[temp.concept]

# 13 Templates [[temp]](./#temp)

## 13.7 Template declarations [[temp.decls]](temp.decls#temp.concept)

### 13.7.9 Concept definitions [temp.concept]

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/templates.tex#L4684)

A [*concept*](#def:concept "13.7.9 Concept definitions [temp.concept]") is a template
that defines constraints on its template arguments[.](#1.sentence-1)

[concept-definition:](#nt:concept-definition "13.7.9 Concept definitions [temp.concept]")  
concept [*concept-name*](#nt:concept-name "13.7.9 Concept definitions [temp.concept]") [*attribute-specifier-seq*](dcl.attr.grammar#nt:attribute-specifier-seq "9.13.1 Attribute syntax and semantics [dcl.attr.grammar]")opt = [*constraint-expression*](temp.constr.decl#nt:constraint-expression "13.5.3 Constrained declarations [temp.constr.decl]") ;

[concept-name:](#nt:concept-name "13.7.9 Concept definitions [temp.concept]")  
[*identifier*](lex.name#nt:identifier "5.11 Identifiers [lex.name]")

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/templates.tex#L4698)

A [*concept-definition*](#nt:concept-definition "13.7.9 Concept definitions [temp.concept]") declares a concept[.](#2.sentence-1)

Its [*identifier*](lex.name#nt:identifier "5.11 Identifiers [lex.name]") becomes a [*concept-name*](#nt:concept-name "13.7.9 Concept definitions [temp.concept]") referring to that concept
within its scope[.](#2.sentence-2)

The optional [*attribute-specifier-seq*](dcl.attr.grammar#nt:attribute-specifier-seq "9.13.1 Attribute syntax and semantics [dcl.attr.grammar]") appertains to the concept[.](#2.sentence-3)

[*Example [1](#example-1)*: template<typename T>concept C = requires(T x) {{ x == x } -> std::[convertible_to](concept.convertible#concept:convertible_to "18.4.4 Concept convertible_­to [concept.convertible]")<bool>;};

template<typename T>requires C<T> // C constrains f1(T) in [*constraint-expression*](temp.constr.decl#nt:constraint-expression "13.5.3 Constrained declarations [temp.constr.decl]") T f1(T x) { return x; }template<C T> // C, as a [*type-constraint*](temp.param#nt:type-constraint "13.2 Template parameters [temp.param]"), constrains f2(T) T f2(T x) { return x; } — *end example*]

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/templates.tex#L4721)

A [*concept-definition*](#nt:concept-definition "13.7.9 Concept definitions [temp.concept]") shall inhabit a namespace scope ([[basic.scope.namespace]](basic.scope.namespace "6.4.6 Namespace scope"))[.](#3.sentence-1)

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/templates.tex#L4725)

A concept shall not have [associated constraints](temp.constr.decl#def:associated_constraints "13.5.3 Constrained declarations [temp.constr.decl]")[.](#4.sentence-1)

[5](#5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/templates.tex#L4728)

A concept is not instantiated ([[temp.spec]](temp.spec "13.9 Template instantiation and specialization"))[.](#5.sentence-1)

[*Note [1](#note-1)*:

A concept-id ([[temp.names]](temp.names "13.3 Names of template specializations")) is evaluated as an expression[.](#5.sentence-2)

A concept cannot be
explicitly instantiated ([[temp.explicit]](temp.explicit "13.9.3 Explicit instantiation")),
explicitly specialized ([[temp.expl.spec]](temp.expl.spec "13.9.4 Explicit specialization")),
or partially specialized ([[temp.spec.partial]](temp.spec.partial "13.7.6 Partial specialization"))[.](#5.sentence-3)

— *end note*]

[6](#6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/templates.tex#L4738)

The [*constraint-expression*](temp.constr.decl#nt:constraint-expression "13.5.3 Constrained declarations [temp.constr.decl]") of a [*concept-definition*](#nt:concept-definition "13.7.9 Concept definitions [temp.concept]") is an unevaluated operand ([[expr.context]](expr.context "7.2.3 Context dependence"))[.](#6.sentence-1)

[7](#7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/templates.tex#L4742)

The first declared template parameter of a concept definition is its[*prototype parameter*](#def:prototype_parameter,concept "13.7.9 Concept definitions [temp.concept]")[.](#7.sentence-1)

A [*type concept*](#def:concept,type "13.7.9 Concept definitions [temp.concept]") is a concept whose prototype parameter
is a type template parameter[.](#7.sentence-2)