Why operator= returns reference not const reference

Question

The original question is related to overloading operator= and I like to share my findings as it was nontrivial for me to find them. I cannot imagine reasonable example to us

Answer 1

One good reason is that one of the requirements in the standard for a class X to be useable in the standard containers is that the expression a = b must have type X& (where a is an lvalue of type X and b is an rvalue of type X).