递归std::函数正在抛出错误

#include <iostream>
#include <functional>

using namespace std;

int main()
{
    std::function<int(int)> x = [](int y) -> int {
        if(y == 0) {
            return 0;
        } else {
            return y + x(--y);
        }
    };
    
    cout << x(3) << endl;
}

main.cpp: In lambda function:
main.cpp:20:24: error: ‘x’ is not captured
             return y + x(--y);
                        ^
main.cpp:16:34: note: the lambda has no capture-default
     std::function<int(int)> x = [](int y) -> int {
                                  ^
main.cpp:16:29: note: ‘std::function x’ declared here
     std::function<int(int)> x = [](int y) -> int {
                             ^

这个抛出错误是因为当编译器到达x(-y)时x没有完全定义吗?谁能告诉我怎么解决这个问题吗。