std::equivalence_relation
From cppreference.com
Defined in header <concepts>


template < class R, class T, class U > concept equivalence_relation = std::relation<R, T, U>; 
(since C++20)  
The concept equivalence_relation<R, T, U>
specifies that the relation R
imposes an equivalence relation on its arguments.
[edit] Semantic requirements
A relation r
is an equivalence relation if
 it is reflexive: for all
x
,r(x, x)
is true;  it is symmetric: for all
a
andb
,r(a, b)
is true if and only ifr(b, a)
is true;  it is transitive:
r(a, b) && r(b, c)
impliesr(a, c)
.
[edit] Notes
The distinction between relation and equivalence_relation
is purely semantic.