鸭子类型与 C++20 concept 中 自带干粮 部分的疑问

```
#include <concepts>
#include <iostream>
#include <type_traits>

class Data {
public:
    Data(int x, int y) : x{x}, y{y} {}
    int x;
    int y;
    void show() {
        std::cerr << x << " " << y << "\n";
    }
};
```

不行：
```
const bool a = requires (Data d{1, 2}) {d.show();};
```

行：
```
const bool a = requires (Data d = Data{1, 2}) {d.show();};
```

是为什么
