std::indirectly_unary_invocable, std::indirectly_regular_unary_invocable

#include <algorithm>
#include <iterator>
#include <print>
#include <ranges>
 
struct IntWrapper
{
 int i;
 
 explicit IntWrapper(int i) : i(i) {}
 IntWrapper(IntWrapper&&) = default;
 IntWrapper& operator=(IntWrapper&&) = default;
};
 
int main()
{
 auto ints = std::views::iota (1, 10);
 auto print = [] (IntWrapper w) { std::print ("{} ", w.i); };
 auto wrap = [] (int i) { return IntWrapper{i}; };
 
 using Proj = std::projected <decltype(ints.begin()), decltype(wrap)>;
 
 // error (evaluated to false) until P2609R3:
 // this was because 'std::iter_value_t<Proj> &' is the same as 'IntWrapper&'
 // which is not convertible to 'IntWrapper' (implicitly deleted copy ctor)
 static_assert(std::indirectly_unary_invocable<decltype(print), Proj>);
 
 // if the compile-time check above evaluates to true, then this is well-formed:
 std::ranges::for_each (ints, print, wrap);
}

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