template<input_iterator I, sentinel_for<I> S,
         class Proj = identity,
         indirect_unary_predicate<projected<I, Proj>> Pred>
constexpr iter_difference_t<I>
    count_if(I first, S last, Pred pred, Proj proj = {});

template<input_range R, class Proj = identity,
         indirect_unary_predicate<projected<iterator_t<R>, Proj>> Pred>
constexpr range_difference_t<R>
    count_if(R&& r, Pred pred, Proj proj = {});

#include <iostream>
#include <algorithm>

namespace range = std::ranges;

int main()
{
    auto list = {1, 2, 3, 1, 2, 3, 3};
    auto count3 = range::count_if(list, [](auto s){return s == 3;});
    auto count7 = range::count_if(list, [](auto s){return s == 7;});
    std::cout << "count3: " << count3 << std::endl;
    std::cout << "count7: " << count7 << std::endl;
    return 0;
}

count3: 3
count7: 0