[temp.concept]

A concept-definition declares a concept.

Its identifier becomes a concept-name referring to that concept within its scope.

The optional attribute-specifier-seq appertains to the concept.

[Example 1: template<typename T> concept C = requires(T x) { { x == x } -> std::convertible_to<bool>; }; template<typename T> requires C<T> / C constrains f1(T) in constraint-expression T f1(T x) { return x; } template<C T> / C, as a type-constraint, constrains f2(T) T f2(T x) { return x; } — end example]

A concept-definition shall inhabit a namespace scope ([basic.scope.namespace]).

A concept is not instantiated ([temp.spec]).

[Note 1:

A concept-id ([temp.names]) is evaluated as an expression.

A concept cannot be explicitly instantiated ([temp.explicit]), explicitly specialized ([temp.expl.spec]), or partially specialized ([temp.spec.partial]).

— end note]

The constraint-expression of a concept-definition is an unevaluated operand ([expr.context]).

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.