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std::indirect_strict_weak_order - cppreference.com

From cppreference.com

template< class F, class I1, class I2 = I1 > concept indirect_strict_weak_order = std::indirectly_readable<I1> && std::indirectly_readable<I2> && std::copy_constructible<F> && std::strict_weak_order <F&, /*indirect-value-t*/<I1>, /*indirect-value-t*/<I2>> && std::strict_weak_order <F&, /*indirect-value-t*/<I1>, std::iter_reference_t<I2>> && std::strict_weak_order <F&, std::iter_reference_t<I1>, /*indirect-value-t*/<I2>> && std::strict_weak_order <F&, std::iter_reference_t<I1>, std::iter_reference_t<I2>>;

 (since C++20)

The concept indirect_strict_weak_order specifies requirements for algorithms that call strict weak orders as their arguments. The key difference between this concept and std::strict_weak_order is that it is applied to the types that I1 and I2 references, rather than I1 and I2 themselves.

Defect reports

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 strict weak order F& 		defined in terms of
/*indirect-value-t*/<I> to
correctly handle such projections
P2997R1 C++20 indirect_strict_weak_order required F& to satisfy
strict_weak_order with std::iter_common_reference_t<I> 		does not require