std::ranges::enumerate_view<V>:: size

#include <cassert>
#include <forward_list>
#include <ranges>
#include <string_view>
int main()
{
    constexpr static auto v1 = {1, 2, 3, 4, 5};
    constexpr auto ev1{v1 | std::views::enumerate};
    static_assert(ev1.size() == 5);
    static_assert(std::ranges::sized_range<decltype(v1)>);
    auto v2 = std::forward_list{1, 2, 3, 4, 5};
    auto ev2 {v2 | std::views::enumerate};
    static_assert(not std::ranges::sized_range<decltype(v2)>);
    // auto size = ev2.size(); // 错误：v2 不是 sized range
    assert(std::ranges::distance(v2) == 5); // 正常，distance 不要求 sized range
                                            // 但此处具有 O(N) 时间复杂度
}