核心问题
章振邦等编《新编英语语法》(下)(上海译文出版社,p.705)说：“能在关系分句中作主语补语的关系代词只有that一个词，that作这种用法，既能指人也能指物，但往往省略。例如：Dr Fordham still talks like the man that he was ten years ago. When Lauren was born, Dorothy decided her daughter would be the singer(that) she always wanted to be. My typewriter is not the machine(that) it was.”
这种说法使我迷惑不解:难道 which 不能在关系从句中作主语补语吗?
词典解答
《新编英语语法》的说法是值得商榷的。实际上，无论在限制性定语从句里，还是在非限制性定语从句里，which也可作主语补语，请看下面的实例：
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She is the perfect accountant her predecessor was not. (CGEL) 她是最优秀的会计师，而她的前任却不是这样。
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This is not the type of modern house my own is. (CGEL) 这不是现代化房屋的样式, 而我的房子却是。
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Anna is vegetarian, no one else is in our family. (CGEL) 安娜是个素食
者，而我们家的其他人都不是。
- She wants low-calorie food, this vegetable curry certainly is. (CGEL) 她需要吃低热量的食物，咖喱这种蔬菜肯定是低热量食物。
此外，CGEL认为，在非限制性关系分句里，关系代词that是极为罕见的。