From cppreference.com
template<classR,classT,classU>conceptequivalence_relation=std::relation<R,T,U>;(since C++20)The concept equivalence_relation<R, T, U> specifies that the
R imposes an
on its arguments.
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 and b, r(a, b) is true if and only if r(b, a) is true;
it is transitive: r(a, b) && r(b, c) implies r(a, c).
Notes
The distinction between
and equivalence_relation is purely semantic.
References
C++23 standard (ISO/IEC 14882:2024):
18.7.6 Concept equivalence_relation [concept.equiv]
C++20 standard (ISO/IEC 14882:2020):
18.7.6 Concept equivalence_relation [concept.equiv]