std::indirect_equivalence_relation
        <iterator> 
 concept indirect_equivalence_relation =
    std::indirectly_readable <I1> &&
    std::indirectly_readable <I2> &&
    std::copy_constructible <F> &&
    std::equivalence_relation 
        <F&, /*indirect-value-t*/<I1>, /*indirect-value-t*/<I2>> &&
    std::equivalence_relation 
        <F&, /*indirect-value-t*/<I1>, std::iter_reference_t <I2>> &&
    std::equivalence_relation 
        <F&, std::iter_reference_t <I1>, /*indirect-value-t*/<I2>> &&
    std::equivalence_relation 
The concept indirect_equivalence_relation specifies requirements for algorithms that call equivalence relations as their arguments. The key difference between this concept and std::equivalence_relation  is that it is applied to the types that I1 and I2 references, rather than I1 and I2 themselves.
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior | 
|---|---|---|---|
| P2609R3 | C++20 | some requirements were defined in terms of std::iter_value_t <I>& which mishandled projections resulting in incompatibility with equivalence relation F& | defined in terms of /*indirect-value-t*/<I> to correctly handle such projections | 
| P2997R1 | C++20 | indirect_equivalence_relationrequired F& to satisfyequivalence_relationwithstd::iter_common_reference_t <I> | does not require |