The language is fundamentally too weak to express the kind of reference and borrowing relationships demanded by CoW.

Suppose I have a string and I take a reference to a character:

    std::string a = f(); // shared
    char& c = a.front();

Should the string be copied or not? One might argue, no taking a reference is different from "writing" so no copying, but one might also argue that reference can be used to change the string later, and the string doesn't know when this would happen so it has to pessimistically make a copy.

You get bugs if you go with the former, and bad performance if you go with the latter.