The name ranges::cpo denotes a customization point object, which is a const function object of a literal semiregular class type. For exposition purposes, the cv-unqualified version of its type is denoted as __cpo_fn.
All instances of __cpo_fn are equal. The effects of invoking different instances of type __cpo_fn on the same arguments are equivalent, regardless of whether the expression denoting the instance is an lvalue or rvalue, and is const-qualified or not (however, a volatile-qualified instance is not required to be invocable). Thus, ranges::cpo can be copied freely and its copies can be used interchangeably.
Given a set of types Args..., if std::declval<Args>()... meet the requirements for arguments to ranges::cpo above, __cpo_fn models
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std::invocable<__cpo_fn, Args...>, std::invocable<const __cpo_fn, Args...>, std::invocable<__cpo_fn&, Args...>, and std::invocable<const __cpo_fn&, Args...>.Otherwise, no function call operator of __cpo_fn participates in overload resolution.
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https://en.cppreference.com/w/cpp/ranges/cpo