C++ implement const method overload

Question

I have recently seen some strange approach on overload a non-const method with a const one.

Lets say we have

const int& foo() const {
  return 42;
}

And now the const version was implemented like this

int& foo() {
  return const_cast<int&>(std::as_const(*this).foo());
}

Is there a reason behind this? And if this is the right way, when should i use this approach?