楼主的问题很专业,其实关于which到底能不能做表语,很多权威语法书都有争议.目前为止,是第三种说法正确.当先行词在主句中做表语,或者,引导词在从句中作表语,符合任一种情况都只能用that.Shenzhen is no longer the city that it used to be .属于引导词在从句中作表语.This isn't the book that I borrowed from the library.属于先行词在主句中作表语.
有争议说which可作表语,但也只有这样的例子,She was fond of dancing,which her brother never was.只能说明是在非限制性定语从句中,which 可作表语.