Why does std::function can implicit convert to a std::function which has more parameter？

I have the following：

void print_str(std::shared_ptr<std::string> str) {
    std::cout << str->c_str() << std::endl;
}

int main() {
    auto str = std::make_shared<std::string>("Hello");
    std::function<void()> f = std::bind(print_str, str);

    f(); // correctly print: Hello

    return 0;
}

I think the type of std::bind(print_str, str) is std::function<void(std::shared_ptr<std::string>)>, but the code above is correctly running. Is there any trick in std::bind?

env: centos, gcc82