requires-clause_s.

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:

In other words,

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 ifthe auto type-specifier appears as one of the_decl-specifier_sthere 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 ofauto 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 ofauto in the decl-specifier_s 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-specifier_s.

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-specifier_s to the table of_simple-type-specifier_s 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-specifier_s 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 the~~The 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 ordecltype(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-specifier_s 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 typevoid, 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 typeT′ replacing T is determined using the rules for template argument deduction. Obtain P from T by replacing the occurrences of _type-constraint_opt autowith 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-parameter_s are appended to the_template-parameter-list after the explicitly declared_template-parameter_s.

[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′, otherwiseE 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.

[Example:

`~~template concept C1 = true;~~ template<template class X> concept C2 = true; template concept C3 = true; template<typename... Ts> concept C4 = true; template<char... Cs> concept C5 = true;

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

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-parameter_s_type-constraint_sin a template-parameter-list. Each of these forms introduces additional_constraint-expression_s that are used to constrain the declaration.

Modify paragraph 3 as follows.

A template’s associated constraints are defined as follows:

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

For purposes of name lookup and instantiation, default arguments,_partial-concept-id_s_type-constraint_s,_requires-clause_s (Clause 12), and _noexcept-specifier_s of function templates and of member functions of class templates are considered definitions; each default argument,_partial-concept-id_stype-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-id_s,_type-constraint_s, _requires-clause_s, or_noexcept-specifier_s. 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 […]

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.

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-specifier_s, and non-static data member initializers, if any, but not their_partial-concept-id_s_type-constraint_sor _requires-clause_s). […]

Modify paragarph 17 as follows.

The _partial-concept-id_s_type-constraint_sand 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 […]