为什么常量返回防止:(A1 = A2） = A3?

我不明白为什么将const添加到返回类型会阻止(a1 = a2) = a3，正如第 2 行的评论所说。
有人可以为我解释一下吗？

// overloaded assignment operator;
// const return prevents: (a1 = a2) = a3
const Array& Array::operator=(const Array& right) {
if (&right != this) { // avoid self-assignment
// for Arrays of different sizes, deallocate original
// left-side Array, then allocate new left-side Array
if (size != right.size) {
delete[] ptr; // release space
size = right.size; // resize this object
ptr = new int[size]; // create space for Array copy
}
for (size_t i{ 0 }; i < size; ++i) {
ptr[i] = right.ptr[i]; // copy array into object
}
}
return *this; // enables x = y = z, for example
}