std::ranges::subrange<I,S,K>::size

constexpr /*make-unsigned-like-t*/<std::iter_difference_t<I>> size() const
    requires (K == ranges::subrange_kind::sized);

#include <functional>
#include <iostream>
#include <ranges>
#include <utility>

int main()
{
    const auto v = {2, 2, 2, 7, 1, 1, 1, 1, 8, 2, 2, 2, 2, 2};
    
    // the value type of views::chunk_by is the ranges::subrange
    
    auto to_pair = [](auto sub) { return std::make_pair(sub[0], sub.size()); };
                                                                 /* ^^^^ */
    auto pairs = v | std::views::chunk_by(std::equal_to{})
                   | std::views::transform(to_pair);
    
    for (auto x : pairs bitor std::views::keys)
        std::cout << x << ' ';
    std::cout << '\n';
    for (auto x : pairs bitor std::views::values)
        std::cout << x << ' ';
    std::cout << '\n';
}

Вывод:

2 7 1 8 2
3 1 4 1 5