Resolved: Cannot assign reference value to result of std::invoke

#include <iostream>
#include <cstdlib>
#include <functional>

struct foo {
    int a;
};

int main() {
    std::vector<foo> foos = {foo{22}};
    std::size_t new_index = -1;
    bool append = (rand() & 1) == 1;
    //compiler error in next line
    foo& new_foo = std::invoke([&foos,&new_index,append](){
        if(append) {
            new_index = foos.size();
            return foos.emplace_back();
        }else {
            new_index = 0;
            return foos[new_index];
        }
    });
    return new_foo.a;
}

[&foos, &new_index, append]()-> foo& { /*..*/ }

[&foos,&new_index,append]() {
    if(append) {
        new_index = foos.size();
        return std::ref(foos.emplace_back());
    } else {
        new_index = 0;
        return std::ref(foos[new_index]);
    }
}