Tested with DMD 2.055

void main() {
    enum foo={
        char[] c="".dup; // works without initialization
        // string c="";  // same with this
        // ubyte[] c=[]; // fine
        auto p = c.ptr;  // error here
        return 0;
    }();
}

Error: CTFE error: cannot cast cast(char*)c to type char*